How to calculate your water intake from body weight

2026-10-30

A practical rule of thumb is about 33 ml of drinks per kilogram of body weight a day, which is roughly half a fluid ounce per pound, plus about 500 ml (17 fl oz) on active days. For a 70 kg (154 lb) adult that's about 2.3 liters (78 fl oz). It's a starting point, not a prescription: no official body sets adult water needs per kilogram, and the formula gets less accurate at the extremes of body weight.

Why use body weight at all?

The official reference values are the same for every adult of a given sex: EFSA suggests 2.0 liters of total water a day for women and 2.5 liters for men (EFSA opinion on water), and the US National Academies 2.7 and 3.7 liters (National Academies). Those are averages. A 50 kg woman and a 100 kg man clearly don't use the same amount of water, and a weight-based estimate is an easy way to make a general guideline a bit more personal.

There is a real basis for it. In a 2022 study of more than 5,600 people that measured water turnover directly, bigger bodies did turn over more water, alongside activity, sex, age and climate (University of Wisconsin–Madison summary). But note the size of the effect, which matters later: 50 kg more body weight added only about 0.7 liters a day.

The formula

Metric: body weight in kg × 33 = ml of drinks per day.

US units: body weight in lb × 0.5 = fl oz of drinks per day.

Then adjust:

Active day (a workout, a physical job, a long walk in the heat): add about 500 ml / 17 fl oz.
Keep it within sensible limits: for most adults, between about 1.5 and 4 liters (50–135 fl oz) of drinks a day.

The 33 ml/kg figure is a convention, not a law of physiology. Calculators online use slightly different factors. We use 33 because for an average adult it lands between the European and US reference values once you subtract the water you get from food.

Worked examples

In kilograms

55 kg, mostly sitting: 55 × 33 = 1,815 ml, so about 1.8 liters.
70 kg, mostly sitting: 70 × 33 = 2,310 ml, so about 2.3 liters. On a training day: about 2.8 liters.
85 kg, active: 85 × 33 = 2,805 ml + 500 ml = about 3.3 liters.
45 kg: 45 × 33 = 1,485 ml, which is just under the floor, so use about 1.5 liters.
120 kg, active: 3,960 + 500 = 4,460 ml. That's above the cap, so use about 4 liters and let thirst tell you if you need more.

In pounds

130 lb: 130 × 0.5 = about 65 fl oz (around 1.9 liters).
180 lb: about 90 fl oz (2.7 liters), or about 108 fl oz on an active day.
250 lb: about 125 fl oz; on active days the formula passes 135 fl oz, so cap it around 135 fl oz (4 liters).

Adjusting for exercise and heat

The flat 500 ml for an active day is deliberately simple. Real sweat losses vary a lot with intensity, duration, temperature and the person. If you train regularly, two better approaches are:

Measure your sweat rate. Weigh yourself before and after a typical session; each kilogram lost is about a liter of fluid. See how to measure your sweat rate.
Follow exercise guidance for before, during and after training.

Hot weather raises needs even when you're not exercising. On very hot days, treat the formula as a minimum and use thirst and urine color to top up.

Where weight-based formulas fall short

Be honest with yourself about what a formula can and can't do.

Weight matters less than the formula implies. The formula adds 33 ml for every extra kilogram, so 50 kg more body weight means 1.65 liters more. The 2022 turnover study found a real difference of only about 0.7 liters for the same gap. This is why a cap makes sense: for people with high body weight, a straight per-kg calculation overshoots.
Muscle and fat aren't the same. Lean tissue holds far more water than fat, so two people of equal weight can have different needs.
Food isn't in the formula. If you eat a lot of fruit, vegetables and soup, you get more water from food and may need to drink less. See how much of your water comes from food.
It's for healthy adults. Children need different amounts by age (see how much water children need). Pregnancy and breastfeeding add to the total.
Some conditions change everything. Heart failure, kidney disease and some medications can mean you should drink less, or a specific amount. Follow your doctor's number, not a formula.

In short, a weight-based number is a reasonable first guess. Your body's signals, mainly thirst and urine color, are how you check it.

Checking whether your number works

Use the formula for a week, then look at the signals:

Your urine is pale yellow most of the day: you're probably about right.
It's often dark, or you get thirsty or headachy in the afternoon: raise the goal by 250–500 ml.
You're constantly running to the bathroom and never thirsty: you can probably lower it.

For more on reading those signals, see how much water you should drink a day.

How Water Tracker suggests a goal

Water Tracker uses this exact approach. When you set it up, you enter your weight and whether you're active, and it suggests a goal of about 33 ml per kg, plus 500 ml if you're active, kept between 1.5 and 4 liters and rounded to whole glasses. You can accept it or set your own number at any time, and show it in glasses, milliliters or ounces. Reminders stop once you reach the goal, so a higher goal on training days doesn't mean extra nagging on rest days.

Bottom line

Rule of thumb: 33 ml per kg (about 0.5 fl oz per lb) of drinks a day.
Add about 500 ml / 17 fl oz on active days; measure your sweat rate if you train a lot.
Keep the result within roughly 1.5–4 liters for adults; formulas overshoot for heavy bodies and undershoot for very light ones.
Treat the number as a starting point and adjust it with thirst and urine color. If you have a medical condition, ask your doctor.

This article is general information, not medical advice. If you have a health condition, are pregnant or take medication, ask your doctor how much you should drink.