Water Intake Calculator

This page produces a rule of thumb. Not a requirement, not a clinical target, and not a figure any health authority publishes — a rule of thumb, of the kind a coach writes on a whiteboard. Both numbers it depends on are exposed as sliders precisely because neither 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. Total 2,975 ml, near enough 3.0 litres, which the tool also shows as 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 honest uncertainty in the method, and it is why an earlier version of this page attributing 35 ml/kg to unnamed "clinical hydration guidelines" was withdrawn. No guideline was ever named because none says it.

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

"Eight glasses a day" is repeated everywhere and rests on almost nothing. It is usually traced to a 1945 US Food and Nutrition Board note that recommended about 2.5 litres of water daily and then observed, in the sentence immediately after, that most of it is already contained in prepared food. The second sentence stopped being quoted.

The trap is what people do next: they debunk the eight glasses and replace it with their own unsourced multiplier, which is the same mistake wearing different clothes. So here is what actual reference bodies say, and how it 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 the total. This calculator has no sex input and does not deduct water from food, so its output is not comparable to either figure and should not be read as agreeing or disagreeing with them.

What the calculation ignores

Climate and altitude, both of which change losses substantially. Fever, vomiting and diarrhoea. Pregnancy and breastfeeding, which raise requirements. The water content of what you eat, which is considerable if your diet is heavy in fruit, vegetables and soup, and negligible if it is not. Sweat rates, which vary enormously between individuals doing identical work — the 350 ml per half hour is a middling guess, not a measurement of you.

Anyone under a fluid restriction for heart failure, kidney disease, cirrhosis or hyponatraemia should ignore this page entirely and use the volume their clinician has set. Restriction exists because excess intake is genuinely dangerous in those conditions.

Better day-to-day signals than a number

For most healthy adults, thirst is an adequate regulator and pale straw-coloured urine is a reasonable check. Very dark urine suggests drinking more; consistently clear urine all day suggests you are drinking more than you need. Drinking to a calculated litre target while ignoring both signals is not a sign of good hydration practice.

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