How to calculate fluid needs for exercise correctly
Baseline needs, exercise losses and hydration recommendations
Daily fluid needs consist of your baseline requirement plus exercise losses. A well-established rule of thumb is 35 ml per kilogram of body weight per day as the baseline value. For a body weight of 70 kg, that works out to 2.45 liters without exercise. With 60 minutes of moderate-intensity exercise (about 700 ml of sweat loss per hour), the requirement rises to 3.15 liters. These reference values match the calculations behind this calculator and form the basis for a practical hydration recommendation.
The amount of sweat lost depends heavily on training intensity, ambient temperature, and individual factors. At low intensity (yoga, walking), the body loses roughly 500 ml per hour. At moderate intensity (easy jogging, cycling), it is around 700 ml, while at high intensity (HIIT, interval training, exercising in the heat) losses can reach 1,000 ml per hour or more. Electrolytes – sodium in particular – are lost through sweat and should be replenished with sports drinks during training sessions longer than 90 minutes.
In practice, sports nutrition experts recommend being well hydrated before training (light-colored urine is a good indicator). During training, drink 150–250 ml every 15–20 minutes. After training, make up the fluid deficit gradually over 2–4 hours. Even a 2% loss of body weight through sweat (1.4 liters for a 70 kg person) measurably reduces athletic performance – making hydration a simple and effective performance tool.