Water Intake Calculator

Estimate how much water to drink daily based on weight, activity and climate.

Please note: For general information only. This is not medical advice — speak to a doctor or registered dietitian before changing your diet, training or medication.

What the Water Intake Calculator does

Water needs scale with body size, activity and climate. A common baseline is 30–35 ml per kilogram of body weight, plus roughly 350 ml for every 30 minutes of exercise, adjusted upward in heat.

Formula

Baseline = Weight (kg) × 0.033 litres
Exercise = (Minutes ÷ 30) × 0.35 litres
Total = (Baseline + Exercise) × Climate factor

Inputs explained

Input	Unit	Required	Notes
Units	one of 2 options	Yes	—
Weight	kg or lb	Yes	Accepts 1 or more.
Exercise per day	minutes	Yes	Accepts 0 or more, up to 600.
Climate	one of 3 options	Yes	—

How to use it

Choose Units and Climate.
Enter Weight and Exercise per day.
Select Calculate.

Worked example

A 70 kg person exercising 45 minutes a day in a warm climate.

Weight: 70 kg
Exercise: 45
Climate: Warm

(2.31 + 0.53) × 1.1 = about 3.1 litres a day.

Frequently asked questions

Is eight glasses a day a real rule?

It is a rough guideline with no strong scientific basis. Actual needs vary with body size, activity, diet and climate — which is what this calculator adjusts for.

Does coffee dehydrate you?

Not at normal intakes. The diuretic effect is mild and outweighed by the fluid in the drink, so coffee counts toward your total.

Method and sources

Method. A baseline scaled to body mass at roughly 30–35 ml per kilogram, plus an allowance for exercise duration and a multiplier for hot conditions.

Assumptions

The reader is a healthy adult with normal kidney and heart function, not on medication affecting fluid balance.
The figure describes drinking water only. Food supplies a substantial share of total water intake — commonly around a fifth to a third — which this does not count.

Limitations

The millilitres-per-kilogram baseline is a widely repeated convention, not an official recommendation. National bodies publish adequate-intake values for *total* water including food, and those are not directly comparable with a drinking-water target.
Requirements vary far more between individuals, climates and activities than any formula captures. Thirst, and urine that is pale rather than dark, remain better day-to-day guides than an arithmetic target.
Drinking substantially beyond thirst is not harmless: exercise-associated hyponatraemia arises from over-drinking, and heart, kidney or liver conditions can require deliberate fluid restriction.

Sources

Your national dietary authority's adequate-intake guidance for total water — Varies by country. What an official intake figure actually is. This calculator deliberately cites none as the basis for its per-kilogram baseline, because that convention is not drawn from one.