How much water should I drink a day?

About 35 ml per kg of body weight is a common starting estimate, plus extra for exercise and hot weather. This calculator applies both adjustments.

Does coffee or tea count?

Yes. Caffeinated drinks in normal amounts contribute to daily fluid intake; the mild diuretic effect does not offset the volume.

Is eight glasses a day correct?

It is a rough rule of thumb, about 1.9 litres, which is too little for many active or larger people and more than enough for some others.

How do I know if I am drinking enough?

Pale-yellow urine and the absence of persistent thirst are the usual practical signs for healthy adults.

Water Intake Calculator

Estimate daily water intake from body weight, exercise time and climate.

Enter your details and your result will appear here.

What Is Water Intake?

This calculator estimates daily water needs from your body weight, how long you exercise and the climate you are in. It reports the result in litres and fluid ounces, and separates total water from what you drink.

Understanding Your Water Intake Result

Water needs scale with body size and with the losses you create: sweat during exercise, and evaporation in heat. A widely used starting point is about 35 ml per kilogram of body weight a day. Roughly 20% of total water intake typically comes from food, particularly fruit, vegetables and soups, which is why the drinking target is lower than the total. The "eight glasses a day" rule is a rounded convenience, not a research finding.

How Is Water Intake Calculated?

Water (ml) = (weight (kg) x 35 + exercise minutes / 30 x 350) x climate factor

Methodology

The baseline is body weight in kilograms multiplied by 35 ml. Exercise adds about 350 ml for every 30 minutes of training. The total is then multiplied by a climate factor of 1.0, 1.1 or 1.2 for temperate, warm or hot and humid conditions. Fluid ounces use 29.5735 ml per ounce.

Worked Example

A 70 kg (154 lb) person who trains for 45 minutes in a warm climate gets a 2,450 ml baseline, plus 525 ml for exercise, multiplied by 1.1, which is about 3.3 litres or 111 fl oz of total water a day. Around 89 fl oz of that would come from drinks.

Important Limitations

Individual needs vary widely with sweat rate, sodium losses, altitude, illness and pregnancy, and this formula captures none of that. Thirst and pale-yellow urine remain the most practical day-to-day guides for healthy adults. Some conditions, including heart failure and advanced kidney disease, require deliberate fluid restriction, and drinking far in excess of losses can cause hyponatraemia. Follow clinical advice over any calculator if you have been given a fluid target.

Frequently Asked Questions

How much water should I drink a day?: About 35 ml per kg of body weight is a common starting estimate, plus extra for exercise and hot weather. This calculator applies both adjustments.
Does coffee or tea count?: Yes. Caffeinated drinks in normal amounts contribute to daily fluid intake; the mild diuretic effect does not offset the volume.
Is eight glasses a day correct?: It is a rough rule of thumb, about 1.9 litres, which is too little for many active or larger people and more than enough for some others.
How do I know if I am drinking enough?: Pale-yellow urine and the absence of persistent thirst are the usual practical signs for healthy adults.