Sodium Loss Calculator for Exercise: How to Estimate Your Sweat Sodium Loss

Written by Tomasz Antczak · Published May 6, 2026 · Updated October 9, 2026

Use a simple sodium loss calculator for exercise to estimate total sweat sodium loss and sodium loss per hour, while understanding the limits of estimates without a measured sweat sodium concentration.

A sodium-loss calculator can do one thing very well:

show you how sweat volume and sweat sodium concentration combine.

What it cannot do is magically discover how salty your sweat is.

That distinction matters.

If you know:

• how much sweat you lost
• your sweat sodium concentration

then sodium loss is straightforward to calculate.

The basic formula is:

Sodium loss = sweat volume × sweat sodium concentration

For example:

1.5 L sweat × 700 mg sodium/L = 1,050 mg sodium lost

But if you know only that you lost:

1.5 L of sweat

you still do not know your exact sodium loss.

Your sweat might contain:

• 400 mg sodium/L
• 800 mg/L
• 1,200 mg/L
• or another value entirely

So this calculator is most useful in two different ways:

1. with a measured sweat sodium concentration, it can estimate your sodium loss from a session
2. without a sweat test, it can show a range of possible outcomes rather than pretending to know one exact number

That is the right way to use it.

For the underlying difference between sweat volume and sweat composition, read Sweat Rate vs Sodium Loss: What to Measure After Hard Training.

Calculate sodium lost during your session

The sodium-loss formula

The simplest version is:

Total sodium loss (mg) = total sweat loss (L) × sweat sodium concentration (mg/L)

Example:

Total sweat loss:

2.0 L

Sweat sodium concentration:

600 mg/L

Calculation:

2.0 × 600 = 1,200 mg

Estimated total sodium loss:

1,200 mg

That is all the calculator is doing.

The difficulty is obtaining reliable inputs.

You need two different measurements

To estimate sodium loss properly, separate:

1. Sweat volume

How much sweat you lost.

Usually expressed in:

litres — L

or, when looking at rate:

litres per hour — L/h

2. Sweat sodium concentration

How much sodium is present in each litre of sweat.

Usually expressed as:

mg/L

or in scientific literature:

mmol/L

These are not interchangeable measurements.

A high sweat rate does not automatically mean highly concentrated salty sweat.

And highly concentrated sweat does not automatically mean a huge total sodium loss if the total sweat volume is small.

Step 1: calculate your sweat loss

For most normal workouts, a practical field estimate is:

Sweat loss ≈ pre-exercise body weight − post-exercise body weight + fluid consumed − urine produced

If food was consumed during a long event, that can also be included in a more detailed calculation.

For example:

Pre-workout weight:

80.0 kg

Post-workout weight:

79.1 kg

Fluid consumed:

0.6 L

Urine:

0 L

Body-weight loss:

0.9 kg

Estimated sweat loss:

0.9 + 0.6 = 1.5 L

For the complete measurement method, read How to Measure Your Sweat Rate Correctly.

Step 2: enter your sweat sodium concentration

If you have a professionally measured or otherwise validated result, use it.

Example:

Sweat sodium concentration:

650 mg/L

Then:

1.5 L × 650 mg/L = 975 mg sodium

Estimated total sodium loss:

975 mg

That is a much more personalised calculation than assuming everybody loses the same amount of sodium.

What if you do not know your sweat sodium concentration?

Then you cannot calculate an exact personal sodium loss.

But you can still calculate scenarios.

This is useful because it shows how sensitive the answer is to the unknown sodium concentration.

Suppose your measured sweat loss was:

1.5 L

Now compare several possible concentrations.

Sweat sodium concentration	Estimated sodium loss
400 mg/L	600 mg
800 mg/L	1,200 mg
1,200 mg/L	1,800 mg

Same sweat volume.

Three very different sodium-loss estimates.

That is why a calculator should not return one apparently exact result unless you actually know the sodium concentration being entered.

These are scenarios, not “sweater types”

Do not interpret:

400 mg/L

as:

low-sodium person

and:

1,200 mg/L

as:

high-sodium person

based only on guesswork.

Those numbers are useful calculator scenarios.

They are not a diagnosis.

Research involving athletes has found substantial variation in predicted whole-body sweat sodium concentration.

One dataset of 506 athletes reported a range of approximately:

18.2–70.8 mmol/L

which converts to roughly:

420–1,630 mg sodium/L

for predicted whole-body sweat.

