What sweat rate tells you
Sweat rate is fluid lost per hour of exercise, in liters. It gives you a drinking target for long sessions and hot races. Rates range from under 0.5 L/h to over 2.5 L/h.
Sweat loss is weight lost plus fluid drunk. A 1 kg drop while drinking 500 ml means 1.5 liters of sweat. Divide by hours.
How much to drink
On long sessions, replace roughly 100% to 150% of hourly sweat loss. A small deficit is normal.
Heat, humidity, intensity, altitude and acclimatization all move sweat rate. Test in race-like conditions and re-test when the climate changes.
Do not overdrink
Drinking far beyond sweat losses dilutes blood sodium and can cause hyponatremia, which can be dangerous. If you gain weight during a session, you are drinking too much. Beyond two hours, pair fluid with sodium.
How to run the sweat-rate weigh-in test
One session and a digital scale, in conditions like your target event.
- 1
Weigh in before
Empty your bladder, strip to minimal dry clothing, and record your weight in kilograms.
- 2
Track your fluid
Measure your bottle and note exactly how many milliliters you drink.
- 3
Train for a set time
Do a steady session of known duration, ideally 60 minutes or more, in race-like conditions.
- 4
Weigh out after
Towel off, return to the same dry clothing, and weigh again immediately.
- 5
Read your rate
Enter both weights, fluid drunk and duration. The calculator returns sweat loss and hourly rate.
Worked example
An athlete weighs in before and after a 60-minute session:
| Weight before | 70.0 kg |
| Weight after | 69.0 kg |
| Fluid consumed | 500 ml |
| Total sweat loss | 1.5 L |
| Sweat rate | 1.5 L/h |
On long sessions this athlete should drink roughly 1.5 to 2.3 liters per hour, with sodium.
Frequently asked questions
How do I calculate my sweat rate?
Calculate sweat rate by weighing yourself before and after a session of known length. Add fluid drunk to weight lost: 1 kg lost plus 500 ml drunk equals 1.5 liters of sweat. Divide by hours.
How much fluid should I drink per hour?
Drink roughly 100% to 150% of your measured hourly sweat loss on long sessions. A small deficit is normal. Adjust for heat, intensity and how you feel.
Can I drink too much water during exercise?
Yes. Drinking far beyond sweat losses dilutes blood sodium and can cause hyponatremia, which can be serious. If you finish heavier than you started, you drank too much. Pair fluid with sodium on long efforts.
Does my sweat rate stay the same?
No. Sweat rate changes with heat, humidity, intensity, altitude and acclimatization. The same athlete can sweat under 1 L/h in cool conditions and over 2 L/h in heat. Test in race-like conditions.
Why do I add the fluid I drank to the weight I lost?
The scale only shows net change. If you sweated 1.5 liters but drank 0.5 liters, you lost only 1 kg. Adding the fluid back recovers the true sweat volume.
Sources
- Sawka et al. (2007), ACSM Position Stand. "Exercise and Fluid Replacement." Med Sci Sports Exerc 39(2):377-390. Source of the sweat-loss method and fluid-replacement guidance. Link
- How Lucy Charles-Barclay Trains for the Heat (and the Hydration Mistake That Cost Her Kona)The 2023 Ironman World Champion prepares for Kona with a six-week heat block, then lost a Kona lead to a sodium overdose. What long-course athletes get wrong about sweat and salt.
- How Wout van Aert Trains (the VLamax Fix Behind His Roubaix Win)Van Aert won the fastest Paris-Roubaix ever, then two Vuelta stages. The training idea behind it is one number his team spent years lowering: VLamax.
- How a Fast Age-Grouper Trains on 8 Hours a Week (With a Full-Time Job)How a fast amateur trains around a job and a family on eight hours a week: two hard sessions, everything else easy, and why the time limit helps.