How Much Water Do You Need? Guidelines and a Simple Estimate

What the guidelines say

Official figures are usually given as total water, meaning everything you take in from drinks and from food. They are "adequate intakes", the amounts that cover most healthy people, rather than strict requirements.

The two organisations set different figures, which shows how much uncertainty surrounds any single answer. Both assume that food provides roughly a fifth of total water intake, since fruit, vegetables, soup, bread and many other foods contain a good deal of water.

The eight-glasses rule

Eight 8-ounce glasses is 64 US fluid ounces, which is about 1.9 litres. That sits near the lower end of the guideline numbers for drinks, so it is a reasonable ballpark for some people but not a target that fits everyone. Tea, coffee, milk, juice and other beverages all count toward fluid intake, and so does the water in food.

Your body also tells you when it needs more. Thirst is a reliable cue for most healthy adults, and pale-yellow urine is a rough sign of adequate hydration, while dark yellow suggests you should drink more. Older adults and people who are ill can have a weaker thirst response, so they may need to drink on a schedule.

A weight-based estimate

A simple rule of thumb often used to estimate adult fluid needs is 30 to 35 millilitres per kilogram of body weight per day. Bigger bodies need more water in the same way they burn more energy.

daily water (mL) = weight (kg) × 30 to 35A rough estimate of total water from drinks and food.

Worked example, 70 kg: 70 × 30 = 2,100 mL and 70 × 35 = 2,450 mL, a range of about 2.1 to 2.5 litres of total water. If roughly 20% comes from food, the drinking share is 80%: 2,100 × 0.8 = 1,680 mL and 2,450 × 0.8 = 1,960 mL. That is about 1.7 to 2.0 litres, or 7 to 8 US cups, because a US cup is about 237 mL.

For 80 kg the same method gives 2,400 to 2,800 mL in total. The water intake calculator adds age, activity and other factors, and the total body water calculator estimates how much water your body holds, which is a different question from how much you drink.

What raises or lowers your needs

• Heat and humidity. Sweating loses water, especially during hot weather or hard exercise.
• Exercise. Longer or harder sessions increase losses. Drinking before, during and after is sensible.
• Altitude. Breathing faster in thin air loses more moisture.
• Illness. Fever, vomiting and diarrhoea can cause rapid fluid loss, so extra fluids may be needed.
• Pregnancy and breastfeeding. Guidelines set higher figures for both.
• Diet. Salty meals can increase thirst, and a diet rich in fruit and vegetables supplies more water from food.

Can you drink too much?

Yes, although it is uncommon. Drinking very large amounts in a short time can dilute the sodium in the blood, a condition called hyponatremia, which can be dangerous. Spreading intake through the day and drinking to thirst avoids this for most people.

People with kidney, heart or liver conditions, or who take certain medicines, may be told to limit fluids. Their advice from a doctor overrides any general figure here.

This article is general education and not medical advice. For personal advice about hydration, especially if you have a medical condition, are pregnant, or train for long periods in the heat, ask a doctor or registered dietitian.