That range illustrates why one universal calculator value is inappropriate.

It does not mean every athlete falls permanently somewhere inside exactly that range.

Converting mmol/L to mg/L

Scientific sweat testing often reports sodium in:

mmol/L

Sports drinks and nutrition labels are more likely to use:

mg

For sodium:

1 mmol sodium ≈ 23 mg

So:

20 mmol/L ≈ 460 mg/L

30 mmol/L ≈ 690 mg/L

40 mmol/L ≈ 920 mg/L

50 mmol/L ≈ 1,150 mg/L

60 mmol/L ≈ 1,380 mg/L

If your sweat-test report gives mmol/L, multiply by approximately:

23

to obtain mg/L.

Example

Sweat test result:

35 mmol/L

Calculation:

35 × 23 = 805 mg/L

So the sweat sodium concentration is approximately:

805 mg/L

If you lost:

1.4 L of sweat

then:

1.4 × 805 = 1,127 mg sodium

Estimated total sodium loss:

about 1,130 mg

Do not pretend the result is precise to the nearest milligram.

Your field measurements are not that exact.

Total sodium loss vs sodium loss per hour

These are different numbers.

Total sodium loss

How much sodium you lost across the entire session.

Sodium loss rate

How quickly you were losing sodium.

Usually expressed as:

mg/hour

To calculate sodium loss rate:

Sweat rate (L/h) × sweat sodium concentration (mg/L)

Example:

Sweat rate:

1.0 L/h

Sweat sodium:

700 mg/L

Sodium loss rate:

700 mg/hour

If the session lasts two hours:

Total sodium loss:

1,400 mg

Why mg/h can be useful

Hourly loss becomes particularly useful when comparing sessions of different durations.

For example:

Session A

Sweat rate:

0.6 L/h

Sweat sodium:

700 mg/L

Sodium loss rate:

420 mg/h

Session B

Sweat rate:

1.3 L/h

Sweat sodium:

700 mg/L

Sodium loss rate:

910 mg/h

The sweat sodium concentration is identical.

The hotter or more demanding session produces more than twice the sodium loss per hour because sweat volume is much greater.

But mg/h is not automatically your replacement dose

This is very important.

If the calculator estimates that you lost:

800 mg sodium/hour

that does not automatically mean:

you must consume exactly 800 mg sodium every hour during exercise.

Loss and replacement are related.

They are not necessarily identical.

Your replacement strategy also depends on:

• session duration
• food before exercise
• food afterwards
• recovery time
• fluid intake
• tolerance
• whether another session is coming soon
• overall dietary sodium

Some sodium can be replaced after exercise through normal food.

The calculator estimates loss.

It does not automatically prescribe intake.

Example: short gym workout

Suppose:

Duration:

60 minutes

Sweat loss:

0.5 L

Assumed sodium scenario:

600 mg/L

Estimated sodium loss:

0.5 × 600 = 300 mg

This is a relatively modest total loss.

If you eat a normal meal afterwards, there may be little reason to create a complicated electrolyte protocol around this workout.

Example: hard Muay Thai session

Before:

86.0 kg

After:

84.9 kg

Fluid consumed:

0.7 L

No urine.

Estimated sweat loss:

1.1 + 0.7 = 1.8 L

Now calculate several sodium scenarios.

400 mg/L

1.8 × 400 = 720 mg sodium

800 mg/L

1.8 × 800 = 1,440 mg

1,200 mg/L

1.8 × 1,200 = 2,160 mg

That range is large.

Without a sweat sodium measurement, claiming:

“You lost exactly 1,440 mg”

would create false precision.

Example: three-hour run

Sweat rate:

0.9 L/h

Duration:

3 hours

Total sweat loss:

2.7 L

Now compare concentrations.

Sweat sodium	Total sodium loss
400 mg/L	1,080 mg
800 mg/L	2,160 mg
1,200 mg/L	3,240 mg

This is where the uncertainty in sweat sodium concentration begins to matter much more.

When sweat volume becomes large, even a modest difference in sodium concentration produces a large difference in estimated total loss.

Example: hot-weather training

Your normal sweat rate:

0.7 L/h

Hot-weather sweat rate:

1.3 L/h

Assume the same sodium concentration for this example:

650 mg/L

Normal:

0.7 × 650 = 455 mg sodium/hour

Hot:

1.3 × 650 = 845 mg/hour

Even if sweat sodium concentration did not change, the higher sweat rate substantially increases sodium loss.

