Before you can win, you need to know how much you lose.Your sweat rate is used to estimate how much fluid you need to replace during and after sport.This can be measured by looking at your weight change over several exercise bouts to get a feel for your own personal sweat rates under different exercise and environmental conditions, where:

Sweat loss (mL) = change in body mass (g) + fluid intake (mL) – urine losses (g)

Weighing yourself, immediately before and after exercise gives you a good idea of how much fluid you actually lose during exercise. The difference in weight represents fluid loss (ie. 1kg loss equals to approximately1L (1000mL) sweat).Once you know how much you lose per hour of exercise, generally it’s best to replace 80% while exercising (so in this case, 800mL).After you finish exercising, you should drink 150% of your fluid deficit. For example, if you are still 400mL down will mean replacing 600mL fluid after exercise.

Sweat rates vary greatly between individuals, with females tending to sweat less than males. For example, even within one sporting team doing similar work, sweat rates can range from 600 – 1200 mL/hr for females, and 800-1400 mL/hr for males. Your personal fluid target should be scheduled, so that you start drinking early on and consistently throughout the exercise bout rather than leaving it all to the later stages. If you leave it too late, it’s likely your stomach won’t be as receptive to absorbing the fluid since the blood flow will have shifted to other areas of your body, such as your muscles. These fluid intake targets should be practiced in training so that you train yourself to drink more, and you can make sure you’re drinking within gastrointestinal (stomach) tolerance.

To calculate how much sweat you lose when training or in competition, you will need to:

1. Weigh yourself (with minimal clothing – eg. no hat, socks, shoes, t-shirt) before exercise

2. Exercise for one hour at your targeted intensity**

3. Track your fluid intake during exercise (measure in mls)

4. Record weight (with minimal clothing) after exercise

** One hour is recommended to get a reliable representation of sweat rate expected

The above method determines your hourly sweat rate by adding the difference in your weight before/after exercise to the fluid consumed (step 3).

Note the environmental conditions on this day, and repeat the measurements on another day when they are different (cooler, warmer). This will give you an idea of how different conditions affect your sweat rate.

Disclaimer: This Method is only a rough estimate of recommended fluid intake. For serious training and competing requirements, please consult a sports dietitian.

References:
1.Broad EM, Burke LM, Cox GR, Heeley P, Riley M. 1996. Body weight changes and voluntary fluid intakes during training and competition sessions in team sports. Int. J. Sport Nutr., 6: 307-320.