Water Intake Calculator
Estimate how much water to drink each day for your weight and activity.
Results update as you type.
About this calculator
This calculator estimates a daily water target from your body weight and activity level. It uses a common guideline of about 33 ml of water per kilogram of body weight, then adds roughly 350 ml for every 30 minutes of exercise to replace what you sweat out. The result is shown in litres and in 250 ml cups.
For example, a 70 kg person who exercises for 30 minutes needs about 70 × 33 = 2,310 ml plus 350 ml, roughly 2.66 litres or about 11 cups a day. A heavier or more active person needs more; a lighter or sedentary one needs less.
Use it as a simple baseline for staying hydrated, then adjust up in hot weather, at altitude, during illness or when pregnant or breastfeeding. Remember all drinks count toward the total and food supplies roughly a fifth of your fluid, so treat the figure as a total fluid target rather than plain water alone.
Frequently asked questions
How much water should I drink a day?
A common guideline is about 30-35 ml per kilogram of body weight, plus extra for exercise. This calculator adds roughly 350 ml per 30 minutes of activity.
Does other drink and food count?
Yes. Food provides around 20% of daily fluid, and all drinks count toward hydration. Treat the number as a total fluid guide, not water alone.
How many glasses of water is that?
The calculator converts your target into 250 ml cups, so about 2.5 litres is roughly 10 cups. The old "8 glasses a day" rule is close for many people but does not scale with body size or activity the way this estimate does.
Should I drink more when I exercise?
Yes. Exercise increases fluid loss through sweat, so this tool adds about 350 ml per 30 minutes of activity. Increase it further for long or intense sessions, especially in heat.
Can I drink too much water?
Rarely, but drinking far more than you need in a short time can dilute blood sodium (hyponatremia), which is dangerous. Spread intake across the day and let thirst and urine colour guide you rather than forcing large amounts at once.
Does the weather change how much I need?
Yes. Hot, humid or high-altitude conditions raise fluid loss, as do illness with fever, pregnancy and breastfeeding. Use the calculator as a baseline and add more in those situations.
API — use this calculator from code
Call this calculator as a free JSON endpoint — no key required. Send the field values below as query parameters or JSON. Read the full API docs →
Endpoint
GET https://calculator.free/api/v1/water-intake/curl
curl "https://calculator.free/api/v1/water-intake/?units=metric&weight=70&activity=30"JavaScript fetch()
const r = await fetch(
"https://calculator.free/api/v1/water-intake/?" + new URLSearchParams({
"units": "metric",
"weight": "70",
"activity": "30"
}));
const data = await r.json();
console.log(data.results);Results are estimates for general guidance only, not financial, medical or tax advice.