The Sweat Rate Formula | How to Calculate Your Sweat Rate

How to Calculate Your Sweat Rate

Every athlete should know their sweat rate so they have a personal hydration target. This simple test shows how much fluid you lose during a session and how much you need to drink each hour to replace it. Once you've done it, you'll have a hydration target you can use in both training and racing.

What you need: accurate bathroom scales, a towel, and your usual drink bottles (marked in ml).

1. Empty your bladder, then weigh yourself (nude or in minimal dry clothing).

2. Pre exercise weight = ________ kg (A)

3. Complete your usual training session and drink as you normally would. Try not to go to the toilet during the session. If you do, note it, as it will affect your result.

4. Record how much fluid you drank during the session.
Fluid consumed = ________ ml (E)

5. At the end of the session, towel dry, empty your bladder, then weigh yourself again (nude or in the same minimal dry clothing).
Post exercise weight = ________ kg (B)

6. Subtract your post exercise weight from your pre exercise weight to find how much weight you lost.
Weight lost = ________ kg (A) minus ________ kg (B) = ________ kg (C)

7. Convert your weight loss to fluid lost. 1 kg of weight lost equals about 1,000 ml of sweat.
________ kg (C) x 1,000 = ________ ml of fluid lost (D)

8. To find your hourly sweat rate, add the fluid you lost (D) to the fluid you drank (E), then divide by the number of hours you trained.
(________ ml (D) + ________ ml (E)) ÷ ________ hours = ________ ml per hour

Worked example: Sam weighs 70.0 kg before a 1.5 hour ride and 69.2 kg after, and drank 750 ml.
Weight lost: 70.0 minus 69.2 = 0.8 kg, which is 800 ml.
Sweat rate: (800 + 750) ÷ 1.5 = about 1,030 ml per hour.

Turning your sweat rate into a drinking plan

Your sweat rate tells you how much you lose, not necessarily how much you must drink. Most athletes aim to limit weight loss to under 2% of body weight during a session rather than replacing every millilitre. For Sam at 70 kg, that is a loss of no more than 1.4 kg.

Never drink more than you sweat. If you weigh more after a session than before, you have overdrunk. Drinking too much plain water during long events can dilute the sodium in your blood, a dangerous condition called hyponatraemia. This is why replacing electrolytes, especially sodium, matters as much as replacing fluid.

Testing in different conditions

Heat, humidity, and higher intensity all increase sweat rate. Measure your sweat rate across several sessions in different conditions (a cool winter morning, a hot humid summer afternoon, an easy session, a hard session) and keep a log.

On race day, check the forecast, go back to your log, and you'll know how much fluid and sodium to plan for.

Heat and humidity in Australia

Australian summers, and especially the humid conditions in Queensland and northern Australia, significantly increase the risk of heat illness. High humidity stops sweat from evaporating, so your body cannot cool itself effectively, even when the temperature doesn't feel extreme. Dangerous heat stress can occur in the mid 20s °C when humidity is very high.

For guidance on training and competing safely in hot conditions, refer to Sports Medicine Australia's Extreme Heat Policy and the Bureau of Meteorology forecast for your event location. Rather than applying a general multiplier, the most reliable approach is to test your sweat rate in conditions similar to your event.

Knowing your sweat rate is a key part of any athlete's performance plan. For a personalised hydration and electrolyte formula built around your sweat rate, talk to the team at Infinit Nutrition Australia.

This information is general in nature and is not medical advice. Athletes with medical conditions, or those unsure of their needs, should consult a doctor or Accredited Sports Nutritionist.