Water Intake Calculator

This water intake calculator estimates how much you should drink in a day and turns the figure into something practical. It starts from your body weight and age, then adds for exercise duration and intensity, for hot, humid or cold climates, for high altitude and illness, and for pregnancy or breastfeeding. You can work in metric or imperial units. The result is shown in millilitres and as a number of glasses at the container size you choose, from a 200 ml small glass up to a one litre bottle, and it is spread across your waking hours between the wake-up and bedtime you enter to give a simple drinking schedule. It also compares your target against published EFSA and US National Academies reference values. It runs in your browser and offers general guidance, not medical advice.

The calculation starts with a base need scaled to body weight and age: 35 ml per kilogram for ages 14 to 30, 33 ml per kilogram for 31 to 55, and 30 ml per kilogram from 56 upward, reflecting the gradual decline in body water with age. Children under 14 use fixed reference values instead, since scaling by weight is unreliable at that stage.

Additions are then stacked on top. Exercise adds 8 ml per minute for light activity such as walking or yoga, 12 ml per minute for moderate work like jogging or a gym session, and 16 ml per minute for intense efforts such as running or HIIT. Climate adds 500 ml for hot and dry conditions, 750 ml for hot and humid, and 250 ml for cold dry air; a temperate climate adds nothing. High altitude and illness each add 500 ml. Pregnancy adds 300 ml and breastfeeding 700 ml, both following EFSA figures.

The total is held above a sensible adult minimum, capped at a safe ceiling, and rounded to the nearest 50 ml.

A 28-year-old weighing 70 kg in a temperate climate falls in the 14 to 30 age band, so the base need is 70 x 35, which is 2,450 ml. A 45 minute moderate gym session adds 45 x 12, or 540 ml. A temperate climate adds nothing, and with no altitude, illness or pregnancy factors the running total is 2,990 ml. Rounded to the nearest 50 ml, the daily target is 3,000 ml, or three litres.

Choosing a 250 ml glass turns that into 12 glasses across the day. With a wake-up time of 07:00 and a bedtime of 23:00, that is a 16 hour waking window, so the schedule works out at roughly one glass every 80 minutes.

Change one input and the figure moves noticeably. Move the same person to a hot, humid climate and 750 ml is added, lifting the target to 3,750 ml, or 15 glasses. Raise the session to an intense hour and the exercise component alone becomes 960 ml.

The target counts total fluid intake, not plain water alone. Tea, coffee, milk, juice and soup all contribute, and food supplies a meaningful share of daily water through fruit, vegetables and cooked grains. The tool also shows the EFSA and US National Academies reference values so you can see where your figure sits against published guidance, including the split between total water and the portion expected to come from drinks.

Pace matters as much as volume. The kidneys clear only around 0.8 to 1 litre per hour, and drinking faster than that over a sustained period risks diluting blood sodium, a condition called hyponatraemia that endurance athletes occasionally encounter. Spread intake through the day and use an electrolyte drink for long, sweaty sessions rather than water alone.

Several conditions change the right answer entirely, including kidney disease, heart failure and some medications, and a few require fluid restriction. Thirst and pale straw-coloured urine remain good day-to-day guides. This is general information, not medical advice.