What the number means
The result is the amount of fluid to drink, counting water, tea, coffee, milk, juice and soup. Food supplies the rest of your water, typically about a fifth of the total.
It is a rule of thumb, not a clinical standard. Nobody has a single correct number, because water needs depend on sweat, diet, health and climate. To keep the estimate honest, here are the official intake figures from the US National Academies, which are based on what healthy people typically drink:
| Group | Total water a day | From drinks a day |
|---|---|---|
| Men, 19 and over | 3.7 L | 3.0 L (about 13 cups) |
| Women, 19 and over | 2.7 L | 2.2 L (about 9 cups) |
| Pregnancy | 3.0 L | 2.3 L |
| Breastfeeding | 3.8 L | 3.1 L |
Your result should land in that neighbourhood on a normal day and above it when you exercise or the weather is hot.
The formula
Fluid (ml) = weight (kg) × 35 + exercise (minutes) × 12 + 500 if the weather is hotOne glass is counted as 250 ml. One US fluid ounce is 29.57 ml.
The 35 ml per kilogram baseline is a long-used clinical rule of thumb for adult fluid needs. The 12 ml per minute of exercise is our allowance for moderate effort. It works out at 720 ml an hour, towards the low end of typical sweat rates of roughly 0.5 to 2 litres an hour. The 500 ml for hot weather is a flat allowance, so heavy sweaters should add more.
Worked example
A man weighing 80 kg who runs for 45 minutes on a hot day.
- Baseline: 80 × 35 = 2,800 ml
- Exercise: 45 × 12 = 540 ml
- Hot weather: + 500 ml
- Total = 3,840 ml, or 3.8 litres (about 15 glasses, 130 US fl oz)
That is more than the usual 3.0 litres from drinks for men, which makes sense for a hard run in the heat.
Are you drinking enough?
- Thirst is a reliable signal for most healthy adults. Drink when you notice it. The thirst signal weakens with age, so older adults may need to drink on a schedule.
- Urine colour is a practical check. Pale yellow suggests good hydration, dark yellow suggests you need more. Some vitamins and foods change colour, so do not rely on it alone.
- Sweat loss is easy to measure. Weigh yourself before and after a hard session. Each kilogram lost is about a litre of fluid to replace.
You can also drink too much. Drinking very large volumes over a short time, which has happened in long endurance events, can dilute blood sodium to a dangerous level. Sip steadily rather than forcing fluid. If you take diuretics or have kidney, heart or liver disease, follow your doctor’s fluid advice instead of this calculator.
What raises your needs
- Hot or humid weather, and heated indoor air in winter.
- Altitude, which increases breathing and fluid loss.
- Fever, vomiting and diarrhoea.
- Pregnancy and breastfeeding (see the table above).
- Eating a lot of salty food or fibre, or drinking alcohol.
Litres, glasses and bottles
Use this table to turn your result into containers you actually own.
| Litres | 250 ml glasses | 500 ml bottles | US fluid ounces |
|---|---|---|---|
| 1.5 L | 6 | 3 | 51 |
| 2.0 L | 8 | 4 | 68 |
| 2.5 L | 10 | 5 | 85 |
| 3.0 L | 12 | 6 | 101 |
| 3.5 L | 14 | 7 | 118 |
| 4.0 L | 16 | 8 | 135 |
Spread it out instead of catching up at night: a glass when you wake, one with each meal, and a bottle at your desk or in your bag for the rest. Drinking a large amount late in the evening mostly means a disturbed night.
These results are general estimates for healthy adults, not medical advice. See the disclaimer for who should check with a professional first.