Water Intake Calculator
The eight-glasses rule is a slogan, not a guideline. Real fluid needs scale with bodyweight, activity, climate and diet. This calculator gives a weight-based starting point, and the notes explain how to adjust it and how to tell whether you are actually hydrated.
Inputs
Result
2.66 litres/day
About 11 glasses of 250 ml
How the water intake calculator works
A common baseline is 30–35 ml per kg of bodyweight per day, which puts a 70 kg adult at roughly 2.1–2.5 litres of total fluid.
Add 400–800 ml per hour of vigorous exercise, more in heat. Endurance athletes may need to weigh themselves before and after training to measure real losses.
Food contributes roughly 20% of total fluid intake, and all drinks count — including tea and coffee, whose mild diuretic effect does not offset their water content.
Worked example: a 72 kg adult training for an hour
- Baseline: 72 × 33 ml = 2,376 ml.
- Exercise addition: 600 ml for one hard hour.
- Total target: about 3.0 litres of fluid.
- Food supplies roughly 20%, so around 2.4 litres needs to come from drinks.
- In hot weather with heavy sweating, add another 500–1,000 ml.
Common mistakes to avoid
Drinking to a fixed number regardless of conditions
Needs vary daily with heat, activity and food. Urine colour and thirst are better real-time signals than a target on a bottle.
Over-drinking during endurance events
Excessive plain water can dilute blood sodium — hyponatraemia — which is dangerous. Long efforts need electrolytes, not just volume.
Assuming coffee dehydrates you
At normal intakes, caffeinated drinks contribute net fluid. They count toward your total.
Frequently asked questions
How much water should I drink a day?
Roughly 30–35 ml per kg of bodyweight in total fluid, adjusted upward for exercise, heat, pregnancy or breastfeeding.
How do I know if I am hydrated?
Pale straw-coloured urine, no persistent thirst, and stable bodyweight across a training session are the practical indicators.
Do other drinks count?
Yes — tea, coffee, milk and juice all contribute. So does food, at about 20% of total intake.
Can I drink too much water?
Yes. Drinking far beyond losses, especially without sodium, can cause hyponatraemia. Extremely rare in normal daily life, a genuine risk in long endurance events.
Does dehydration affect performance?
Losses above about 2% of bodyweight measurably reduce endurance and cognitive performance, which is roughly 1.4 kg for a 70 kg person.
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