Water Intake Calculator
A daily water range from your weight, your exercise and the temperature — with the sum written out, so you can argue with it.
The result is total water for the day, including the water in food and in other drinks. It is an estimate for planning, not a prescription — see the limits below.
Where the range comes from
| Component | Rule used here | Why |
|---|---|---|
| Baseline | 30–35 ml per kg of body weight | A common clinical rule of thumb: 70 kg gives 2.1–2.5 L, close to the 2.0–2.5 L total-water reference intake for adult women |
| Exercise | +350–500 ml per 30 minutes | Roughly 0.7–1.0 L per hour of sweating, a mid-range figure for moderate activity in temperate conditions |
| Heat | +10–20% on the subtotal | Sweat losses rise with temperature and humidity; the percentage keeps the estimate proportional to body size |
The arithmetic is deliberately simple and shown under the result, so the number can be checked and adjusted rather than trusted. The published guidelines behind the 30–35 ml/kg range are explained on the how-much-water page.
Why a range instead of one number
Healthy kidneys handle a wide intake: they can concentrate urine when water is short and excrete up to about a litre an hour when it is plentiful. Two people of the same weight can be perfectly hydrated on amounts half a litre apart. A single number would be false precision, so the tool returns the interval its inputs support.
Roughly a fifth of total water comes from food — fruit, vegetables, soup, yoghurt, rice, pasta. Someone eating a plant-heavy diet may need fewer glasses than someone eating mostly dry food, at the same total. The result already includes that share; it is not a target for plain water alone.
Turning litres into a day
- Start with 400–500 ml after waking. Overnight losses are real and the first drink is the easiest one to keep.
- Keep a bottle in sight. Visible containers get emptied; hidden ones do not. A 750 ml bottle refilled twice covers most of a desk day.
- Drink with meals. Three meals with a glass each is 600–750 ml without any reminders.
- Add 400–600 ml around exercise, before and after rather than all at once.
- Check the colour, not the clock. Pale straw is the useful signal; clear urine usually means you are ahead, not behind.
This is not medical advice. The estimate assumes healthy kidneys, a normal heart and no fluid-restricting medication. If you have kidney, heart or liver disease, take diuretics, are pregnant or breastfeeding, or are preparing for an endurance event, ask a clinician who knows your history — a web page cannot see any of that.
Questions
Is eight glasses a day a real rule?
It is folklore with a plausible origin: eight 8-ounce glasses is about 1.9 L, which sits inside the reference range for many adults but is not a guideline anyone published as a universal rule. Body size, food, climate and activity change the number far more than the slogan admits.
Does coffee or tea dehydrate you?
Not at ordinary intakes. Moderate caffeine consumption does not produce a net fluid loss worth correcting for; the drinks count toward the total. Very high doses of caffeine in concentrated form are a different matter, and alcohol does increase fluid loss.
Can I drink too much water?
Yes. Drinking well beyond thirst can dilute blood sodium (hyponatraemia), which is dangerous and occasionally fatal. It is rare in ordinary life and mostly seen in endurance events, crash diets and some psychiatric conditions. The kidneys can excrete roughly 0.8–1 L per hour; sustained intake above that outpaces them.
Does the calculator work in pounds?
Convert first: pounds divided by 2.205 gives kilograms. 154 lb is about 70 kg, which is the default in the form.
Should I drink more when I am ill?
Often yes — fever, vomiting and diarrhoea all increase losses, and the usual advice is to replace them with fluids and electrolytes. But some conditions (heart failure, advanced kidney disease) require restriction instead, which is exactly why the tool asks you to treat its output as a starting point.