About the water intake calculation
The "eight glasses a day" rule ignores that a 50 kg person and a 100 kg person have very different needs. Scaling to bodyweight and adding for sweat loss gets much closer. Treat the result as a starting point and let thirst and urine colour fine-tune it.
Formula
Water (ml) = weight(kg) × 35 + (exercise hours × 500)
This includes water from food and other drinks, not just plain water.Worked example
A 70 kg person doing 30 minutes of exercise: (70 × 35) + (0.5 × 500) = 2,450 + 250 = 2,700 ml, or 2.7 litres.
Frequently asked questions
Does tea, coffee and food count?
Yes. Food typically supplies 20-30% of total intake, and caffeinated drinks are still net hydrating at normal intakes — the mild diuretic effect does not offset the fluid they contain.
Can I drink too much water?
Yes, though it is rare. Drinking several litres far faster than the kidneys can excrete — roughly 0.8-1.0 L per hour — can dilute blood sodium and cause hyponatraemia, which is dangerous. Endurance athletes are the usual at-risk group.
How do I tell if I am drinking enough?
Pale straw-coloured urine is the practical test. Dark yellow suggests you need more; completely clear suggests you can ease off. Thirst is a reliable signal for most healthy adults under normal conditions.
Sources
- EFSA Panel on Dietetic Products — Dietary reference values for water (2010)