💧 Workout Sweat Rate & Electrolyte Calculator
Calculate hourly fluid loss sweat rate (L/hr), percentage dehydration, and sodium electrolyte replacement requirements from pre and post-workout body weight.
What Workout Sweat Rate & Electrolyte Calculator Does
Sweat rate is measured, not estimated: weigh yourself before and after a session, add whatever you drank, and divide by the time. This calculator does that arithmetic and converts the result into an hourly rate, the percentage of body mass lost, an estimated sodium loss and a replacement volume.
The fluid numbers here are solid. Body mass lost during exercise is almost entirely water, and one kilogram of mass lost corresponds to roughly one liter of sweat — that is the standard sports-science assumption and the reason this test works with nothing but a scale.
The sodium figure is a different matter and deserves to be read with suspicion. It comes from multiplying your sweat volume by an assumed concentration of 900 mg per liter. Real measured values in athletes range from around 200 to 2,000 mg per liter, which means the honest answer for any individual is a range spanning a factor of ten. The table below shows what that does to the number on screen.
How to Use Workout Sweat Rate & Electrolyte Calculator
- Enter pre-workout and post-workout body weights in kg
- Input fluid consumed during the workout and exercise duration in minutes
- Review hourly sweat rate in liters, dehydration percentage, and rehydration targets
Formula Used by Workout Sweat Rate & Electrolyte Calculator
Sweat rate
sweat volume (ml) = (pre-weight − post-weight in kg) × 1,000 + fluid drunk (ml); rate (L/h) = volume ÷ 1,000 ÷ hours
- pre − post
- Body mass lost, in kilograms. Weigh nude or in the same dry clothing, after towelling off
- fluid drunk
- Everything consumed during the session. It replaced sweat already lost, so it must be added back
- × 1,000
- 1 kg of body mass lost ≈ 1 liter of sweat
Worked example
The defaults: 75.0 kg before, 73.8 kg after, 500 ml drunk, 60 minutes.
- Mass lost: 75.0 − 73.8 = 1.2 kg = 1,200 ml
- Add fluid drunk: 1,200 + 500 = 1,700 ml of sweat
- Rate: 1.7 L ÷ 1 hour
Result: 1.70 liters per hour — a high but entirely ordinary rate for an hour of hard work in warm conditions. Without the 500 ml, the same session would have read 1.2 L/h and understated the loss by 29%.
Body mass lost as a percentage
dehydration % = (pre-weight − post-weight) ÷ pre-weight × 100
- dehydration %
- The figure performance guidance is written against, because it scales with body size
Worked example
The same 1.2 kg lost from a 75 kg starting weight.
- 1.2 ÷ 75 = 0.016
Result: 1.6% of body mass. The conventional threshold is 2%, above which endurance performance is generally observed to decline — for this athlete that is 1.5 kg, so this session finished just inside it.
Sodium lost, and the range around it
sodium (mg) = sweat volume (L) × sweat sodium concentration (mg/L)
- concentration
- Assumed here at 900 mg/L. Measured values in athletes span roughly 200–2,000 mg/L and are strongly individual
Worked example
1.7 liters of sweat, at the low, assumed and high ends of the measured range.
- At 200 mg/L: 1.7 × 200 = 340 mg
- At 900 mg/L: 1.7 × 900 = 1,530 mg
- At 2,000 mg/L: 1.7 × 2,000 = 3,400 mg
Result: 340 mg to 3,400 mg from the identical session. This is why sweat sodium testing exists as a commercial service, and why a single default figure should be treated as a placeholder rather than a result.
The Same 1.7 Liters of Sweat, Across the Measured Sodium Range
Sweat sodium concentration is highly individual and only modestly trainable. The daily-limit column uses the 2,300 mg figure in the Dietary Guidelines for Americans.
| Sweat sodium | Sodium lost this session | As % of a 2,300 mg daily limit |
|---|---|---|
| 200 mg/L (low) | 340 mg | 15% |
| 400 mg/L | 680 mg | 30% |
| 600 mg/L | 1,020 mg | 44% |
| 900 mg/L (this tool's assumption) | 1,530 mg | 67% |
| 1,200 mg/L | 2,040 mg | 89% |
| 1,500 mg/L | 2,550 mg | 111% |
| 2,000 mg/L (high) | 3,400 mg | 148% |
How Long to Sweat Out 1,000 mg of Sodium?
