Water Intake Calculator

Water makes up roughly 60 percent of an adult body and is involved in almost everything: regulating temperature, carrying nutrients, lubricating joints and flushing waste. Unlike most nutrients, there is no single official daily requirement, because your needs depend on your size, how much you sweat and what you eat. This calculator builds a personal estimate from three inputs.

The baseline used here is 35 millilitres per kilogram of body weight. A 70 kg person therefore starts at about 2.45 litres a day. That is higher than the often-quoted eight glasses, and closer to the Institute of Medicine's general guidance of about 2.7 litres of total water a day for women and 3.7 litres for men — remembering that "total water" includes what comes from food and from other drinks, not just plain water.

On top of the baseline, exercise and heat add real volume. You lose roughly 0.35 litres per 30 minutes of moderate activity through sweat, and more in hot or humid conditions where sweating is less effective at cooling you. This calculator adds both. Note the deliberate framing as fluids rather than water: tea, coffee, milk, juice and soup all count towards hydration, and the old idea that coffee dehydrates you is largely a myth at normal intakes.

The practical test is not the number on this page but the colour of your urine. Pale straw means you are well hydrated; dark amber means drink more; completely clear means you can ease off. Thirst is a decent guide for most healthy adults, though it lags behind need during intense exercise and becomes less reliable with age. If you have a heart, kidney or liver condition, or take diuretics, follow your doctor's advice rather than a general calculator.

This calculator returns estimates for general information only. It is not medical advice, diagnosis or treatment, and it cannot replace an assessment by a qualified health professional. Individual results vary, and figures such as calorie needs, body-fat percentage and heart-rate zones are approximations based on population averages rather than measurements of you.