How much water should I drink a day by weight?

The clinical rule of thumb is 30–35 ml per kg daily — about 2.3 L of total fluid for a 70 kg adult, before exercise and climate. Since food covers roughly a fifth, the poured amount is nearer 1.8–2.1 L.

Is the 8-glasses-a-day rule true?

It is a memorable coincidence, not evidence — eight 250 ml glasses is 2 L, which happens to land near the answer for a mid-sized adult. Weight-based calculation explains why a 50 kg and a 100 kg person should not share a target.

How much extra water does exercise require?

Roughly 350 ml per 30 minutes of moderate activity, more in heat or heavy sweat. For sessions beyond an hour, sports guidance adds electrolytes; for ordinary workouts, water and thirst are enough.

Can you drink too much water?

Yes — flushing litres far beyond need, especially rapidly during endurance events, risks hyponatremia, a dangerous sodium dilution. The calculator's targets sit well inside safe territory; the lesson is that more is not automatically better.

Do coffee, tea and food count toward hydration?

All of them. Caffeinated drinks hydrate despite the mild diuretic effect, and food — fruit, vegetables, soups — typically supplies about 20% of daily fluid, which is exactly the offset this calculator applies.

Water Intake Calculator

A daily water target from your weight, exercise and climate — shown in litres, glasses and bottles, with the share food already covers subtracted.

The Water Intake Calculator runs entirely in your browser. Weight and activity inputs are processed on your device and never uploaded.

Open the TDEE Calculator

About Water Intake Calculator

Water Intake Calculator starts from the guideline clinical references actually use — roughly 30 to 35 millilitres per kilogram of body weight per day; this tool uses the 33 ml midpoint — instead of the folklore eight glasses that fits nobody in particular. Exercise adds about 350 ml per half hour of activity, hot or humid climates add half a litre, and pregnancy and breastfeeding add the officially recommended 300 and 700 ml supplements. Then comes the honest part most calculators skip: about a fifth of daily fluid arrives in food — fruit, vegetables, soups, yoghurt — so the headline number here is what you actually need to pour, which is why it looks lower than cruder tools. It is translated into 250 ml glasses and 500 ml bottles, with an hourly sipping rhythm that ends two hours before bed.

Features

Weight-based baseline at 33 ml/kg, in kg or lb
Exercise supplement of 350 ml per 30 minutes
Hot-climate and pregnancy or breastfeeding adjustments
Food-water offset (≈20%) so the target is drinkable, not inflated
Results in litres, US fluid ounces, glasses and bottles
Hourly sipping schedule from waking to two hours before bed
Full breakdown of where the number comes from
Everything computed on your device

How to use the Water Intake Calculator

Enter your weight in kg or lb
Add daily exercise minutes and pick your climate
Toggle pregnancy or breastfeeding if they apply
Read the drink target and the sipping rhythm

Example

Input

70 kg · 30 min exercise · temperate

Output

Drink ≈ 2.1 L — about 9 glasses or 4.3 bottles

Total need 2.66 L, of which food typically covers a fifth.

Common errors & troubleshooting

The target is lower than other calculators give. — Most tools report total fluid need and let you assume it must all be drunk. This one subtracts the ~20% food typically contributes, because the useful number is what goes in the glass.
Coffee and tea were left out of the count. — They count. The diuretic effect of normal caffeine intake is far too small to cancel the fluid — a mug of tea is roughly a mug of water for hydration purposes.
Following the schedule on a long run felt wrong. — Endurance exercise has its own rules — drink to thirst, and consider electrolytes past the hour mark. The daily schedule is for ordinary days, not race days.
Urine colour disagrees with the calculator. — Trust the body. Pale straw means hydration is fine regardless of millilitre counts; dark yellow means drink more even if you hit the number. The target is a plan, urine colour is the feedback loop.

Frequently asked questions

How much water should I drink a day by weight?: The clinical rule of thumb is 30–35 ml per kg daily — about 2.3 L of total fluid for a 70 kg adult, before exercise and climate. Since food covers roughly a fifth, the poured amount is nearer 1.8–2.1 L.
Is the 8-glasses-a-day rule true?: It is a memorable coincidence, not evidence — eight 250 ml glasses is 2 L, which happens to land near the answer for a mid-sized adult. Weight-based calculation explains why a 50 kg and a 100 kg person should not share a target.
How much extra water does exercise require?: Roughly 350 ml per 30 minutes of moderate activity, more in heat or heavy sweat. For sessions beyond an hour, sports guidance adds electrolytes; for ordinary workouts, water and thirst are enough.
Can you drink too much water?: Yes — flushing litres far beyond need, especially rapidly during endurance events, risks hyponatremia, a dangerous sodium dilution. The calculator's targets sit well inside safe territory; the lesson is that more is not automatically better.
Do coffee, tea and food count toward hydration?: All of them. Caffeinated drinks hydrate despite the mild diuretic effect, and food — fruit, vegetables, soups — typically supplies about 20% of daily fluid, which is exactly the offset this calculator applies.

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