Water Intake Calculator — Daily Water Needs
Estimate how much water to drink a day from your weight, exercise and climate, beside the IOM figure for your age and sex. A general guide, not a strict quota.
Spread over the hours awake, the part to drink is about 116 ml an hour — one cup (237 ml) every 123 minutes or so.
Signs you need more than you are drinking
- Feeling thirsty
- Dark yellow, strong-smelling urine
- Peeing less often than usual
- Feeling dizzy or lightheaded
- Feeling tired
- A dry mouth, lips and tongue
Thirst and pale-straw urine are better guides than any formula, including this one.
Plan for a race or a hard session
- Before: 350–490 ml, drunk slowly at least 4 hours ahead (5–7 ml per kg of body weight).
- During: about 700 ml an hour, in sips rather than all at once.
- After: about 1.5 L for every kg the session took off — weigh yourself before and after to know.
Source: American College of Sports Medicine position stand, “Exercise and Fluid Replacement” (2007).
🔒 Calculated in your browser. A general guideline (~33 ml/kg + exercise + climate) — not medical advice. Needs vary; some conditions require different intake.
How much water a day
Enter your weight and, optionally, your daily exercise minutes and whether you’re in a hot climate. The tool estimates a daily target using a common guideline (about 33 ml per kg, plus ~350 ml per 30 minutes of exercise, plus a hot-climate allowance) and shows it in litres, ounces and cups, with the part to drink also counted in bottles. Add your age and sex — and, for women, pregnancy or breastfeeding — and it also shows the Institute of Medicine’s Adequate Intake for that life stage, which does not use weight, exercise or climate.
A guide, not a rule
Hydration needs vary with activity, health, pregnancy and diet, and food and other drinks count too. Use this as a starting point, not a strict quota, and follow medical advice for any condition that affects fluid intake. Everything runs in your browser.
The arithmetic behind the number
Three terms, all of them rules of thumb rather than findings. Body weight contributes 33 mL per kilogram. Exercise adds 350 mL for every 30 minutes you enter. A hot climate multiplies the total by 1.1. That is the whole of the weight-based model — you can reproduce any figure it shows with a calculator. It is offered because a rule of thumb you can see is more useful than one you cannot, not because those three numbers are settled science.
What it leaves out
Roughly a fifth of most people's water comes from food, and the total does not subtract it, so treat it as fluid from all sources rather than glasses of plain water to get through; the "Litres / day to drink" figure beside it is what is left once food's share is taken off. Tea, coffee and juice count; the diuretic effect of ordinary amounts of caffeine is smaller than the water that came with it. Kidney or heart conditions and many medications all change the answer, and none of them is an input here; pregnancy and breastfeeding are, but they move only the Adequate Intake figure, not the weight-based one. Drinking far more than you need is not harmless either — it dilutes blood sodium, which at extremes is dangerous. Thirst and pale-straw urine remain better signals than any formula. Under the answer, “Signs you need more than you are drinking” lists the others to watch for, and “Plan for a race or a hard session” gives what to drink before, during and after, from the American College of Sports Medicine's position stand.
Frequently asked questions
How much water should I drink a day?
A common guideline is ~33 ml per kg of body weight, more with exercise and heat. For a 70 kg person that’s about 2.3 litres — but needs vary.
Where does 33 mL per kg come from?
It is a common clinical rule of thumb, roughly equivalent to the older "eight glasses" advice once you scale it to body size. It is not derived from a trial, and no formula can be — requirements vary with diet, climate, activity and health.
Do tea and coffee count towards it?
Yes. At ordinary intakes the fluid outweighs the mild diuretic effect of the caffeine. So does the water in food, which the main total does not subtract, so it errs on the generous side — the "Litres / day to drink" figure is the one with food's share taken off.
Can I drink too much water?
Yes. Taking in far more than the kidneys can clear dilutes blood sodium — hyponatraemia — which is rare but serious. This tool is a general guide, not medical advice; if you have a kidney or heart condition, ask a doctor rather than a calculator.
Does it take age, sex or pregnancy into account?
Yes, as a second figure. Give an age and a sex and the page adds the "Adequate Intake for this age and sex" from the Institute of Medicine — the total, and the part from drinks — beside the weight-based answer; for a woman, a "Pregnancy" select switches it to the pregnant or breastfeeding row. That figure is a population average for healthy people and ignores weight, exercise and climate, which is why it sits beside the first answer rather than replacing it.
Does it say how much to drink during a workout, or how to spread it over the day?
Yes. With exercise minutes entered, a tile gives the session's own share — the millilitres to drink during the exercise, and the rate an hour — which is the part of the daily total the exercise added, shown apart from it. The “Hours awake” box spreads the part to drink, the total less food's share, evenly over your waking day, as millilitres an hour and one cup every so many minutes.
Does it help plan a race day, or say when I am not drinking enough?
Yes. Under the answer, “Plan for a race or a hard session” works out how much to drink before the session for the weight you entered, about how much an hour during it, and how much afterwards for each kilogram it took off, following the American College of Sports Medicine’s position stand. “Signs you need more than you are drinking” lists the signs of dehydration — thirst, dark yellow urine, peeing less often, dizziness, tiredness, a dry mouth.