How the estimate works
A practical starting point is about 35 ml per kilogram of body weight, plus roughly 10 ml per minute of exercise, adjusted up in heat. For a 70 kg person doing 30 minutes in a warm climate, that's around 3.0 litres. A smaller example: a 55 kg person doing 20 minutes of exercise in a cool climate needs roughly (55×35 + 20×10) × 1.0 ≈ 2.1 litres.
Where this baseline comes from
There's no single universal formula for fluid needs — they genuinely vary by body size, sweat rate, diet, and climate. The 35 ml/kg figure is a commonly used clinical rule of thumb, broadly in line with general adequate-intake guidance from bodies like the US National Academies of Sciences, which puts total daily fluid (including food) at roughly 3.7 litres for men and 2.7 litres for women — most of that from drinks, with the rest from food.
Signs you're well hydrated
The simplest check is urine colour — pale straw means you're on track; dark yellow means drink more.
Tips to hit your target
- Start the day with a glass before coffee.
- Keep a bottle visible.
- Drink around meals and exercise.
- Water-rich foods count too.
Limitations
This is a general estimate, not medical advice. People with kidney, heart, or liver conditions, or who are pregnant or breastfeeding, often have different fluid needs and should follow guidance from their doctor rather than a generic calculator.