How Much Water Should I Drink a Day? What the Guidelines Say

By HSP · Published October 5, 2026 · 5 min read · Health & Fitness

“Drink eight glasses of water a day” is one of the most repeated pieces of health advice, but it is not a precise scientific rule. How much you need depends on your size, how active you are, the climate, what you eat and your health. This guide explains what the main guidelines actually say, how to estimate a personal target and how to tell if you are drinking enough. It is general information, not medical advice.

What the official guidelines say

In its Dietary Reference Intakes for water, the U.S. National Academies (then the Institute of Medicine) set an adequate intake for healthy adults aged 19 to 30 of:

Two points are often missed. First, these figures are total water, including the water in food such as fruit, vegetables, soup and dairy. Food typically supplies around a fifth of it, so the amount you need to drink is lower. Second, an adequate intake is not a minimum that everyone must reach; it is the typical intake of healthy people. The report notes that most healthy people meet their daily needs by letting thirst guide them, with more needed during heat and physical activity.

Where the “8 glasses” idea fits

Eight 8-ounce glasses is 64 US fl oz, or about 1.9 litres. For many adults, that is a reasonable amount of drinks once you add the water from food, which is why the rule of thumb has lasted. But a large, active person in a hot climate will need more, and a small, inactive person may need less. Treat it as a starting point, not a target that fits everyone.

A personal estimate based on body weight

A common way to estimate daily fluid needs is about 30 to 35 ml per kilogram of body weight. Our water intake calculator uses 33 ml per kg as a base, then adds:

• 350 ml for every 30 minutes of exercise, to replace sweat losses.
• 0.5 litres for hot or humid climates.

Example: a 70 kg person needs about 70 × 33 = 2,310 ml (2.3 litres) on a rest day. With 30 minutes of exercise, that rises to about 2.66 litres, or roughly 90 US fl oz. On a hot day, add another half litre. This is an estimate of total fluids, so drinks such as milk, tea and juice count too.

Things that increase your needs

• Exercise: sweat losses vary widely; weighing yourself before and after a long workout is one way to see how much you lose.
• Heat, humidity and altitude: you lose more water through sweat and breathing.
• Illness: fever, vomiting and diarrhoea increase losses; follow medical advice, which may include oral rehydration solutions.
• Pregnancy and breastfeeding: the National Academies set higher adequate intakes for these stages.
• High-protein or high-fibre diets may also increase fluid needs.

Do tea, coffee and food count?

Yes. All drinks contribute to your total fluid intake. Caffeinated drinks have a mild diuretic effect, but for people who drink them regularly the fluid in the drink generally outweighs this. Water-rich foods such as cucumber, watermelon, oranges, yoghurt and soups also help. Water is still the best everyday choice because it has no calories or sugar; sugary drinks add calories that are easy to overlook, which matters if you are trying to manage your weight (see how many calories to lose weight).

How to tell if you are drinking enough

• Thirst: for most healthy people, drinking when thirsty and with meals is enough.
• Urine colour: pale yellow usually means you are well hydrated; dark yellow can be a sign to drink more.
• Signs of dehydration include headache, tiredness, dizziness and passing small amounts of dark urine.

Older adults can feel less thirsty, so it can help to drink regularly through the day. Babies, children, older people and people with some health conditions are more vulnerable to dehydration.

Can you drink too much water?

It is rare, but drinking very large amounts in a short time can dilute the sodium in your blood, a condition called hyponatraemia. It has been seen in endurance athletes who drink far more than they sweat. There is no benefit in forcing down water well beyond your needs. People with kidney, heart or liver conditions may be told to limit fluids, so follow your doctor’s advice if that applies to you.

Simple habits that help

• Keep a refillable bottle with you and note how many times you refill it.
• Have a glass of water with every meal.
• Drink before, during and after exercise.
• Flavour water with lemon, mint or cucumber if plain water feels boring.
• Eat plenty of fruit and vegetables, which also supply water.

Hydration is one part of overall health. To set calorie targets, try the calorie calculator, and to check whether your weight is in a healthy range, use the BMI calculator alongside our guide to what BMI means.