Water Intake Calculator

This calculator estimates a daily drinking target from your body weight, then adjusts it for activity and climate. For a 70 kg person who is moderately active in a temperate climate, it gives 2.5 litres a day, about 10 glasses of 250 ml.

How the target is calculated

Scaling by body weight means larger people get a larger target; the adjustments reflect the extra water lost through sweat during exercise and in heat.

Worked examples

• 70 kg, sedentary, temperate: 70 × 0.033 = 2.31 L (about 9 glasses).
• 70 kg, moderate activity, temperate: 2.31 × 1.1 = 2.54 L (about 10 glasses).
• 70 kg, active, hot climate: 2.31 × 1.2 × 1.15 = 3.19 L (about 13 glasses).

Daily water by body weight

Litres per day:

What official guidelines say

Health authorities set an adequate intake for total water, which includes water from food as well as drinks:

The National Academies found that about 80% of total water comes from drinks and 20% from food. On that basis, the U.S. figures mean roughly 2.2 litres a day from drinks for women and 3.0 litres for men. For average-sized adults the calculator's results are broadly in line with these figures. The different numbers reflect different populations and methods; none is a precise requirement for an individual.

Does it have to be plain water?

No. Milk, tea, coffee, juice and other drinks all count towards your fluid intake, and so does water in food. Fruit and vegetables such as watermelon, strawberries, cantaloupe, cabbage and squash are mostly water. Plain water is still the best everyday choice because it has no calories or sugar.

When you need more water

• Exercise: drink before, during and after. Sweat losses vary widely between people and conditions, so long or hard sessions may need much more.
• Hot or humid weather, and high altitude: sweating and breathing increase water loss.
• Illness: fever, vomiting and diarrhoea cause fluid loss. Oral rehydration solutions can help; seek medical advice if you can't keep fluids down.
• Pregnancy and breastfeeding may increase the fluids you need.

How to tell if you're drinking enough

• You rarely feel thirsty.
• Your urine is colourless or light yellow.
• Signs you may need more: dark urine, headaches, tiredness and reduced exercise performance; severe dehydration can cause dizziness and confusion.

The National Academies concluded that most healthy people meet their daily needs through normal drinking habits and by letting thirst guide them.

Can you drink too much water?

Yes, though it is uncommon. Drinking very large amounts in a short time, which mainly happens during long endurance events, can dilute the sodium in your blood (hyponatraemia), which can be dangerous. There's no need to force down far more than you're thirsty for. People with heart failure or kidney disease may be told to limit fluids; follow your doctor's advice.

Is 8 glasses a day right?

Eight 250 ml glasses is 2 litres. It's an easy goal to remember and is close to many people's needs, but some need less and others more. The calculator gives 9 glasses for a sedentary 70 kg adult and 13 for the same person active in a hot climate.

This calculator is for healthy adults. Children, older adults, pregnant or breastfeeding women and people with medical conditions may have different needs; ask a doctor if you're unsure. For other daily targets, see the calorie calculator and the protein intake calculator.

Sources

• National Academies of Sciences (2004): Dietary Reference Intakes for water, potassium, sodium, chloride and sulfate
• European Food Safety Authority (2010): dietary reference values for water
• Nordic Nutrition Recommendations 2023: fluid and water balance
• Mayo Clinic: Water, how much should you drink every day?