Sweat Rate Calculator

Why your sweat rate matters

Sweating is how your body gets rid of the heat your working muscles produce, and the volume can be surprisingly large. According to the National Athletic Trainers’ Association (NATA), adult sweat rates during exercise range from about 0.5 to 4 litres an hour — an eightfold spread between people doing what looks like the same workout. That is why blanket advice such as “drink a cup every 15 minutes” can leave one runner short while another over-drinks.

Both extremes matter. The American College of Sports Medicine (ACSM) advises preventing fluid losses of more than 2% of body weight, because beyond that point endurance performance tends to suffer. NATA notes that core temperature rises by roughly 0.15–0.2 °C for every 1% of body weight lost, and the extra heat strain makes the same pace feel harder. At the other extreme, drinking far more than you sweat during long events can dilute blood sodium and cause exercise-associated hyponatremia, a potentially fatal condition. Both organisations therefore recommend individual drinking plans — and a good plan starts with knowing your own sweat rate.

How to use this calculator

The test needs nothing more than an accurate scale and a marked water bottle. It works best on a typical session of about an hour, in conditions you expect to repeat:

1. Empty your bladder, then weigh yourself in minimal clothing just before you start. Use the same scale throughout — ideally one that reads to 0.1 kg or 0.2 lb.
2. Exercise as normal and keep track of everything you drink. Note how much is left in your bottle at the end.
3. Note any urine you pass during the session. Most people won’t need to go; if you do, estimate the volume and enter it in the optional box.
4. Towel off and weigh again straight after, in the same minimal, dry clothing. Sweat-soaked clothes can hide several hundred grams of fluid.
5. Enter both weights, the fluid you drank, any urine and the duration in minutes. Use the units toggle to switch between metric (kg, mL) and imperial (lb, fl oz).

The results show your sweat rate in litres and fluid ounces per hour, your total sweat loss, how much of your body weight you finished down, and a rehydration target for after the session. If you lost more than 2%, the tool also estimates how much you would need to drink per hour in a similar session to stay near that line, and a step-by-step table shows exactly how the numbers add up.

How it's calculated

The calculator uses the sweat-rate equation from NATA’s 2017 position statement. It rests on one practical fact: over a single session, almost all short-term weight change is water, and 1 kg of body mass corresponds to about 1 litre of fluid (1 lb is about 15 fl oz).

Sweat loss (L) = weight before (kg) − weight after (kg) + fluid drunk (L) − urine passed (L)
Sweat rate (L/h) = sweat loss ÷ exercise duration (hours)

Fluid you drank is added back because it replaced some of what you sweated; urine is subtracted because that weight left your body without being sweat. The percentage of body weight lost is the weight change divided by your starting weight, which is the figure the 2% guideline refers to.

For recovery, ACSM guidance advises about 1.25–1.5 litres of fluid for every kilogram lost (roughly 19–23 fl oz per pound), and NATA describes replacing 100–150% of the deficit when recovery time is short. The extra above 100% covers the urine your kidneys produce when you drink a large volume quickly. With more than 12 hours before your next session, NATA notes that normal eating and drinking usually restores balance on its own.

Worked example

Maya weighs 160.0 lb before a 75-minute run on a warm morning and 157.0 lb afterwards. She drank one 16 fl oz bottle and didn’t stop to urinate.

• Weight lost: 3.0 lb = 1.36 kg, or about 1.36 L of fluid.
• Add the fluid she drank: 16 fl oz = 0.47 L, so her sweat loss is 1.83 L (62 fl oz).
• Sweat rate: 1.83 L ÷ 1.25 hours = 1.47 L/h (50 fl oz per hour).
• Body weight lost: 3.0 ÷ 160 = 1.9% — just inside the 2% guideline.
• Rehydration: 1.36 kg × 1.25–1.5 L = 1.7–2.0 L (58–69 fl oz) over the next few hours.

Her bottle replaced only about a quarter of what she sweated. On a two-hour run at the same pace and temperature she would sweat about 2.9 L, while 2% of her body weight is about 1.45 L — so she would need to drink roughly 1.5 L during the run, or about 25 fl oz an hour. That is still well under her 50 fl oz-an-hour sweat rate, which keeps her clear of over-drinking.

Limitations & tips

Scale accuracy matters. An error of just 0.2 lb (0.1 kg) changes the result of a one-hour test by about 3 fl oz (90–100 mL) per hour, so use the same reliable scale, weigh in the same dry clothing and towel off first.

Small non-sweat losses are ignored. You also lose a little water through breathing and a little mass from the fuel you burn. Over an hour these are minor, but in very long sessions the method will slightly overstate sweat loss.

Conditions change the answer. Sweat rate rises with temperature, humidity, intensity and heavier clothing or equipment, and NATA notes that sweat losses increase as you acclimatize to heat. Re-test when the season or your training changes, and don’t rely on a cool-weather result for a hot race.

Sweat isn’t just water. NATA reports sodium losses during exercise ranging from 0.2 to 7.3 g an hour. For long or very sweaty sessions, sodium from a sports drink or salty food helps your body hold on to the fluid you drink.

Don’t chase zero. Finishing slightly lighter is normal and expected. This calculator is designed for healthy people doing everyday training and is not medical advice; if you have a heart or kidney condition, have been told to limit fluids, or feel dizzy, confused or unwell during or after exercise, get medical help.

Sources & references

• American College of Sports Medicine — Position Stand: Exercise and Fluid Replacement (Sawka et al., Med Sci Sports Exerc, 2007) — source of the advice to prevent body-weight losses above 2%, to measure individual sweat rates from pre- and post-exercise weight, and to use customised fluid plans.
• National Athletic Trainers’ Association — Position Statement: Fluid Replacement for the Physically Active (McDermott et al., J Athl Train, 2017) — source of the sweat-rate equation, the 0.5–4 L/h range, sodium losses, the 2–5% and >5% categories, and 100–150% post-exercise replacement.
• The Effect of Fluid Intake Following Dehydration on Subsequent Athletic and Cognitive Performance: a Systematic Review and Meta-analysis (Sports Medicine – Open, 2017) — summarises the ACSM recommendation of 1.25–1.5 L of fluid per kg of body mass lost.