Enter your weight, how long you exercise and whether the weather is hot. The calculator estimates how much water to drink in a day, in litres and glasses.
Results are estimates for planning. Check the figures before you make a financial or health decision.
How the Water intake works
The estimate adds three parts together:
- Baseline: 33 ml for each kilogram of body weight. Pounds are converted to kilograms first.
- Exercise: 350 ml for every 30 minutes of exercise.
- Hot weather: an extra 500 ml if it is hot or humid.
The total is shown in litres, as glasses of 250 ml and as US fluid ounces, with the three parts listed separately.
About a fifth of the water you take in comes from food, so you may need a little less from drinks. The NHS gives a general guide of about 6 to 8 glasses a day. This tool will often give a higher figure than that, because it counts total water including food and adjusts for weight and activity.
Needs differ in pregnancy, breastfeeding, illness and with some medical conditions or medicines, and the tool does not allow for these. It accepts weights of 20 to 300 kg.
Worked example
Using the default values in the calculator above:
| Input | Value |
|---|---|
| Weight unit | Kilograms |
| Weight | 68 |
| Exercise each day | 30 min |
| Weather | Mild or cool |
Water to drink each day: 2.6 litres
| Glasses of 250 ml | 10 |
|---|---|
| US fluid ounces | 88 |
| Baseline for your weight | 2,244 ml |
| Extra for exercise | 350 ml |
| Extra for hot weather | 0 ml |
Tips and common mistakes
Use the result as a rough target, not a strict rule. Thirst and pale urine are good everyday signs. Tea, coffee, milk and juice also count towards your total. Drink more in hot weather, when you are ill with a fever, vomiting or diarrhoea, and during long exercise. Speak to a GP if you have kidney, heart or other conditions that affect fluid intake. General information, not medical advice.
Frequently asked questions
How much water should I drink a day?
It depends on weight, activity and weather. The NHS suggests about 6 to 8 glasses a day as a general guide, and this tool gives a personalised estimate.
Does tea or coffee count?
Yes. Most drinks contribute to your fluid intake, as does water from food, which provides roughly a fifth of the total.
Can you drink too much water?
Yes, though it is uncommon. Drinking very large amounts in a short time can be harmful, so spread intake through the day and follow medical advice if you have a condition.