For heat-specific planning, read Hydration in Hot Weather: How to Train in Heat Without Crashing.

Heat acclimation complicates the calculation

Sweat sodium concentration itself can also change.

With heat acclimation, the body can become better at conserving sodium through the sweat glands.

So over time you may experience:

• increased sweat production
• lower sweat sodium concentration

at the same time.

That means hot-weather sodium loss is determined by the interaction of both changes.

Again:

volume × concentration

is what matters.

Example: before and after acclimation

Before acclimation:

Sweat rate:

0.8 L/h

Sweat sodium:

800 mg/L

Sodium loss rate:

640 mg/h

After acclimation:

Sweat rate:

1.2 L/h

Sweat sodium:

600 mg/L

Sodium loss rate:

720 mg/h

The sweat became less salty.

But total sodium loss per hour still increased because sweat volume rose.

Why salt marks cannot complete the calculator

You may see white residue on:

• shirts
• hats
• shorts
• skin

That can suggest salt is being left behind as sweat evaporates.

But it cannot tell you whether your concentration is:

500 mg/L

or:

900 mg/L

or:

1,300 mg/L

So do not enter an invented number into the calculator simply because your shirt has white marks.

Use the result as evidence that sodium loss may deserve attention.

Not as a quantitative sweat test.

Sweat stinging your eyes is not a measurement

The same applies if sweat:

• tastes very salty
• stings your eyes
• leaves visible crystals

Those are qualitative observations.

They cannot provide an exact mg/L value.

Cramps cannot complete the calculator either

Muscle cramps do not reveal sweat sodium concentration.

Exercise-associated cramps are multifactorial.

They may relate to:

• neuromuscular fatigue
• exercise intensity
• previous cramp history
• environmental stress
• fluid and sodium losses in some circumstances

Do not type:

1,000 mg/L

into a calculator because you cramped during a workout.

That number has not been measured.

Headache is not a sodium test

A headache after exercise can occur with:

• dehydration
• heat
• fatigue
• insufficient food
• caffeine withdrawal
• illness
• exercise-associated hyponatremia

It cannot tell you how much sodium you lost.

How accurate is sweat-rate measurement?

Whole-body sweat rate can be estimated relatively well using change in body mass, corrected for:

• fluid intake
• food intake where relevant
• urine or other measurable output

But field testing still contains measurement error.

Potential problems include:

• wet clothes
• different scales
• delayed weighing
• spilled drinks
• inaccurate bottle measurement
• unrecorded urine
• non-sweat changes in body mass

For normal athletes, the goal is useful approximation.

Not laboratory precision.

How accurate is sweat sodium testing?

That depends heavily on the method.

Sweat sodium measurement can be influenced by:

• body location
• skin cleaning
• collection device
• sample contamination
• collection duration
• storage
• analysis technique

Local sweat samples do not automatically equal whole-body sweat concentration.

A result from one patch should be interpreted according to the validation method used by that testing system.

When professional sweat sodium testing is most useful

Testing becomes more useful when sodium loss could meaningfully affect a planned strategy.

Examples include:

• marathon or ultramarathon racing
• long cycling events
• triathlon
• prolonged training in heat
• very high sweat rates
• repeated same-day training
• tournaments
• athletes trying to build a repeatable competition hydration plan

For a normal:

45–60 minute gym session

the added precision may have little practical value.

Sweat rate should usually come first

Before paying for sodium testing, know approximately:

how much you sweat.

That is because even a precise sweat sodium result is incomplete without sweat volume.

If your sweat sodium concentration is:

800 mg/L

but you do not know whether you lose:

0.4 L

or:

2.0 L

you still do not know total sodium loss.

For most recreational athletes:

measure sweat rate first.

Scenario calculator table

If you do not know your sweat sodium concentration, this table shows several plausible calculation scenarios.

It does not classify you.

Sweat loss	At 400 mg/L	At 800 mg/L	At 1,200 mg/L
0.5 L	200 mg	400 mg	600 mg
1.0 L	400 mg	800 mg	1,200 mg
1.5 L	600 mg	1,200 mg	1,800 mg
2.0 L	800 mg	1,600 mg	2,400 mg
2.5 L	1,000 mg	2,000 mg	3,000 mg
3.0 L	1,200 mg	2,400 mg	3,600 mg

Look at how wide the range becomes as sweat volume increases.

