Water Intake Calculator

What this page gives you is a rule of thumb, the kind a coach writes on a whiteboard. It isn't a requirement or a clinical target, and no health authority publishes it. Both numbers it depends on are sliders because neither one is a standard, and moving them changes the answer by litres.

Two multipliers you can change

The default calculation is millilitres per kilogram of body weight, plus an allowance for exercise:

base       = body weight (kg) x 35 ml       [slider range 20-60]
exercise   = (minutes / 30) x 350 ml        [slider range 0-800]
total      = base + exercise

For 70 kg and 45 minutes of exercise: 70 x 35 = 2,450 ml from body weight, and 45 / 30 = 1.5 blocks at 350 ml each = 525 ml for the session. That's 2,975 ml in total, near enough 3.0 litres, which the tool also shows in fluid ounces and as roughly twelve 250 ml cups.

Slide the per-kilogram figure to 25 and the same person gets 2.3 litres. Slide it to 50 and they get 4.0. That spread is the real uncertainty in the method. It's also why we withdrew an earlier version of this page that credited 35 ml/kg to unnamed "clinical hydration guidelines". No guideline was ever named, because none says it.

The eight-glasses problem, and the trap that follows it

"Eight glasses a day" gets repeated everywhere and rests on almost nothing. It's usually traced to a 1945 US Food and Nutrition Board note that recommended about 2.5 litres of water daily, and then pointed out in the very next sentence that most of it is already contained in prepared food. People stopped quoting the second sentence.

The trap is what comes next. People debunk the eight glasses and then replace it with their own unsourced multiplier, which is the same mistake again. So here's what the actual reference bodies say, and how that differs from the number above.

The European Food Safety Authority sets adequate total water intake at 2.5 litres a day for men and 2.0 litres for women. The US National Academies set 3.7 litres for men and 2.7 litres for women. Both figures are sex-specific, and both are *total* water, including the water in food, which typically supplies around a fifth of it. This calculator has no sex input and doesn't deduct water from food, so you can't compare its output with either figure, and you shouldn't read it as agreeing or disagreeing with them.

What the calculation ignores

It ignores climate and altitude, both of which change losses substantially, and fever, vomiting and diarrhoea. It ignores pregnancy and breastfeeding, which raise requirements. It also ignores the water content of what you eat, which is considerable if your diet is heavy in fruit, vegetables and soup and negligible if it isn't. And sweat rates vary enormously between people doing identical work, so the 350 ml per half hour is a middling guess and has nothing to do with your own body.

If you're under a fluid restriction for heart failure, kidney disease, cirrhosis or hyponatraemia, ignore this page entirely and use the volume your clinician has set. Those restrictions exist because drinking too much is genuinely dangerous in those conditions.

Better day-to-day signals than a number

For most healthy adults, thirst does the job well enough and pale straw-coloured urine is a reasonable check. Very dark urine suggests you should drink more. Urine that's clear all day, every day, suggests you're drinking more than you need. Hitting a calculated litre target while ignoring both of those signals doesn't mean you're hydrating well.

Fluid needs go up and down with body size, which is the one thing this rule does capture. If you also want a sense of how your weight relates to your height, the BMI calculator covers that. Everything here is worked out in your browser and nothing leaves the device, though your own browser may remember the sliders. No medically qualified person has reviewed this page.