Water Intake Calculator
How much water you need scales with your body size and how much you sweat. This gives a practical daily starting point from your weight and exercise time, in litres, cups and bottles.
| Body weight | kg |
| Exercise per day | minutes |
Result
Your result will appear here.
How it works
Water needs scale roughly with body mass, because a larger body has more tissue to perfuse and more heat to lose. A widely used rule of thumb is 35 ml per kilogram of body weight per day for a healthy adult in a temperate climate, before exercise.
Exercise adds a separate allowance. Sweat rates vary enormously with intensity, clothing and heat, but about 400 ml per half hour of moderate activity is a reasonable planning figure. The calculator caps the exercise allowance at five hours a day, because beyond that the answer depends on measuring your own sweat loss rather than on a formula.
Formula
The two parts are added together, then converted to litres and containers.
- Baseline = body weight in kg × 35 ml
- Exercise extra = (minutes ÷ 30) × 400 ml
- Daily target = baseline + exercise extra
- Litres = millilitres ÷ 1,000
Worked example
A 70 kg person who exercises for 30 minutes: baseline 70 × 35 = 2,450 ml, plus 400 ml for the session, giving 2,850 ml a day — about 2.85 litres, or eleven and a half 250 ml glasses.
Assumptions
The formula assumes a healthy adult, a mild climate, and ordinary daily activity outside the exercise you enter. Pregnancy, breastfeeding, fever, vomiting, diarrhoea, kidney or heart conditions, and certain medicines all change fluid requirements, and hot humid weather can double sweat losses. Food typically supplies around 20% of total water intake, so drinks alone need not reach the full number.
Reading the result
Treat the number as a starting point and check it against your body. Pale straw-coloured urine and no persistent thirst suggest you are in range; dark urine, headaches or dizziness on standing suggest you are short. Drinking far beyond your needs is not harmless — it dilutes blood sodium — so spread intake through the day rather than forcing large volumes at once, and drink to thirst during long endurance events.
For general information only, not medical advice. Follow your doctor's guidance if you have a condition affecting fluid balance.
How to use the Water Intake Calculator
It takes a few seconds, and every figure is calculated in your browser.
- Enter your body weight.
- Enter your daily exercise in minutes.
- Press Calculate — the result appears in the dark panel beside the form.
- Use Copy, CSV, Share or Print under the result if you want to keep it.
Water Intake Calculator — frequently asked questions
What formula does the Water Intake Calculator use?
The two parts are added together, then converted to litres and containers. Baseline = body weight in kg × 35 ml Exercise extra = (minutes ÷ 30) × 400 ml Daily target = baseline + exercise extra Litres = millilitres ÷ 1,000
How does the Water Intake Calculator work?
Water needs scale roughly with body mass, because a larger body has more tissue to perfuse and more heat to lose. A widely used rule of thumb is 35 ml per kilogram of body weight per day for a healthy adult in a temperate climate, before exercise. Exercise adds a separate allowance. Sweat rates vary enormously with intensity, clothing and heat, but about 400 ml per half hour of moderate activity is a reasonable planning figure.
What assumptions and limitations should I know about?
The formula assumes a healthy adult, a mild climate, and ordinary daily activity outside the exercise you enter. Pregnancy, breastfeeding, fever, vomiting, diarrhoea, kidney or heart conditions, and certain medicines all change fluid requirements, and hot humid weather can double sweat losses. Food typically supplies around 20% of total water intake, so drinks alone need not reach the full number.
Can you walk through a worked example?
A 70 kg person who exercises for 30 minutes: baseline 70 × 35 = 2,450 ml, plus 400 ml for the session, giving 2,850 ml a day — about 2.85 litres, or eleven and a half 250 ml glasses.
Last updated 10 November 2026 · Formula, worked example and assumptions are documented above, and every result is calculated in your browser.