Hydration is more than water

When you sweat you lose water and electrolytes, mostly sodium. Lose enough and training feels harder than it should. The American College of Sports Medicine's position is that the goal is to avoid losing more than about 2% of body weight from water loss during exercise.

Weigh yourself, once

The simplest test there is: weigh yourself before and after a typical session, without clothes. Every kilogram lost is roughly a liter of sweat. Do it a few times in different weather and you will know your own numbers better than any chart.

After training, the ACSM suggests drinking about 1.5 liters of fluid for every kilogram lost, because you keep losing some as you rehydrate.

Sodium varies a lot

Sweat sodium is very different from person to person. A review by Lindsay Baker of the Gatorade Sports Science Institute found large differences between athletes, which is why some people finish a session with white salt lines on their shirt and others do not. If that is you, plain water may not be enough on long or hot days.

A practical approach

• Start sessions hydrated: drink with meals and check that your urine is pale.
• For sessions under an hour in mild weather, water is usually fine.
• For long sessions, heat or heavy sweating, add electrolytes.
• Weigh before and after to learn what you actually lose.

More is not always better. Drinking far beyond your losses can be dangerous too. Match what you lose.