Water intake calculator
NHS advice is 6 to 8 glasses of fluid a day, about 1.5 to 2 litres. Enter your weight and activity level to turn that into a personal daily target in litres and glasses.
How it works
The NHS advice is the starting point: 6 to 8 glasses of fluid a day, which works out at roughly 1.5 to 2 litres. That is the same for most adults regardless of size. To make it personal, 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 how active you are. Moderate activity adds around 350 ml and heavy activity around 700 ml, to cover fluid lost through sweat. The result is never allowed to fall below the NHS lower anchor of about 1.5 litres.
Treat the number as a target to aim near, not a strict quota. Fluid also comes from food, so the figure is on top of a normal diet. The clearest sign you are drinking enough is pale straw-coloured urine. Dark urine, headaches or feeling thirsty all suggest drinking more, and you should drink more in hot weather or when unwell.
Worked example
Take someone who weighs 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. Someone lighter or less active would sit closer to the NHS 1.5 litre baseline.
Common questions
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.
Do tea and coffee count towards fluid?
Yes. The NHS counts tea, coffee and other drinks towards your daily fluid, though water, lower-fat milk and sugar-free drinks are the healthier choices. Alcohol does not count, because it makes you pass more fluid.
Should I drink more when exercising?
Yes. You lose fluid through sweat, so you need more when active or in hot weather. This calculator adds a rough amount for moderate and heavy activity on top of the baseline, but listen to thirst and drink more if your urine is dark.
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 personal guide around that anchor.
Can you drink too much water?
Very rarely, drinking far more than needed in a short time can dilute the salts in your blood, which is dangerous. For almost everyone the practical risk is drinking too little. Spread your intake through the day rather than all at once.