About the Sweat Rate Calculator
How much you sweat is one of the most individual numbers in sport. Two runners on the same course on the same day can lose fluid at rates that differ by a factor of three or more. Generic advice about how much to drink cannot account for that, but a simple sweat rate test can: weigh yourself before and after a session, account for what you drank, and you know how much fluid you lost per hour.
This sweat rate calculator does the arithmetic. Enter your weight before and after exercise, the time, the fluid you drank and any urine passed, and it gives your sweat rate per hour, the total sweat lost, the percentage of bodyweight lost, and a drinking plan for similar sessions.
How to Use the Sweat Rate Calculator
Weigh yourself before exercise, after going to the toilet, in minimal clothing.
Exercise as normal, noting how much you drink and any urine you pass.
Towel off and weigh yourself again in the same clothing straight afterwards.
Enter the two weights, the exercise time, the fluid drunk and the urine passed.
The Formula
weight lost = weight before − weight after
sweat loss = weight lost (1 kg ≈ 1 litre) + fluid drunk − urine passed
sweat rate = sweat loss ÷ hours of exercise
% lost = weight lost ÷ weight before × 100Step-by-Step Examples
A one-hour run. Weight falls from 70.0 to 69.2 kg, and 500 ml was drunk.
weight lost 70.0 − 69.2 = 0.8 kg
sweat loss 0.8 + 0.5 = 1.3 L
sweat rate 1.3 ÷ 1 h = 1.30 L an hour
% lost 0.8 ÷ 70 = 1.1% of bodyweightA 90-minute session with a toilet stop. Weight falls from 70 to 69 kg; 750 ml was drunk and 200 ml of urine passed.
sweat loss 1.0 + 0.75 − 0.2 = 1.55 L
sweat rate 1.55 ÷ 1.5 h = 1.03 L an hourWhy 1 kg Equals 1 Litre
Sweat is mostly water, and a litre of water weighs almost exactly a kilogram, so weight lost over an hour or two of exercise is essentially fluid. Fat loss over the same period is tiny by comparison — a hard hour of exercise typically burns only a few tens of grams of fat — so the rapid drop on the scale after a workout is water, and it comes back when you drink.
What Affects Sweat Rate?
Temperature and humidity are the biggest factors: sweat rates can double between a cool morning and a hot, humid afternoon. Intensity matters too, as harder efforts produce more heat. Larger people and those who are fitter and heat-acclimatised tend to sweat more and earlier, which helps them stay cool. Clothing, wind and sun also play a part. That is why it is worth repeating the test in different conditions — a cool-weather rate and a hot-weather rate — to cover the sessions you actually do.
Using Your Sweat Rate
Once you know your rate, you can plan how much to drink. Most guidelines aim to keep weight losses under about 2 percent of bodyweight during exercise, since performance often suffers beyond that, especially in the heat. Many people can comfortably drink about 0.4 to 0.8 litres an hour, so heavy sweaters may not be able to replace everything during exercise; they should start well hydrated and make up the rest afterwards, drinking about 1.5 times the weight lost. Never aim to gain weight during exercise, which signals overdrinking.
Sweat Rate and Sodium
A high sweat rate means high sodium losses too. People who sweat heavily, or whose sweat leaves white salty marks on their clothes and skin, often benefit from sports drinks or electrolyte supplements in sessions longer than about two hours, rather than plain water alone.
Doing the Test Well
Choose a session that represents your normal training, at least an hour long, so the weight change is large enough to measure accurately on household scales. Use scales that read to 0.1 kg or 0.2 lb, and weigh yourself on the same scales before and after. Empty your bladder before the first weigh-in. During the session, drink from a bottle you can measure, and avoid spilling or pouring water over yourself. Afterwards, towel off thoroughly before weighing, because sweat trapped in hair and clothing hides some of the loss. Record the temperature and humidity along with the result, so you can build up a picture of your sweat rate across the conditions you train and race in.
Sweat Rate in Children and Older Adults
Children sweat less than adults and rely more on other ways of losing heat, so they can overheat more easily in hot conditions; regular drink breaks and shade matter for them. Older adults often feel less thirsty even when they need fluid, so a planned drinking routine, rather than waiting for thirst, is especially useful during exercise in warm weather.
Understanding Your Result
The headline is your sweat rate per hour.
The total sweat line gives the fluid lost over the whole session.
The weight lost line gives the loss as a percentage of bodyweight and whether it is within the usual 2 percent target.
The drinking plan line suggests how much to drink during and after similar sessions.
When Should You Use This Calculator?
Use it before planning hydration for a race or long training day.
Use it to compare how much you sweat in cool and hot weather.
Use it to check whether your current drinking habits keep up with your losses.
Common Mistakes
Weighing in sweaty clothes. Wet clothing adds weight; towel off and change or use the same dry kit.
Forgetting what you drank. Every sip adds to the sweat total.
Testing only once. Rates change a lot with weather and effort.
Treating the loss as fat. It is water and returns when you drink.