Water intake calculator
There is no single official UK fluid target, so this calculator starts from the NHS guideline of 6 to 8 glasses a day, about 1.5 to 2 litres, then applies a common rule-of-thumb formula based on weight and activity to turn that anchor into a daily figure. Individual needs vary, and results are general information, not medical advice.
How is the water intake figure worked out?
The NHS guideline is the starting point: 6 to 8 glasses of fluid a day, which works out at roughly 1.5 to 2 litres.
That NHS figure is the same for most adults regardless of size.
To personalise it, this tool applies a common rule of thumb of about 33 millilitres of fluid per kilogram of body weight, then adds a fixed amount for activity level. This rule of thumb is not an official NHS figure, and it is shown only as a guide.
Moderate activity adds around 350 ml and heavy activity around 700 ml in this tool, to allow for fluid lost through sweat.
The figure shown is never allowed to fall below the NHS lower anchor of about 1.5 litres.
| Element | Amount |
|---|---|
| NHS baseline | 1.5 to 2 litres |
| Weight adjustment | About 33 ml per kg |
| Moderate activity | Plus 350 ml |
| Active | Plus 700 ml |
Fluid also comes from food, so this figure sits on top of a normal diet.
This is general information rather than medical advice, and a doctor should be consulted about personal hydration needs, particularly in hot weather, illness, pregnancy or breastfeeding.
What would the target be for an example weight and activity level?
Take a weight of 70 kg with a moderate activity level.
The weight part gives 70 times 33, which is 2,310 ml, and moderate activity adds 350 ml, for a total of 2,660 ml.
That is 2.7 litres, or about 13 glasses of 200 ml across the day.
A lighter or less active person would sit closer to the NHS 1.5 litre baseline, and individual needs vary.
Common questions
How is the water intake figure worked out?
This tool starts from the NHS figure of 6 to 8 glasses of fluid a day, roughly 1.5 to 2 litres, then applies a common rule of thumb of about 33 ml of fluid per kilogram of body weight, plus a fixed amount for activity level. The rule-of-thumb part is not an official NHS figure, and individual needs vary. The result is never shown below the NHS lower anchor of 1.5 litres.
What would the target be for an example weight and activity level?
Take a weight of 70 kg with a moderate activity level. The weight-based part gives 70 times 33, and the moderate activity adjustment adds 350 ml, for a total of 2,660 ml. That is 2.7 litres, or about 13 glasses of 200 ml across the day. A lighter or less active person would sit closer to the NHS 1.5 litre baseline.
How much water should I drink a day?
NHS advice is 6 to 8 glasses of fluid a day, which is roughly 1.5 to 2 litres. That covers water, lower-fat milk and sugar-free drinks, including tea and coffee. This tool adjusts that figure upwards slightly for body weight and activity as a guide, and individual needs vary.
Do tea and coffee count towards fluid?
Yes. The NHS counts tea, coffee and other drinks towards daily fluid, though water, lower-fat milk and sugar-free drinks are the healthier choices. Alcohol does not count, because it makes the body pass more fluid.
Should I drink more when exercising?
Fluid is lost through sweat, so more is generally needed when active or in hot weather. This calculator adds a rough amount for moderate and heavy activity on top of the baseline. This is general information only, and a doctor should be consulted about personal hydration needs.
Is the weight-based figure official NHS advice?
No. The NHS anchor is 6 to 8 glasses a day for everyone. The weight and activity adjustment used here is a common rule of thumb of about 33 ml per kilogram, shown only as a guide around that anchor, and individual needs vary.
Can you drink too much water?
Rarely, drinking far more than needed in a short time can dilute the salts in the blood, which the NHS notes can be dangerous. For most people the more common issue is drinking too little. This is general information, not medical advice, and a doctor should be consulted about any concern.
Sources & Rate Verification
Rates verified against official statutory guidance on 6 September 2026.