Water calculator
Estimate how much water to aim for each day from your body weight. A simple baseline — drink more in heat or during hard workouts.
Approx. need ≈ 35 ml × weight (kg)
Your details
kg
A baseline guide: 35 ml of water per kilogram of body weight. Your need rises in hot weather and with intense workouts.
Daily water need
2.5L/day
≈ 10 glasses of 250 ml
Results are for reference only and are not medical advice. For individual guidance, consult a doctor or dietitian.
Next step
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How this calculator works
Briefly and honestly: the science behind it, how far you can trust it, and its limits.
How it works
The calculator multiplies your weight by ~35 ml, giving a practical reference for daily fluid needs.
Approx. need ≈ 35 ml × weight (kg)
What it’s based on
Plainly: the “35 ml per kg” rule has no primary scientific source — it is a common practical convention, and we will not attach a citation it does not have. The actual reference values look different: EFSA sets an adequate intake for total water (drinks + food) of 2.5 L/day for men and 2.0 L/day for women, and the NHS suggests 6–8 glasses of fluid a day.
✓
Is it validated?
There is no single “correct” water norm; “35 ml/kg” is a convenient approximation, not a clinical standard. It only lines up with the reference values in the middle of the weight range: 70 kg gives 2.45 L (close to EFSA’s 2.5 L for men), 60 kg gives 2.1 L (close to 2.0 L for women). Outside that range the linear rule drifts away from the guidelines, because EFSA sets its value by sex, not by kilograms. Some fluid also comes from food.
Can you rely on it?
Follow your body’s signals: thirst and urine colour. Needs rise with heat, exercise and some conditions — then drink more than the reference.
Results are for reference only and are not medical advice. For individual guidance, consult a doctor or dietitian.
For the curious
Deeper: why it works this way, what it does for your body, and what happens if you get it wrong.
Water is involved in almost everything: transporting nutrients, regulating temperature, lubricating joints and keeping you focused. An adult body is 50–60% water, and we constantly lose it — through breathing, sweat and urine.
“35 ml per kg” is a handy starting point, but real needs vary: in heat and during training you lose more, and some fluid comes from food (vegetables, fruit, soups). So the formula is a reference, not a strict rule.
Even 1–2% dehydration lowers focus and endurance and often masquerades as hunger. Forcing fluids isn’t necessary either — the most reliable signals come from your body: follow thirst and a light urine colour.
Worked examples
Concrete numbers for common situations — so you can see at a glance how to read the result.
Person, 60 kg
- Inputs
- 60 kg × 35 ml
- Result
- ≈ 2.1 L of fluid/day
Part of this comes from food — vegetables, fruit and soups.
Person, 80 kg
- Inputs
- 80 kg × 35 ml
- Result
- ≈ 2.8 L of fluid/day
In heat or during training the need rises — drink more than the reference.
Person, 100 kg
- Inputs
- 100 kg × 35 ml
- Result
- ≈ 3.5 L of fluid/day
This is an approximation, not a norm: the best signals are thirst and a light urine colour.
FAQ
Frequently asked questions
How much water should I drink per day?
A common rule of thumb is about 35 ml per kg of body weight: for 70 kg that’s ≈ 2.5 L of fluid. There is no single “correct” norm, and some fluid comes from food.
Do tea and coffee count?
Yes. Moderate tea and coffee add to your overall fluid balance. Plain water is still the best choice without extra sugar or calories.
How do I know I’m drinking enough?
Watch your body’s signals: no strong thirst and a pale-yellow urine colour usually mean you’re well hydrated. A dark colour is a cue to drink more.
Can I drink too much water?
In extreme cases yes — a very large volume in a short time can disrupt electrolyte balance. For most people this isn’t a concern: drink to thirst, not by force.
What to read and do next
Go deeper with articles and turn the numbers into a plan in the app.
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