That is the main lesson of the table.

Sodium loss calculator by hour

You can also combine sweat rate with sodium concentration.

Sweat rate	400 mg/L	800 mg/L	1,200 mg/L
0.5 L/h	200 mg/h	400 mg/h	600 mg/h
0.75 L/h	300 mg/h	600 mg/h	900 mg/h
1.0 L/h	400 mg/h	800 mg/h	1,200 mg/h
1.25 L/h	500 mg/h	1,000 mg/h	1,500 mg/h
1.5 L/h	600 mg/h	1,200 mg/h	1,800 mg/h

Again:

these are arithmetic scenarios.

They are not recommended sodium doses.

A calculator cannot know how much sodium you should consume

This distinction is worth repeating.

The calculator estimates:

loss

It does not calculate:

the exact amount you should ingest.

Replacement is influenced by:

• exercise duration
• food
• recovery time
• tolerance
• next training session
• environmental conditions
• individual health considerations

Do not turn a loss estimate directly into a supplement dose without context.

Do you need to replace 100% of sodium loss?

Not always.

Imagine you lose an estimated:

700 mg sodium

during a workout.

You finish training.

You eat a normal meal containing:

• bread
• meat
• cheese
• sauce
• soup

That meal may already replace a substantial amount of sodium.

There is no requirement that every milligram be replaced in a sports drink before you leave the gym.

Food is part of sodium replacement

Sodium comes from many normal foods.

Examples include:

• bread
• cheese
• soups
• sandwiches
• sauces
• salted potatoes
• rice dishes
• many prepared foods

A hydration calculator should not make you forget that athletes eat.

When deliberate sodium replacement becomes more useful

It becomes more relevant when:

• the total sweat loss is large
• exercise lasts several hours
• the environment is hot
• you sweat heavily
• another session follows soon
• normal food cannot be consumed yet

That is when electrolyte drinks or other sodium-containing options can be practical.

Read Electrolytes for Workouts: When You Need Them.

Two sessions in one day

Imagine:

Morning Muay Thai sweat loss:

1.6 L

Evening gym sweat loss:

0.8 L

Total:

2.4 L

If your sweat sodium concentration were:

700 mg/L

estimated daily training sodium loss would be:

2.4 × 700 = 1,680 mg

Now the short recovery window makes deliberate fluid and sodium replacement more relevant.

For the full strategy, read Two Workouts a Day: Simple Hydration and Recovery Plan.

Do not solve sodium loss by drinking unlimited water

Sodium loss sometimes leads people to the wrong conclusion:

“I lost a lot, so I should drink as much as possible.”

Fluid and sodium are related.

But excessive fluid intake can become dangerous during prolonged exercise.

Exercise-associated hyponatremia is strongly associated with consuming fluid faster than it is lost and excreted.

Electrolyte drinks do not make unlimited drinking safe.

Read Can You Drink Too Much Water During Exercise? Hyponatremia Explained.

Sodium in an electrolyte drink does not guarantee a perfect match

Suppose your estimated sodium loss is:

1,500 mg

and your drink contains:

500 mg/L.

That does not automatically mean:

drink exactly three litres.

Three litres could substantially exceed your fluid requirement.

Fluid intake should still respect:

• sweat loss
• thirst
• exercise duration

Sodium concentration and fluid volume must be considered together.

Check the label correctly

If you use an electrolyte product, look for:

sodium

not only:

electrolytes

and not only:

salt.

Products vary enormously.

Check:

• sodium per serving
• serving size
• final volume of the mixed drink

Sodium vs salt

These terms are related but not identical.

Table salt is:

sodium chloride

A useful approximate food-label conversion is:

salt ≈ sodium × 2.5

and:

sodium ≈ salt ÷ 2.5

So:

1,000 mg sodium ≈ 2.5 g salt

Do not accidentally treat:

1 gram of salt

as:

1 gram of sodium.

Example using a drink label

Suppose an electrolyte drink provides:

500 mg sodium per litre.

You drink:

750 mL

Sodium consumed:

0.75 × 500 = 375 mg

That is separate from the calculation of what you lost.

If your estimated sodium loss was:

1,200 mg

you consumed:

375 mg

during exercise.

That does not automatically mean you must immediately consume the remaining:

825 mg

as a supplement.

Food and post-workout intake also matter.

