Water Intake Calculator — Daily Water Needs
Estimate how much water to drink a day from your weight, exercise and climate — in litres, ounces and cups. A general guide, in your browser.
🔒 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.
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 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 this figure does not subtract it, so treat the total as fluid from all sources rather than glasses of plain water to get through. Tea, coffee and juice count; the diuretic effect of ordinary amounts of caffeine is smaller than the water that came with it. Pregnancy, breastfeeding, kidney or heart conditions and many medications all change the answer, and none of them is an input here. 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.
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.
Does coffee or food count?
Yes — food and other drinks contribute to hydration. This tool estimates total water; you don’t need to drink it all as plain water.
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 this total does not subtract, so it errs on the generous side.
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.