This is the question people arrive with most often. At a 1.7 L/h sweat rate the answer depends entirely on concentration, which is the point.
| Sweat sodium concentration | Time to lose 1,000 mg at 1.7 L/h |
|---|---|
| 400 mg/L | 88 minutes |
| 900 mg/L | 39 minutes |
| 1,500 mg/L | 24 minutes |
What a Percentage of Body Mass Actually Is
For a 75 kg athlete. Performance guidance is written in percentages because the absolute volume depends on body size.
| Body mass lost | Kilograms | Equivalent sweat | Reading |
|---|---|---|---|
| 1% | 0.75 kg | 750 ml | Well within normal session losses |
| 2% | 1.50 kg | 1,500 ml | The conventional threshold for performance decline |
| 3% | 2.25 kg | 2,250 ml | Meaningful impairment likely, especially in heat |
| 4% | 3.00 kg | 3,000 ml | Substantial; replace aggressively afterwards |
| 5% | 3.75 kg | 3,750 ml | Well beyond what a session should produce unreplaced |
What an Hourly Rate Means Over a Long Session
Unreplaced losses across three hours, as a percentage of a 75 kg body mass. This is why sweat rate matters more the longer the event.
| Sweat rate | Loss over 3 hours | Body mass lost if nothing is drunk |
|---|---|---|
| 0.5 L/h | 1.5 L | 2.0% |
| 1.0 L/h | 3.0 L | 4.0% |
| 1.7 L/h (the default here) | 5.1 L | 6.8% |
| 2.0 L/h | 6.0 L | 8.0% |
| 2.5 L/h | 7.5 L | 10.0% |
How to Read Your Result
The sodium number is a placeholder until you measure it
Everything else on this page is arithmetic on a scale reading and is as good as your scale. The sodium figure is arithmetic on an assumption about your physiology, and that assumption varies tenfold between people, changes with heat acclimation, and is only loosely predicted by how salty your sweat tastes or whether you leave white marks on a cap. If sodium replacement matters to what you are doing — long events, heat, a history of cramping — the useful version of this number comes from a measured sweat test, not from any calculator.
Measure it more than once, because it is not one number
Sweat rate is a property of the session, not of the athlete. The same person can double their hourly rate between a cool morning run and the same effort in heat and humidity, and it rises with intensity, with acclimation and with body size. A single test tells you what you lost that day in those conditions. Repeating it in the conditions you are actually preparing for — race heat, race intensity — is the difference between a number and a plan.
Drinking to replace every gram is not the goal either
The replacement figure here is 125% of what was lost, which reflects the standard recommendation to over-replace after a session because some of what you drink is passed as urine rather than retained. During exercise, though, matching sweat rate liter for liter is often neither achievable nor advisable: the gut has a limited absorption rate, and drinking substantially more than you lose carries its own risk. Exercise-associated hyponatraemia comes from over-drinking plain water, not from under-drinking, and it is more dangerous than the mild dehydration most athletes are trying to avoid.
What the scale is not telling you
The test treats every gram of mass lost as sweat. It is not quite: some mass is lost as water vapor in breath and as substrate oxidized for fuel, and any urine passed during the session is counted as sweat unless you account for it — this calculator has no urine field, so a bathroom stop inflates the result. Food eaten during a session cuts the other way. For a one-hour test these are small; for a six-hour ultra they are not.
Limitations & Accuracy Notes
- No urine input. Standard protocols subtract urine produced during the session; this tool does not, so any bathroom stop is counted as sweat and inflates the rate.
- Sodium is estimated from a flat 900 mg/L assumption. Measured individual values span roughly 200–2,000 mg/L — see the table above, and treat the on-screen figure as the midpoint of a very wide range.
- No potassium, chloride or magnesium, which are also present in sweat, and no product or dosing recommendation of any kind.
- Metric only. Weights must be entered in kilograms and fluid in milliliters.
- One session at a time. There is no history, no averaging across tests, and no adjustment for temperature, humidity or acclimation state — all of which move sweat rate substantially.
- Assumes 1 kg of body mass lost equals 1 liter of sweat, and ignores respiratory water loss and substrate oxidation. Reasonable over an hour, less so over very long durations.
- Not medical guidance. Heat illness and exercise-associated hyponatraemia are both medical emergencies and neither is something a calculator can screen for.
Frequently Asked Questions
How is exercise sweat rate calculated?
How much sodium is lost in sweat?
How do I measure my sweat rate?
Why does sweat rate vary so much between people?
Can I drink too much water?
Do I need an electrolyte drink?
Is thirst a reliable guide?
Is this medical advice?
References & Further Reading
- Baker LB — Sweating Rate and Sweat Sodium Concentration in Athletes, Sports Med 2017;47(Suppl 1):111-128 — Review of measured sweating rates and sweat sodium concentrations, and of how much they vary within and between individuals — the basis for the range published above
- ACSM Position Stand — Exercise and Fluid Replacement, Med Sci Sports Exerc 2007;39(2):377-390 — The position stand behind the 2% body-mass threshold and the post-exercise over-replacement guidance
- NATA Position Statement — Fluid Replacement for the Physically Active, J Athl Train 2017;52(9):877-895 — Athletic training guidance on measuring sweat rate, individualizing fluid plans, and the risks at both ends including hyponatraemia