Do not use fake precision

Suppose the calculator produces:

1,347 mg

But your body scale reads only to:

0.1 kg

and your sweat sodium value is an estimate.

Reporting:

1,347 mg

suggests a precision that does not exist.

A more honest result would be:

approximately 1.3 g sodium

or:

roughly 1,300–1,400 mg

depending on the quality of the inputs.

An estimate can still be useful

Not knowing the exact answer does not make the calculator pointless.

Imagine you learn that your plausible sodium-loss range for a hot three-hour event is somewhere around:

1,200–3,600 mg

That immediately tells you the problem is potentially much larger than during your ordinary 45-minute gym session.

The estimate helps you understand scale.

That is valuable.

When the range is too wide

If the uncertainty is so large that it changes your actual competition plan, that is when a proper sweat sodium test becomes more useful.

For example:

Possible loss estimate:

1.2 g vs 3.5 g

during an important long-distance event.

That difference may justify collecting better data.

For a short recreational workout, it probably does not.

Do not copy another athlete’s sweat sodium number

Your training partner may have:

• a different sweat rate
• different sweat sodium concentration
• different body size
• different heat acclimation
• different environment

Their:

900 mg/L

does not become your:

900 mg/L

because you train together.

Can sweat sodium change in the same person?

Yes.

It can vary with factors including:

• heat acclimation
• exercise conditions
• sweat rate
• testing method

Do not assume one number is an immutable lifetime value.

What about wearable sweat sensors?

Wearable technology is developing rapidly.

Some devices attempt to estimate:

• sweat rate
• sodium concentration
• electrolyte loss

They may become increasingly useful.

But a precise-looking number on a screen is only as useful as the underlying measurement.

Before relying on it, understand:

• whether it measures local or whole-body sweat
• whether it has been independently validated
• how it handles regional differences
• whether the measurement is appropriate for your sport

A simple calculator workflow

If you know total sweat loss

Use:

Total sodium loss = sweat loss × sodium concentration

If you know sweat rate

Use:

Sodium loss rate = sweat rate × sodium concentration

Then:

Total loss = sodium loss rate × exercise duration

If you do not know sodium concentration

Use several scenarios.

Do not choose one arbitrary value and label it your exact result.

Example using both methods

Sweat rate:

1.1 L/h

Duration:

2 hours

Sweat sodium:

750 mg/L

Method 1

Total sweat volume:

1.1 × 2 = 2.2 L

Then:

2.2 × 750 = 1,650 mg

Method 2

Hourly sodium loss:

1.1 × 750 = 825 mg/h

Then:

825 × 2 = 1,650 mg

Same answer.

Common mistake: confusing mg/L with mg/h

If your sweat sodium is:

700 mg/L

that does not mean you lose:

700 mg every hour.

You lose 700 mg per litre of sweat.

If you sweat:

0.5 L/h

then:

350 mg/h

If you sweat:

1.5 L/h

then:

1,050 mg/h

Common mistake: confusing total loss with hourly loss

If a three-hour session costs:

2,400 mg sodium total

that does not mean:

2,400 mg/hour.

Always check the unit.

Common mistake: assuming sweat rate tells you sweat sodium

It does not.

These are separate measurements.

Common mistake: using salt marks as a number

Salt marks can be a clue.

They cannot provide mg/L.

Common mistake: treating cramps as a sweat test

Cramps are not a quantitative sodium measurement.

Common mistake: replacing every milligram during exercise

Some replacement can happen afterwards through food and drinks.

Common mistake: ignoring water balance

A sodium strategy still has to fit an appropriate fluid strategy.

Common mistake: thinking electrolyte drinks prevent hyponatremia regardless of volume

They do not.

Overdrinking remains a problem.

Common mistake: copying an exact online number

Population averages are not personalised measurements.

Common mistake: believing the calculator is more accurate than its inputs

A calculator can perform perfect arithmetic on uncertain numbers.

The result remains uncertain.

Practical guide by training type

Normal gym workout

Usually start with:

• normal hydration
• water as needed
• normal meals afterwards

Detailed sodium calculations are rarely necessary.

Hard Muay Thai

Measure sweat rate.

If sweat losses are consistently large, sodium becomes more relevant.

Long-distance running

Both sweat rate and sodium concentration can become increasingly useful because losses accumulate over several hours.

Hot-weather training

Retest your sweat rate.

Do not assume winter values still apply.

