What this calculator does
It gives a rough daily fluid estimate from body weight and exercise time, using 35 ml per kilogram plus 500 ml per hour of exercise.
This is a planning number, not a prescription. Official adequate intakes are population figures, not individual targets, and they include water from food.
The estimate
Fluids (ml) = weight (kg) × 35 + (exercise minutes ÷ 60) × 500At 70 kg with 30 minutes of exercise: 2,450 + 250 = 2,700 ml, or 2.7 litres.
At the same weight with no exercise: 2.45 litres.
A note on the evidence
Hydration needs vary widely. The Institute of Medicine’s adequate intake is about 3.7 litres of total water for men and 2.7 litres for women, including food. The 35 ml/kg rule is a common simplification, not a law. Climate, pregnancy, breastfeeding, illness and medication all change the picture.
What this calculator does not do
It does not diagnose dehydration or overhydration, and it is not a target for people with kidney, heart or liver conditions, or for endurance athletes who need electrolytes. If a clinician has given you a fluid restriction, follow that instead. Thirst and urine colour remain useful everyday signals.
Where the 35 ml figure comes from
It is a common rule of thumb, not a regulatory figure. The Institute of Medicine’s adequate intake is a population total, including food, and it is higher for men than women. Scaling by weight is a convenience, not a measured requirement.
What changes the number
Heat, altitude, exercise intensity, pregnancy, breastfeeding, fever and some medications increase needs. Kidney, heart and liver conditions can require restriction. Urine colour and thirst are useful everyday checks, but they are not measurements. When in doubt, drink to thirst and spread it through the day.
The estimate is not a target for every day.
Frequently asked questions
How is the estimate calculated?
It uses 35 ml per kilogram of body weight per day, plus about 500 ml for each hour of exercise. That is a planning heuristic, not an official requirement.
Does food count?
Yes. Food typically supplies around a fifth of total water intake. The number shown is for fluids; you do not need to drink all of it.
Is eight glasses a day a rule?
No. It is a rough slogan. Needs vary with body size, climate, activity, pregnancy, breastfeeding and health conditions.
Can I drink too much water?
Yes, though it is uncommon. Drinking very large amounts quickly can dilute sodium dangerously. Spread intake through the day and follow thirst unless a clinician has given you a specific target.