Water Intake Calculator

This water intake calculator estimates a daily hydration target from your weight and exercise. How much water you need depends mostly on your body size and how much you sweat. This calculator estimates a daily target from your weight and exercise, then shows it in litres, millilitres, US cups and fluid ounces so it maps onto whatever bottle or app you use.

The estimate

baseline  = weight(kg) × 35 ml
exercise  = (minutes ÷ 30) × 350 ml
daily     = baseline + exercise

The 35 ml-per-kg baseline is a practical middle ground; the exercise term replaces sweat losses.

Use it as a guide, not a rule

About a fifth of your water comes from food, and heat, altitude, illness, pregnancy and breastfeeding all raise your needs. The most reliable signals are thirst and pale-yellow urine.

Worked example

A 70 kg person exercising 30 minutes a day:

baseline = 70 × 35 = 2,450 ml
exercise = (30 ÷ 30) × 350 = 350 ml
daily    = 2,800 ml

The same target in the other units this tool prints: 2.8 litres, 11.7 US cups of 240 ml, and 95 fluid ounces. An hour of exercise instead of half an hour adds another 350 ml and takes it to 3,150 ml — the exercise term scales linearly with minutes, so a long training day genuinely changes the answer.

Why this is not the eight-glasses rule

The familiar advice to drink eight 8-ounce glasses comes to about 1.9 litres, which is close to the baseline for a 54 kg person and well short of it for anyone heavier. It has the advantage of being memorable and the disadvantage of ignoring body size entirely, which is the largest single determinant of how much water you turn over.

The per-kilogram approach used here scales with you. It also does not double-count what food provides: roughly a fifth of daily water intake normally comes from what you eat, and drinks such as tea and coffee count too despite their mild diuretic effect, since the fluid they contain comfortably exceeds what the caffeine costs. Plain water should still make up most of it.

When the estimate is wrong

It is a population average applied to one person, and several things push your real requirement away from it. Hot or humid weather and altitude both increase losses. Illness with a fever increases them sharply. Pregnancy and breastfeeding raise needs substantially. A high-protein or high-fibre diet changes them, as does heavy sweating from a physical job rather than from deliberate exercise.

The reliable feedback signals are simpler than any formula: thirst, and urine that is pale yellow rather than dark. Drinking to those signals is enough for most healthy people, and this calculator is most useful as a check on whether you are broadly in the right range. There is such a thing as too much — very high intake over a short period can dilute blood sodium — so spread it across the day instead of chasing the target in one sitting. BMR Calculator uses the same weight input if you are working through the whole set.

Private and precise

This calculator produces estimates from published formulas and is not medical advice. For anything about your health, talk to a doctor or a registered dietitian.