Sweat Loss Percentage Calculator
Weight lost during exercise is almost all water, so dividing by your starting weight turns it into a percentage of dehydration. Starting at 72 kg, drinking 500 mL and finishing at 70.6 kg means 1.4 kg or 1.94% gone, while the sweat you actually produced is 1,900 mL once the drink is added back. Replacing it takes about 1.5 L per kilogram lost — 2,100 mL here, spread over the next few hours. The neighbouring sweat-rate tool divides the same measurements by time to give an **hourly rate**, which is what you plan the next session around. This one leaves time out and reads the **deficit** you finished with.
Your numbers
Body mass lost
1.94%
Total sweat loss
1,900mL
Fluid to replace
2,100mL
What it means
You are 1.94% down. Two percent is where performance starts to slip, so aim to drink a little more per hour next time.
Formula
Body mass lost = (Weight before exercise (kg) − Weight after exercise (kg)) ÷ Weight before exercise (kg) × 100
The 2% line comes from prolonged exercise in the heat; a short cool session is far less affected. Over long events the lost weight is not purely water either — burned carbohydrate and fat, and losses through breathing, make up part of it. You need a scale that reads to 0.1 kg, and zero loss is not the goal: drinking heavily enough to prevent any loss carries a risk of hyponatraemia.
What to enter
| Input | Default | Accepted range |
|---|---|---|
| Weight before exercise (kg) (kg)Weight taken just before you start, after the toilet and in light clothing. | 72 | 0 ~ 250 |
| Weight after exercise (kg) (kg)Weight taken right after, towelled dry and in the same clothing. | 70.6 | 0 ~ 250 |
| Fluid drunk (mL) (mL)Fluid you drank during the session. | 500 | 0 and up |
Step by step
Quick reference table
Results when only Weight before exercise (kg) (kg) changes and everything else stays put.
| Weight before exercise (kg) (kg) | Body mass lost (%) | Total sweat loss (mL) | Fluid to replace (mL) |
|---|---|---|---|
| 36 | 0 | 500 | 0 |
| 54 | 0 | 500 | 0 |
| 72 | 1.94 | 1,900 | 2,100 |
| 108 | 34.63 | 37,900 | 56,100 |
| 144 | 50.97 | 73,900 | 110,100 |
What each result means
| Result | At default values |
|---|---|
| Body mass lost (%)The weight lost during the session as a share of your starting weight. | 1.94 |
| Total sweat loss (mL)Weight lost plus what you drank — the sweat you actually produced. | 1,900 |
| Fluid to replace (mL)What to drink over the next few hours to make the lost weight back. | 2,100 |
Common mistakes
The 2% line comes from prolonged exercise in the heat; a short cool session is far less affected. Over long events the lost weight is not purely water either — burned carbohydrate and fat, and losses through breathing, make up part of it. You need a scale that reads to 0.1 kg, and zero loss is not the goal: drinking heavily enough to prevent any loss carries a risk of hyponatraemia.
Glossary
- Weight before exercise (kg)
- Weight taken just before you start, after the toilet and in light clothing.
- Weight after exercise (kg)
- Weight taken right after, towelled dry and in the same clothing.
- Fluid drunk (mL)
- Fluid you drank during the session.
- Body mass lost
- The weight lost during the session as a share of your starting weight.
- Total sweat loss
- Weight lost plus what you drank — the sweat you actually produced.
- Fluid to replace
- What to drink over the next few hours to make the lost weight back.
Frequently asked questions
Q. How is Sweat Loss Percentage Calculator calculated?
Body mass lost = (Weight before exercise (kg) − Weight after exercise (kg)) ÷ Weight before exercise (kg) × 100 — Weight lost during exercise is almost all water, so dividing by your starting weight turns it into a percentage of dehydration. Starting at 72 kg, drinking 500 mL and finishing at 70.6 kg means 1.4 kg or 1.94% gone, while the sweat you actually produced is 1,900 mL once the drink is added back. Replacing it takes about 1.5 L per kilogram lost — 2,100 mL here, spread over the next few hours. The neighbouring sweat-rate tool divides the same measurements by time to give an **hourly rate**, which is what you plan the next session around. This one leaves time out and reads the **deficit** you finished with.
Q. Can you walk through an example?
With Weight before exercise (kg) 72kg, Weight after exercise (kg) 70.6kg, Fluid drunk (mL) 500mL, the answer is Body mass lost 1.94%.
Q. What do I need to enter?
Enter Weight before exercise (kg), Weight after exercise (kg), Fluid drunk (mL). The result recalculates as you type, and an empty box counts as zero.
Q. How much does the answer move if I change a number?
Changing only Weight before exercise (kg) (kg) moves the answer to Weight before exercise (kg) (kg) 36 → Body mass lost (%) 0 and Weight before exercise (kg) (kg) 144 → Body mass lost (%) 50.97. The table below lays out five steps.
Q. How are the numbers rounded?
Money is shown to the nearest whole unit, percentages to one decimal place and everything else to two. What you see is rounded; the calculation itself carries the unrounded value forward.
Q. Anything to watch out for?
The 2% line comes from prolonged exercise in the heat; a short cool session is far less affected. Over long events the lost weight is not purely water either — burned carbohydrate and fat, and losses through breathing, make up part of it. You need a scale that reads to 0.1 kg, and zero loss is not the goal: drinking heavily enough to prevent any loss carries a risk of hyponatraemia.
Related calculators
These calculators help you understand health figures; they do not replace diagnosis or prescription. See a clinician if you have symptoms.