Two-a-day training

Recovery time is shorter, so deliberate replacement becomes more useful.

FAQ

What is the formula for sodium loss during exercise?

Sweat volume × sweat sodium concentration = estimated sodium loss

Can I calculate sodium loss without a sweat sodium test?

You can estimate a range.

You cannot know your exact individual sodium loss from sweat volume alone.

What sweat sodium value should I enter?

Use your measured value if you have one.

If you do not, use several plausible scenarios rather than pretending one estimate is your exact concentration.

Is 500 mg/L a normal sweat sodium concentration?

It is a plausible value for some athletes.

It is not a universal value.

Is 1,000 mg/L high?

It is within values observed in athletes, but interpreting it depends on the testing method and individual context.

What does mmol/L mean?

It is another unit used to express sodium concentration.

For sodium:

1 mmol/L ≈ 23 mg/L

How do I convert 40 mmol/L?

40 × 23 ≈ 920 mg/L

What is the difference between mg/L and mg/h?

mg/L describes sodium concentration in sweat.

mg/h describes how much sodium you lose per hour.

How do I calculate mg/h?

Sweat rate in L/h × sodium concentration in mg/L

Do I need to replace 100% of calculated sodium loss?

Not automatically.

Food, recovery time and the total session all matter.

Does a calculator tell me how much sodium supplement to take?

No.

It estimates loss.

Replacement strategy is a separate decision.

Are salt marks enough to know my sodium concentration?

No.

They can suggest meaningful salt loss but cannot quantify it.

Do cramps mean I need more sodium?

Not necessarily.

Exercise-associated cramps have multiple possible causes.

Does headache mean sodium loss?

No.

Headache is nonspecific and can even occur with overhydration and hyponatremia.

Can I calculate sodium loss from body-weight change?

Body-weight change helps calculate sweat volume.

You still need sweat sodium concentration.

Should I get a professional sweat test?

It may be worthwhile if you have large sweat losses during long, hot or competition-focused training and the extra precision would change your strategy.

Should I test sweat rate first?

For most recreational athletes:

yes.

It is inexpensive, practical and immediately useful.

Can my sodium concentration change?

Yes.

Factors such as heat acclimation and testing conditions can affect sweat sodium concentration.

Can too much sodium be harmful?

More is not automatically better.

People with medical conditions or prescribed sodium restrictions should follow individual medical guidance rather than a generic sports calculator.

Sources and further reading

The formulas, ranges and limitations in this guide are informed by published research on whole-body sweat rate, sweat sodium concentration and sports hydration.

• Baker LB. Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra/Interindividual Variability. Sports Medicine. 2017;47(Suppl 1):111–128. PMID: 28332116. DOI: 10.1007/s40279-017-0691-5.

• Baker LB et al. Normative Data for Regional Sweat Sodium Concentration and Whole-Body Sweating Rate in Athletes. Journal of Sports Sciences. 2016;34(4):358–368. PMID: 26070030. DOI: 10.1080/02640414.2015.1055291.

• Barnes KA et al. Normative Data for Sweating Rate, Sweat Sodium Concentration, and Sweat Sodium Loss in Athletes: An Update and Analysis by Sport. Journal of Sports Sciences. 2019;37(20):2356–2366. PMID: 31230518. DOI: 10.1080/02640414.2019.1633159.

• McDermott BP et al. National Athletic Trainers’ Association Position Statement: Fluid Replacement for the Physically Active. Journal of Athletic Training. 2017;52(9):877–895. PMID: 28985128. DOI: 10.4085/1062-6050-52.9.02.

A calculator is only as accurate as the numbers entered into it.

Use the result to understand the scale of your losses.

Do not confuse mathematical precision with physiological certainty.

Final takeaway

The sodium-loss calculation itself is simple:

sweat volume × sweat sodium concentration = total sodium loss

or:

sweat rate × sweat sodium concentration = sodium loss per hour

The difficult part is knowing your sweat sodium concentration.

If you have a measured result, the calculator becomes much more personalised.

If you do not, use several scenarios and treat the answer as a range.

Do not invent an exact sodium concentration from:

• salt marks
• cramps
• headache
• sweat volume
• somebody else’s test

And remember:

estimated sodium loss is not automatically the exact amount you must replace during exercise.

For most recreational training, start with sweat rate, normal food and sensible hydration.

Add precision when the duration, heat, sweat volume or competition demands make that precision genuinely useful.