Water Intake Calculator

Checked 15 October 2026. What was checked. How these pages are maintained.

A simple water target

One common rule is about 35 ml per kilogram of body weight (roughly half an ounce per pound), then a bump for heat or training. 75 kg × 35 ml is about 2.6 litres. Thirst, urine colour, illness, and medical conditions matter more than any formula. This is not medical advice.

Worked example

The form uses 35 ml per kilogram, then a day-type factor. At 70 kg a desk day (factor 1) is 2.45 litres. Light activity (1.15) is about 2.82 litres, roughly 12 cups of 240 ml. A hot day or hard training (1.3) is about 3.19 litres. A 30 ml rule you saw elsewhere will not match.

Thirst, urine colour, and a clinician’s advice beat a webpage. Heart or kidney conditions can mean a fluid limit. Not medical advice.

35 millilitres per kilogram, then a day-type bump

The base is 35 ml for each kilogram of body weight. At 70 kg on a desk day the factor is 1, so the target is 70 × 35 = 2,450 ml, which is 2.45 litres, about 10.2 cups of 240 ml. Light activity uses 1.15, so the same 70 kg becomes 2,450 × 1.15 = 2,817.5 ml, about 2.82 litres and 11.7 cups. A hot day or hard training uses 1.3, about 3.19 litres at 70 kg. Pounds are converted to kilograms with 2.20462 before the 35 ml is applied. A rule of thumb you saw elsewhere at 30 ml per kilogram will not match this page. Thirty millilitres was an older round number. The form uses 35.

Coffee and tea count as fluid for most people. The page does not subtract them, and it does not add a glass for every coffee. It only scales the 35 ml rule.

Thirst still outranks the litre

Urine colour, thirst, heat, a salty meal, and a clinician’s instruction all beat a body-weight formula. Heart failure, kidney disease, and some medicines come with a fluid limit. Forcing litres because a calculator said 2.82 can be the wrong direction. Do not “catch up” by drinking a gallon at once. This is a sketch of an ordinary day, not a hydration plan for a race and not medical advice. The protein calculator is the same kind of body-weight sketch for a different nutrient, with the same limit: a clinician wins when a condition is involved.

Common questions