Water Intake Calculator

By Sofia Andersen · Last updated 15 October 2026

Eight glasses. A gallon. Half your body weight in ounces. You have heard a few versions, and they cannot all be right, because a 60 kg office worker and a 90 kg roofer in August do not need the same amount of water. The honest answer starts with your weight, then adjusts for what kind of day you are having.

Enter your numbers

Without JavaScript: About 30-35 ml per kg of body weight per day. Rough guide: weight in lb x 0.5 = fluid ounces.

Example: 70 kg = about 2,100-2,450 ml per day.

Turn JavaScript on to use the interactive calculator.

Result

General information only. This is not medical advice, a diagnosis, or treatment. Some conditions and medicines change how much fluid is right for you, so talk with a qualified health professional if you are unsure. Urgent symptoms need urgent care, not a calculator.

The short version

Most adults need somewhere around 30 to 35 millilitres of water per kilogram of body weight each day. For a 70 kg person, that works out to roughly 2.1 to 2.5 litres. The calculator below turns your own weight into a personal starting point, then adjusts for activity and heat.

Enter your weight and pick your activity level: Low, Moderate, or High. Three activity levels cover the usual cases: Low for a quiet day, Moderate for an active day, and High for hard training or heat, with the High level adding the exercise top-up automatically. The calculator shows a range rather than one magic number, in litres and in cups, because needs change day to day. Results show litres and cups together, rounded to one decimal place for litres and to the nearest half-cup.

Food counts too. Fruit, vegetables, soups, and yoghurt all carry water, so the number includes what you get from meals, not just the glass in your hand.

A day in numbers

The baseline is weight times a small factor:

Daily water (litres) = weight (kg) × 0.030 to 0.035

That factor is just 30 to 35 millilitres per kilogram. The calculator uses 0.033, the middle of the range, as its headline figure and shows the full range beside it. A US cup is 240 ml, so divide millilitres by 240 to get cups.

Weight	Midpoint for a normal day	With an hour of hard training in warm weather
60 kg	About 2.0 litres	About 2.5 litres
70 kg	About 2.3 litres (9 to 10 cups)	About 2.8 litres
90 kg	About 3.0 litres	About 3.5 litres

For the 70 kg row: low end 70 × 30 = 2,100 ml, or 2.1 litres. High end 70 × 35 = 2,450 ml, or 2.45 litres. Middle 70 × 33 = 2,310 ml, or 2.3 litres. In cups, that is 2,310 ÷ 240 = 9.6, so about 9 to 10 cups across the whole day, food included.

Then come the adjustments. An hour of hard exercise typically adds about 500 ml, sometimes more in heat. A hot, humid day adds more on top. Pregnancy and breastfeeding raise needs as well. On a cool, quiet day at a desk, the low end of the range is usually plenty.

Thirst, colour, and the top end

The calculator gives a planning number, not a prescription. Thirst is still a decent guide for most healthy adults. Urine colour helps too: pale straw is a reasonable sign you are drinking enough, while dark yellow usually means you need more. Clear as water all day, every day, can mean you are overdoing it, though a pale colour on its own is rarely a problem.

It is possible to drink too much, especially during long endurance exercise, when drinking far more than you sweat can dilute sodium in the blood. That is rare in everyday life, but it is one reason a range beats a single big target. And if you are told to limit fluids for a heart, kidney, or liver condition, or you take medicines that affect fluid balance, your clinician’s guidance comes first.

Hydration also sits beside food. If you are tracking intake for weight or energy, the calorie calculator covers the food side, the BMI calculator gives the quick weight-and-height picture, and the BMR calculator shows what your body uses at rest. The body fat percentage calculator estimates body fat from tape measurements. Or browse all health calculators and all calculators on CalcKits.

Frequently asked questions

How much water should I drink a day?

A common starting point is 30 to 35 ml per kilogram of body weight, which is about 2.1 to 2.5 litres for a 70 kg adult. Add more for exercise, heat, pregnancy, or breastfeeding, and let thirst and urine colour fine-tune it.

Does coffee count toward water intake?

Yes, mostly. Coffee, tea, and other drinks all contribute fluid, and the mild diuretic effect of caffeine does not cancel that out at normal intakes. Water is still the simplest choice for most of the day.

Is the eight-glasses rule true?

It is a handy reminder, not a rule. Eight glasses is about 1.9 litres, which suits many smaller or quieter adults but runs low for larger people or hot, active days. Your weight and your day matter more than a fixed glass count.

Can you drink too much water?

Yes, though it is uncommon in everyday life. Drinking far more than you sweat, especially during long endurance exercise, can dilute sodium in the blood. A sensible range, plus thirst, is a better guide than forcing a large fixed target.

Does food count toward my water intake?

Yes. Fruit, vegetables, soups, and yoghurt all carry water, and the calculator’s number includes what you get from meals, not just the glass in your hand.

Do I need extra water when I exercise?

Usually, yes. An hour of hard exercise typically adds about 500 ml, sometimes more in heat. That is why the training preset adds an exercise top-up automatically.

This page offers general information, not medical advice, a diagnosis, or treatment. It cannot replace a qualified health professional who knows your history, and if symptoms worry you or anything feels urgent, get urgent care rather than waiting on a calculator result.