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Sweat Rate Calculator
This page turns two weigh-ins, what you drank, what you passed and the session length into your sweating rate in litres per hour, your whole-session fluid loss, and the hourly and whole-session amount of water to drink next time.
Your whole-body sweating rate
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1. Sweat rate
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- Whole-body sweating rate, in litres per hour. The equation and its source are printed below.
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2. Whole-session fluid loss
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3. How much water should I drink next time
The session you measured has already happened, so this is a plan for your next session of the same length and conditions, not a way to drink backwards. If you came here looking for a hydration calculator or for how much water should I drink calculator, this is the part that answers it, on this same page.
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- Recommended amount per hour, across that next session.
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The third stage gives one recommended drinking amount and a separate upper limit, not a range. The recommended amount is the former lower bound, your whole-session sweat loss minus two percent of your pre-exercise body mass, floored at zero so it is never negative, shown as one number per hour and one number for the whole session. The upper limit is the sweat rate itself, and drinking more than that number is drinking too much. The recommended value and the upper limit are given separately; this is our arithmetic from the two quoted statements, not a rule published by either source. When the recommended amount is zero, the page says: Your sweat rate is low, so drink to thirst. The number is still printed.
Sweat sodium, for context only. In the review cited below, "Local sweat [Na+] typically ranges from approximately 10 to approximately 90 mmol/L, while whole-body sweat [Na+] is approximately 10–70 mmol/L". The same review calls whole-body washdown the reference method for sweat sodium loss, notes that local absorbent-patch measurements are more practical in the field but are usually not a valid direct substitute for whole-body values, and recommends regression equations to predict whole-body sweat sodium from local sweat sodium. This page has no sodium field, no sodium number in any result, and no sodium advice.
Formulas and their sources, quoted verbatim
- Whole-body sweating rate
- WBSR = [Body massPRE-EX – (Body massPOST-EX – fluid intakeEX + urine outputEX)]/exercise duration — Table 3, Baker et al., "Sweating Rate and Sweat Sodium Concentration in Athletes", PMC5371639. Whole-session fluid loss is the numerator of that same equation, and the hourly rate is that numerator divided by the duration in hours.
- Mass to volume, and what is not corrected
- The printed equation mixes kilograms of body mass with litres of fluid. The review does not state a density conversion in that table, so this page applies the equation exactly as printed: one kilogram of mass change is treated as one litre of fluid. The same review says "SR calculations based on change in body mass should be corrected for metabolic mass loss and respiratory water loss, particularly when exercise lasts several hours (e.g, >2–3 h)", and that trapped sweat in clothing can lead to underestimations of SR. Those corrections are not applied here because there are no fields for them.
- Recommended drinking amount
- "The goal of drinking during exercise is to prevent excessive (>2% body weight loss from water deficit) dehydration and excessive changes in electrolyte balance to avert compromised performance." — ACSM Position Stand, "Exercise and Fluid Replacement", Medscape mirror. Also "drinking sufficient fluid during exercise to prevent dehydration from exceeding 2% body weight" — PubMed 10410838.
- Upper limit on drinking
- "Importantly, it is emphasized that during exercise individuals should avoid drinking more fluid than the amount needed to replace their sweat losses." — ACSM Position Stand, "Exercise and Fluid Replacement", Medscape mirror.
- The derivation, stated as a derivation
- Let S be whole-session sweat loss, M pre-exercise body mass in kilograms and D the volume drunk. The end-of-session deficit is S minus D. Keeping that deficit at or below two percent of M gives D at or above S minus 0.02 M; not drinking more than the sweat loss gives D at or below S. The recommended value and the upper limit are given separately, not as a range; this is our arithmetic from the two quoted statements, not a rule published by either source.
- A published practical check, quoted not modelled
- "A practical recommendation is to drink small amounts of fluid (150-300 mL) every 15 to 20 minutes of exercise, varying the volume depending on sweating rate." — PubMed 10410838. The page does not turn this into an extra input or a third number.
- Unit switches
- "pound (avoirdupois) (lb) — kilogram (kg) — 4.535 924 E-01" and "fluid ounce (U.S.) (fl oz) — milliliter (mL) — 2.957 353 E+01" — NIST Special Publication 811, Appendix B.8. Canonical values are held in kilograms and millilitres; switching units never re-reads the displayed number.
- Sweat sodium concentration range
- "Local sweat [Na+] typically ranges from approximately 10 to approximately 90 mmol/L (Fig. 2a; see also Maughan and Shirreffs [46], Baker et al. [54], Patterson et al. [55], Shirreffs and Maughan [87], and Verde et al. [92]), while whole-body sweat [Na+] is approximately 10–70 mmol/L (predicted, Fig. 2b; measured in Baker et al. [54], Patterson et al. [55], and Shirreffs and Maughan [87])." — PMC5371639.
Three worked examples
Every row below is recomputed on load by the same function that drives the calculator above, from the inputs printed beside it. Nothing is typed in by hand.
Every number above is worked out from the five entries you type, in this browser, when you type them. Nothing is uploaded, nothing is saved, and every formula is printed with its source on this page.
Questions
What does a sweat rate calculator work out?
It takes body mass before and after exercise, the fluid you drank, any urine you passed, and the session duration. From those five entries it works out whole-body sweating rate in litres per hour, the total fluid lost over the whole session, and one recommended hourly and whole-session drinking amount with a separate upper limit for your next comparable session. The three are worked out in that order from the same inputs, so changing one entry changes all three.
How is whole-body sweating rate calculated?
The equation is quoted verbatim, with its source, in the formulas section below. Body mass difference, plus fluid drunk, minus urine passed, all divided by exercise duration. Body mass is in kilograms and the result is in litres per hour. The printed equation mixes kilograms and litres, and the review gives no density conversion in that table, so this page applies it exactly as printed and says so in that section. The review also lists respiratory water loss, metabolic mass loss and sweat trapped in clothing as corrections to consider; this page has no inputs for them and does not apply them.
How much water should I drink during exercise?
This page gives one recommended drinking amount and a separate upper limit for your next session of the same length and conditions, not a range. The recommended amount is the volume that keeps the remaining body mass loss at or below two percent of your pre-exercise mass, from the ACSM position stand and from PubMed 10410838, floored at zero so it is never negative. The upper limit is your measured sweat loss, because the same position stand says to avoid drinking more fluid than needed to replace sweat losses; drinking more than that upper limit is drinking too much. The recommended value and the upper limit are given separately; this is our derivation from those two published statements, not a rule printed by either source, and the reasoning is shown in the formulas section. It is arithmetic, not individual medical advice.
Does this page tell me how much sodium to replace?
No. There is no sodium input, no sodium number in any result above, and no sodium heading. One short note gives the published sweat sodium concentration range and how the literature estimates it, so you can see why replacement advice is left out. Sweat sodium is not part of any calculation here.
What does this page deliberately not do?
It does not model respiratory water loss or metabolic mass loss, it does not correct for sweat trapped in clothing, it does not give a training plan or medical advice, and it does not print or export anything. The review cited in the formulas section lists those first two as corrections to consider, especially for sessions longer than two to three hours, and this page has no inputs for them. Every number is worked out only from the five fields you fill in.