How to use the water Intake Calculator
- Choose metric or imperial units and enter your body weight.
- Enter how many minutes of exercise you do on a typical day.
- Choose your climate. Pick hot or humid if you are in a hot season or spend time in heat.
- The result updates instantly as you type. There is no button to press.
- Use Copy result to copy the figures, or Copy link to share a link that reopens the water Intake Calculator with the same inputs.
How it works
How much water you need depends on your size, how much you sweat and how hot it is. A larger body has more tissue to keep hydrated, exercise loses water through sweat and breath, and heat raises sweat losses even at rest. This calculator combines those three factors into one daily estimate.
It starts with a widely used rule of thumb of about 35 ml per kilogram of body weight per day (roughly half an ounce per pound). It then adds about 350 ml for every 30 minutes of exercise and 500 ml for hot or humid conditions. The result is your total fluid target. Tea, coffee, milk, soup and water-rich foods such as fruit and vegetables all count towards it. Food typically provides about a fifth of the water we take in.
For comparison, the European Food Safety Authority suggests total water intakes of about 2.0 litres a day for women and 2.5 litres for men from all sources, and the US National Academies suggest about 2.7 and 3.7 litres. The simplest everyday check is the colour of your urine: pale straw-coloured usually means you are well hydrated. People with heart, kidney or liver conditions, and those told to restrict fluids, should follow their doctor's advice instead.
Formula
Converted with 1 US fl oz = 29.57 ml and 1 US cup = 236.6 ml. Imperial weights are converted at 1 lb = 0.4536 kg first.
Example
A person weighing 70 kg who exercises for 30 minutes a day in a temperate climate needs about 70 × 35 = 2,450 ml for body weight plus 350 ml for exercise: 2.8 litres in total, about 95 US fl oz or 11 glasses of 250 ml. On a hot day the target rises to 3.3 litres. If food supplies about 20%, that leaves roughly 2.2 litres to drink.
Frequently asked questions
Do coffee and tea count towards water intake?
Yes. Although caffeine is a mild diuretic, studies show that moderate amounts of coffee and tea still contribute to hydration overall.
Can you drink too much water?
Yes. Drinking very large amounts quickly, especially during long endurance events, can dilute sodium in the blood (hyponatremia), which can be dangerous. Spread intake through the day and use electrolyte drinks for long, sweaty exercise.
Is the '8 glasses a day' rule correct?
It is a reasonable rough guide for many adults, but needs vary. Larger, more active people and those in hot climates need more, while smaller, less active people may need less.
Disclaimer: This calculator provides general estimates for education only and is not medical advice, diagnosis or treatment. Formulas are based on population averages and may not fit your body, age, pregnancy or health conditions. Talk to a doctor, midwife or registered dietitian before making decisions about your health.