Daily Water Intake Calculator

Daily water intake needs vary with body weight, physical activity, climate, and health.
The old “8 glasses a day” rule is an oversimplification. A 55 kg office worker and a 95 kg roofer in August do not have remotely the same requirement.

The baseline this calculator uses:

Baseline daily water = Body weight (kg) × 33 mL
That is roughly half a fluid ounce per pound. For a 70 kg adult it comes to 2.3 litres to drink, which sits between the European and the US reference figures further down.

Activity and climate are then applied as multipliers on top of that baseline:

The multipliers are rules of thumb, not measurements. Adult sweat rates run from 0.5 to 4.0 litres an hour (NATA, the National Athletic Trainers’ Association, 2017), so on a hard training day the honest method is the bathroom scale: weigh before and after, and each kilogram lost is about a litre of water to put back.

Cold surprises people. You lose more water than you would guess in cold air, because it holds very little moisture, so every breath you exhale is more humid than the air you took in, and dry indoor heating does the same thing all winter.

Worked example:

A 170 lb (77 kg) person who trains 3 to 5 days a week and lives somewhere hot and humid:

• Baseline: 77 × 33 mL = 2.5 L (86 fl oz)
• Moderately active: × 1.30 gives 3.3 L
• Hot and humid: × 1.15 gives 3.8 L per day (129 fl oz, about 15 glasses)

Official totals, from all sources including food:

Those US totals usually come out higher than this calculator, for two reasons. About 20% of your daily water arrives in food, and the panel below adds that back in. Watermelon is roughly 92% water, cucumber 96%, and a bowl of soup counts for most of its volume.
The rest of the gap is that the US figures are not a requirement at all. The Institute of Medicine set them at the median intake of Americans in a national survey, so they describe what people drink rather than what anyone needs. The European Food Safety Authority sets its adequate intake lower, at 2.5 L for men and 2.0 L for women from all sources, and adds about 300 mL a day for pregnancy and 700 mL for breastfeeding. Add those to the figure below.

How to tell whether you have it right: urine that is pale straw, six to eight trips to the bathroom a day, and no mid-afternoon headache. Dark amber means you are behind. Completely colorless all day usually means you are overdoing it.

The limit that matters is hourly, not daily. Healthy kidneys clear roughly 0.8 to 1.0 L an hour, and drinking faster than that dilutes blood sodium, which is how hyponatremia happens to endurance athletes. If heat or long training puts you at 5 L a day or more, spread it out and make some of it an electrolyte drink or salted food rather than more plain water.

This rule is for adults. Children’s needs are set by age instead: the US figures are about 1.3 L a day from all sources at ages 1 to 3, and 1.7 L at 4 to 8.