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Water Intake Calculator

Enter your weight and how long you exercise. This gives a daily water guideline; thirst, urine colour, and climate still matter.

How the estimate is built

The base figure is 35 ml per kilogram of body weight. On top of that, roughly 350 ml is added for every 30 minutesof exercise to cover sweat losses. The total is your fluid target for the day from all sources — water, tea, coffee, milk, and the water in food such as fruit and soup all count.

When to drink more

This is a starting point, not a medical prescription. Adjust to thirst and urine colour.

Worked example

A 70 kg person who does 45 minutes of exercise: base = 35 × 70 = 2,450 ml; exercise = (45 ÷ 30) × 350 = 525 ml; total ≈ 3.0 litres of fluid for the day, from all sources. Since fruit, vegetables, soup, milk, tea and coffee typically supply 20% or more of daily fluid, that might mean drinking around 2.3–2.4 litres as actual drinks.

Rough daily fluid by body weight (at rest)

Body weight≈ Daily fluidIn cups (240 ml)
50 kg / 110 lb1.8 L~7
70 kg / 154 lb2.5 L~10
90 kg / 198 lb3.2 L~13
110 kg / 243 lb3.9 L~16

Add exercise, heat, altitude, fever, and pregnancy/breastfeeding on top.

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Frequently asked questions

How much water should I drink a day?
A common guideline is about 35 ml per kilogram of body weight, plus extra for exercise, heat, and altitude. Food and other drinks also count toward your total fluid, so you rarely need to drink the whole amount as plain water.
Is the '8 glasses a day' rule right?
It is a rough average, not a rule. Eight 8-ounce glasses is about 1.9 litres, which suits a smaller, lightly active adult. Larger or more active people need more; the weight-based estimate here scales with you.
Can I drink too much water?
Rarely, but yes. Drinking very large volumes in a short time can dilute blood sodium (hyponatraemia), which is dangerous. Spread intake across the day and let thirst guide you.
How do I know if I'm hydrated?
Pale straw-coloured urine and infrequent strong thirst are good signs. Dark yellow urine, headaches, dizziness and poor concentration can indicate you need more, especially in warm weather or after exercise.
Does coffee or tea dehydrate you?
No. The mild diuretic effect of the caffeine in normal servings is more than offset by the water in the drink, so coffee and tea count toward your daily fluid. Alcohol is the exception — it genuinely increases fluid loss.
Do I need electrolytes or just water?
For everyday activity, plain water and a normal diet are fine. During prolonged or heavy sweating — long runs, hot-weather work, endurance events — replacing sodium (and to a lesser extent potassium) matters, because drinking large volumes of plain water while losing a lot of salt is what causes hyponatraemia.
Should I drink to a schedule or to thirst?
For most healthy adults, thirst is a reliable guide under normal conditions. A schedule helps if you tend to forget, if you're older (thirst sensitivity declines with age), or during structured exercise. Use the estimate below as a daily target and spread it out rather than drinking a lot at once.

Last reviewed: September 2026. Figures and formulas are checked against their published sources; see the site's data notes.