Proper hydration during training affects performance, concentration and safety. Even slight dehydration lowers results and increases the risk of overheating. In this guide we show how much water to drink before, during and after exercise and how to estimate your own fluid losses.
Why is hydration so important?
During exercise the body loses water mainly through sweat, cooling the body in the process. Losing as little as 2 percent of body weight in fluids can noticeably worsen performance and endurance. With sweat we also lose electrolytes, above all sodium, which is why during long exercise water alone is not always enough to replace everything.
How much to drink before, during and after?
- Before training: about 400–600 ml of water 2–3 hours before exercise, so you start well hydrated.
- During: roughly 150–350 ml every 15–20 minutes, depending on intensity and temperature.
- After training: replace the water you lost — ideally based on the real drop in body weight.
How to estimate sweat losses?
The simplest method is weighing yourself before and after training (without clothing, after wiping off the sweat):
- Weigh yourself before training.
- Weigh yourself right after training.
- The difference in weight in kilograms is approximately the number of litres of fluid lost (1 kg ≈ 1 litre).
- Add the water drunk during training to find the total sweat loss.
Worked example
Suppose a runner who weighed 75.0 kg before training and 73.8 kg after it, drinking 0.5 litre of water during the session:
- Weight drop: 75.0 − 73.8 = 1.2 kg, that is about 1.2 litres.
- We add the water drunk: 1.2 + 0.5 = 1.7 litres of actual sweat loss.
- After training it is worth replacing about 1.2–1.8 litres (usually 1.2–1.5 litres are taken for each kilogram lost).
Such a test, repeated under different conditions, lets you learn your own sweat rate and plan hydration better.
When to reach for electrolytes?
For exercise shorter than an hour, water is usually enough. When training lasts longer, takes place in the heat or you sweat heavily, it is worth adding an isotonic drink providing sodium and carbohydrates. This helps maintain electrolyte balance and energy levels.
Beware of overhydration
Drinking in advance is not a good idea. Excess water without electrolytes can lead to hyponatraemia, a dangerous drop in the concentration of sodium in the blood. Drink according to thirst and real losses, not beyond measure.
These recommendations are general in nature and do not replace medical consultation. Individual needs depend on fitness, climate and state of health — if in doubt, consult a specialist.
Calculate your water needs during exercise: Hydration calculator →