Sweat Rate Calculator
Calculate your sweat rate in litres and fluid ounces per hour from pre and post exercise weights, with replacement volumes and an estimated sodium loss.
Sweat rate
1.50 L/h
That is 50.7 fl oz per hour, or 1,500 mL (50.7 fl oz) across the whole 1.00 hour session.
sweat = (pre − post) × 1000 + fluid in − urine out = 1,500 mL; rate = 1,500 ÷ 1.00 = 1,500 mL/h
Rehydration plan
| Measure | Metric | Imperial |
|---|---|---|
| Total sweat lost | 1,500 mL | 50.7 fl oz |
| Drink during exercise, per hour | 1,500 mL/h | 50.7 fl oz/h |
| Replace afterwards at 125% of the deficit | 1,250 mL | 42.3 fl oz |
| Replace afterwards at 150% of the deficit | 1,500 mL | 50.7 fl oz |
| Sodium lost per hour | 1,724 mg/h | 4.38 g salt/h |
| Sodium lost in total | 1,724 mg | 4.38 g salt |
Sodium mass = sweat volume (L) × your sweat sodium concentration (mmol/L) × 22.99 mg/mmol. Salt (sodium chloride) is 39.34% sodium by mass, which is where the gram-of-salt column comes from. Replacing 125–150% of the fluid deficit is the ACSM position-stand recommendation when there is time to rehydrate before the next session.
Estimates only, not medical advice, and this diagnoses nothing. A single weigh-in session measures that session: sweat rate changes with heat, humidity, clothing, intensity and how heat-acclimated you are, so repeat the test in the conditions you actually race in. The method also treats every gram of weight lost as sweat, when a little is respiratory water and substrate oxidation. Over-drinking is genuinely dangerous — exercise-associated hyponatraemia comes from drinking more than you sweat, so do not exceed your measured rate. Anyone with kidney disease, heart failure or on medication affecting fluid balance should ask a clinician before changing their hydration routine.
What is the Sweat Rate Calculator?
Sweat rate equals your pre-exercise weight minus your post-exercise weight, plus any fluid you drank, minus any urine passed, divided by the hours you exercised. A kilogram of weight lost is treated as a litre of sweat.
- Sweat rate in L/h and fl oz/h from the standard pre and post weigh-in method
- Percentage of body mass lost, banded against ACSM dehydration thresholds
- Replacement volumes at both 125% and 150% of the fluid deficit
- Sodium loss in milligrams and grams of salt, per hour and in total
- Metric and imperial entry with automatic conversion when you switch
- Runs entirely in your browser — your training data never leaves your device
How to use the Sweat Rate Calculator
- 1
Choose metric or imperial units, and enter your sweat sodium concentration if you have had a sweat test.
- 2
Enter your weight before exercise, taken nude or in dry underwear after emptying your bladder.
- 3
Towel dry, weigh yourself again, and enter the post-exercise weight.
- 4
Add the fluid you drank during the session and any urine passed, then the exercise duration in hours and minutes.
- 5
Read the sweat rate, the dehydration band, and the rehydration plan table showing what to drink during and after.
About the Sweat Rate Calculator
The ByteTools Sweat Rate Calculator runs the standard weigh-in protocol used by sports scientists and athletic trainers. Weigh yourself before and after a session, note what you drank and any urine passed, enter the duration, and it returns your sweat rate in litres and fluid ounces per hour along with the percentage of body mass you lost.
It goes on to the practical part: how much to drink during a session of the same length, and how much to replace afterwards at the 125 to 150% of deficit that the American College of Sports Medicine position stand recommends. Set your own sweat sodium concentration and it converts that into milligrams of sodium and grams of salt lost per hour.
All calculations happen in your browser and nothing is uploaded. These are estimates for training planning only, not medical advice, and they diagnose nothing. Over-drinking is genuinely dangerous — exercise-associated hyponatraemia comes from drinking more than you sweat — so do not exceed your measured rate.
Frequently asked questions
How do you calculate sweat rate?
Take your pre-exercise weight minus your post-exercise weight, convert that to millilitres (1 kg lost = 1,000 mL), add the fluid you drank, subtract any urine passed, then divide by the hours you exercised. Losing 1 kg while drinking 500 mL over an hour gives a sweat rate of 1.5 L/h.
What is a normal sweat rate?
Most people fall between 0.5 and 2.0 litres per hour, with 1.0 to 1.5 L/h typical for moderate exercise in warm conditions. Elite athletes in heat have been measured above 3 L/h. Your own figure changes with temperature, humidity, clothing, intensity and heat acclimation, so measure it in the conditions you actually compete in.
How much should I drink after exercise?
The ACSM position stand recommends replacing 125 to 150% of the fluid deficit when there is time before the next session, because some of what you drink is lost as urine. Both figures are shown in the rehydration table above so you can pick a target.
How much sodium do you lose in sweat?
Sweat sodium typically runs between 20 and 80 mmol/L, which is about 460 to 1,840 mg of sodium per litre. At 50 mmol/L, losing 1.5 litres costs roughly 1,700 mg of sodium — around 4.4 g of salt. A sweat test gives your own concentration; the default here is a middle-of-the-road estimate.
How much does dehydration affect performance?
Endurance performance is generally measurably impaired somewhere around 2% of body mass lost, and the effect grows from there, especially in heat. Under 1% is considered minimal. That said, the evidence for the exact threshold is messier than the round number suggests, and drinking to thirst works well for many people.
Is my weight data private?
Yes. The calculator is JavaScript running locally in your browser, so nothing you enter is uploaded, logged or saved, and it continues to work with no internet connection.
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