If you want to know how much water you should drink when running, you’ll need to know how much you sweat. Sweat rates vary from runner to runner. Some people sweat as little as 200ml of fluid per hour, whereas others can sweat as much as 3 litres per hour!

To complicate things, sweat rates can vary significantly within the same runner. If you’re running in -30°C you will sweat far less than you will in +30°C. You will also sweat far more when running at race-pace than at your easy-pace.

As you can see, trying to give generic advice on how much water you should drink when running is a fool's errand. So what we really need is a strategy to work out how much water you need. To work that out, we need to get a picture of how much fluid you lose when you’re running.