There is no single magic number
You have probably heard “drink eight glasses of water a day”. It is a handy reminder, but official bodies do not set one exact amount for everyone. Your needs depend on your size, how active you are, the weather, your diet and your health. What they do publish are reference values: amounts that healthy groups of people typically take in when they are well hydrated.
This calculator starts from those reference values (or a simple body-weight rule if you prefer), then adds extra for exercise and hot weather. Every number is labelled with where it comes from.
The official reference values
Two expert bodies have reviewed the evidence. Both give total water, which includes water in food as well as drinks.
| Group | US National Academies (IOM) | of which from drinks | EFSA (Europe) |
|---|---|---|---|
| Men | 3.7 L | 3.0 L | 2.5 L |
| Women | 2.7 L | 2.2 L | 2.0 L |
| Pregnant | 3.0 L | 2.3 L | 2.3 L |
| Breastfeeding | 3.8 L | 3.1 L | 2.7 L |
The US figures come from the Institute of Medicine’s 2004 report on water and electrolytes, which notes that about 80% of total water usually comes from drinks and 20% from food. The European Food Safety Authority’s 2010 opinion gives lower figures, and adds 300 mL a day in pregnancy and 700 mL while breastfeeding. Both apply to adults at moderate temperatures and activity levels. In the UK, the NHS suggests 6 to 8 cups or glasses of fluid a day.
The calculator shows the range between the EFSA and IOM values as your baseline, then works out the part to drink.
The body-weight rule of thumb
Many people prefer an estimate that scales with size. A common rule is:
This is a rule of thumb, not an official reference value. It gives smaller people a lower figure and larger people a higher one. If you are pregnant or breastfeeding, the calculator adds EFSA’s 0.3 L or 0.7 L on top.
Exercise and heat
Sweat losses vary hugely between people. The American College of Sports Medicine’s 2007 position stand on fluid replacement cites 0.4 to 0.8 litres an hour as a starting range during exercise, with the lower end for lighter people in cool weather and the higher end for heavier people in the heat. Its main advice is to avoid losing more than 2% of your body weight through sweat during a session. The calculator shows what 2% is for you.
For hot or humid days, the calculator adds a rule-of-thumb 0.5 to 1 litre. If you work outdoors in the heat, you may need more.
Worked examples
A woman, 65 kg, 30 minutes of exercise, mild weather. Baseline total water: 2.0 to 2.7 L. From drinks: 80% of that is 1.6 to 2.16 L. Exercise adds 0.5 h × 0.4 to 0.8 L = 0.2 to 0.4 L. Drinks: about 1.8 to 2.6 L a day, or roughly 8 to 11 cups of 8 oz.
A man, 80 kg, using the body-weight rule, 1 hour of exercise on a hot day. Baseline: 80 × 30 to 35 mL = 2.4 to 2.8 L. From drinks: 1.92 to 2.24 L. Exercise adds 0.4 to 0.8 L and heat adds 0.5 to 1.0 L. Drinks: about 2.8 to 4.0 L a day. He should try not to lose more than 1.6 kg (2% of 80 kg) in sweat during that hour.
Signs you are getting enough
- You rarely feel thirsty.
- Your urine is pale yellow, not dark.
- You do not get headaches or feel dizzy on hot or active days.
Tea, coffee, milk and other lower-sugar drinks count towards your fluid. Water-rich foods such as fruit, vegetables, soup and yoghurt help too.
When to be careful
More is not always better. Drinking very large amounts quickly can dilute the sodium in your blood (hyponatraemia), which is a real risk in long endurance events. People with kidney disease, heart failure or liver disease, and those taking diuretics, may have been told to limit fluids. Older adults can feel less thirsty and may need reminders. If any of this applies to you, follow your doctor’s advice rather than this estimate.
Next steps
Convert between litres, cups and fluid ounces with the volume converter. To estimate how much energy your body uses at rest, try the BMR calculator, and for your full daily needs the TDEE calculator.