Water makes up about half to two thirds of adult body weight and is involved in almost everything the body does: carrying nutrients, regulating temperature, cushioning joints and clearing waste through the kidneys. You lose it constantly through urine, sweat, breath and stool, so it has to be replaced daily. How much is enough varies with body size, climate and activity, but expert panels have set reference levels for healthy people that are a useful starting point. This calculator shows those levels for your age, sex and life stage.
The values are Adequate Intakes (AIs): the amounts that keep most healthy people hydrated, not rigid requirements. Your own needs may be higher or lower.
How to use the water intake calculator
- Choose the group that matches you: age band for children and teens, adult man or woman, pregnancy or breastfeeding.
- Read the total water figure, which includes the water in food.
- Read the beverage figure, which is the part expected to come from drinks of all kinds.
- Adjust for conditions. Add more if you exercise hard, live somewhere hot or are unwell.
Water intake formula
Water from food ≈ total AI − beverage AI (about 20% of total)
Glasses = beverage litres × 1,000 ÷ 250
There is no equation behind the values: they are the median water intakes of healthy people in US national surveys (NHANES III), as set by the Institute of Medicine (now the National Academies) in Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate (2005). The survey data showed that around 80 percent of total water comes from drinks and 20 percent from food.
Worked example
An adult woman aged 30.
- Total water AI = 2.7 L a day (about 91 fl oz).
- Beverages = 2.2 L; food provides the rest, about 0.5 L.
- In 250 mL glasses, 2.2 L is 8.8 glasses.
An adult man: total 3.7 L, beverages 3.0 L (12 glasses of 250 mL), food 0.7 L.
Adequate intakes by group
| Group | Total water (all sources) | Total beverages (incl. drinking water) |
|---|---|---|
| Girls 9–13 y | 2.1 L | 1.6 L |
| Boys 9–13 y | 2.4 L | 1.8 L |
| Girls 14–18 y | 2.3 L | 1.8 L |
| Boys 14–18 y | 3.3 L | 2.6 L |
| Women 19+ | 2.7 L | 2.2 L |
| Men 19+ | 3.7 L | 3.0 L |
| Pregnant women | 3.0 L | 2.3 L |
| Breastfeeding women | 3.8 L | 3.1 L |
What changes your needs
- Heat and humidity. Sweat losses can be several litres a day in hot conditions.
- Exercise. Replace fluids lost in sweat; weighing yourself before and after a session shows how much was lost.
- Illness. Fever, vomiting and diarrhoea increase losses and may require oral rehydration.
- Diet. Fruit, vegetables and soup supply a lot of water; salty or high-protein diets can raise needs.
- Pregnancy and breastfeeding. Higher AIs reflect added fluid needs for the baby and milk production.
Signs of good hydration
Most healthy adults can rely on thirst and on urine colour: pale straw yellow is typical when well hydrated, and dark yellow suggests you need more. Consistently very dark urine, dizziness or little urine are signs to drink more or seek medical advice. People with heart failure, kidney disease or on diuretics may have an individual fluid allowance from their clinician that overrides these values. See also the Calorie Calculator and TDEE Calculator for nutrition planning and the Calories Burned Calculator for activity.
This page is for information only and is not medical advice. If you have a heart condition, take medicines that affect heart rate, or are new to exercise, check with a doctor before starting a training plan.
Sources
- Institute of Medicine. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. National Academies Press, 2005.
- US National Academies of Sciences, Engineering, and Medicine. Dietary Reference Intakes tables: Elements (Total Water and Macronutrients).
Frequently asked questions
How much water should I drink a day?
The US National Academies set an Adequate Intake (AI) of about 3.7 litres of total water a day for men and 2.7 litres for women aged 19 and over. That includes water from all drinks and from food. About 3.0 L for men and 2.2 L for women is expected to come from beverages.
Is the “8 glasses a day” rule right?
Eight 250 mL glasses is 2 L, which is close to the beverage figure for women but below the figure for men. There is no scientific basis for one number for everyone. The AI values are levels that keep most healthy people well hydrated, not strict requirements.
Do coffee and tea count?
Yes. Caffeinated drinks contribute to fluid intake in normal amounts, and so do milk, juice and soup. Water is not the only source, and the AI includes all beverages.
When do I need more?
Hot weather, high altitude, exercise, fever, vomiting, diarrhoea, pregnancy and breastfeeding all raise needs. Thirst is a reliable guide for most healthy adults, and pale yellow urine is a rough check. People with kidney, heart or other conditions may have fluid limits and should follow medical advice.
Can I drink too much water?
Rarely, but it can happen. Drinking very large amounts of water in a short time can dilute blood sodium (hyponatraemia), which is dangerous. Drink according to thirst, spread through the day, rather than forcing large volumes.