Sweat Rate Calculator
Work out how much you sweat per hour from your weight before and after exercise, what you drank and the time, with sodium lost if you know your sweat sodium.
Result
Enter weight before exercise and weight after exercise to see the result.
https://calcgallery.com/fitness-training/sweat-rate-calculator/
How to use the Sweat Rate Calculator
- Empty your bladder, then weigh yourself with as little clothing as possible. Enter that as the weight before.
- Exercise as you normally would. Note how much you drink, and how long the session lasts including the warm-up and cool-down.
- Towel off and weigh yourself again in the same clothing on the same scale. Enter that as the weight after.
- Under more options, add any urine passed or food eaten during the session, and your sweat sodium concentration if a sweat test has given you one.
- Read your sweat rate per hour and the total sweat lost.
The method
A sweat test in the field is a mass balance. Whatever went into your body during the session, and whatever left it other than sweat, is accounted for, and the rest of the weight change is sweat. Lindsay Baker of the Gatorade Sports Science Institute sets it out in Sports Science Exchange #161 (2016):
sweat loss = weight before − weight after + fluid drunk + food eaten − urinesweat rate = sweat loss ÷ exercise timesodium = sweat loss × sweat sodium × 22.99 mg per mmolWeight is converted to volume at one kilogram per litre, the density of water, as Baker’s worked example does. Food eaten during the session is counted the way GSSI’s own fluid loss calculator counts it.
What it does not correct for
- Breathing and fuel use. You lose water in your breath, and the carbohydrate and fat you burn leave as carbon dioxide and water. Both take weight off that this method counts as sweat, so the sweat rate reads a little high. The effect grows with longer and harder sessions.
- Sweat trapped in clothing or hair. Towel off before the second weigh-in.
- Scale precision. A scale that reads to 0.1 kg or 0.2 lb makes a short session’s figure much more precise than one that reads to the whole pound.
Using the figures
A sweat rate is a measurement of one session in one set of conditions. The page shows the numbers and does not grade them. How much of that loss to replace, and how fast, is a personal decision that depends on the event and the person; a sports dietitian or physician can help with it.
With your numbers
- Sweat = 2.6 lb weight lost + drinks and food − urine = 63.88 fl oz, taking 1 kg as 1 L
- Sweat rate = 63.88 fl oz ÷ 1:00:00 = 63.88 fl oz per hour
Quick reference
An estimate: breathing and the fuel you burn also take off a little weight, which this method counts as sweat.
| Example | Sweat rate, per hour |
|---|---|
| 1 h run, 150 to 148 lb, 16 fl oz drunk | 46.68 fl oz |
| 1 h run, 165 to 162.4 lb, 24 fl oz drunk | 63.88 fl oz |
| 2 h ride, 180 to 177 lb, 40 fl oz drunk | 43.01 fl oz |
| 45 min, 70 to 69.2 kg, 500 mL drunk | 1.73 L |
| 1 h, 60 to 59.3 kg, nothing drunk | 0.7 L |
| 1:45, 82.5 to 81 kg, 1.1 L drunk, 0.3 L urine | 1.31 L |
Where the numbers come from
The fixed figures this calculator relies on, and the published source each one was checked against. How the calculators are built and checked
2 sources
NIST Chemistry WebBook, water at 4 °C and 0.101325 MPa: 999.9749 kg/m³ (IAPWS-95), https://webbook.nist.gov/cgi/fluid.cgi?ID=C7732185, rounded to the conventional 1000 kg/m³ of NIST SP 811 (2008) Appendix B, centimeter of water, conventional = 98.0665 Pa.
Used for
- The conventional density of water, 1000 kg/m³. Pure water at 4 °C, its densest, is 999.97 kg/m³.
CIAAW, Standard Atomic Weights 2024 (https://www.ciaaw.org/atomic-weights.htm), the table of Prohaska et al., "Standard atomic weights of the elements 2021 (IUPAC Technical Report)", Pure Appl. Chem. 94(5) 573-600 (2022), Table 1, with the 2024 revisions of Gd, Lu and Zr.
Used for
- Standard atomic weight of sodium.
Common questions about the Sweat Rate Calculator
- How do I calculate my sweat rate?
- Weigh yourself before and after exercise, add back what you drank and ate, take off any urine you passed, and divide by the time. One gram of weight lost is taken as one millilitre of sweat. This is the method the Gatorade Sports Science Institute describes for testing in the field, and the page does the arithmetic for you.
- Why do I need to weigh myself nude or in minimal clothing?
- Sweat soaked into clothing still weighs something. If you weigh yourself dressed after exercise, the damp clothes hide part of the loss and the sweat rate comes out low. Weighing in minimal, dry clothing both times avoids this.
- Does the weather change my sweat rate?
- Yes. Sweat rate changes with heat, humidity, intensity, clothing and fitness, so one test describes one set of conditions. Repeating the test in the conditions you race or train in gives figures that match them.
- How is the sodium figure worked out?
- Only from a sweat sodium concentration you enter, usually from a sweat patch test. The page multiplies the litres of sweat by that concentration and converts millimoles of sodium to grams with sodium’s atomic weight. The page does not assume a typical concentration, because it varies widely from person to person.
- What does the weight lost percentage mean?
- It is the drop in body weight from before to after, as a share of the starting weight, before anything you drank is added back. The page reports the number only; it does not say whether it is too much.