What this calculator works out
This calculator multiplies your body weight by a millilitres-per-kilogram rate, adds an allowance for exercise, and returns a daily fluid target in litres.
It is the simplest of the two hydration tools on this site. If you want a figure that accounts for age, sex, weather and what you have already drunk, use the Water Intake Calculator instead.
Enter your details
Before you use the result
- Around 30 to 35 ml per kilogram of body weight is the commonly cited range for adults in a temperate climate.
- The exercise allowance should reflect fluid lost through sweat. Roughly 500 ml per hour of moderate exercise is a reasonable starting figure, more in heat.
- This is total fluid from all sources, not water alone. Tea, coffee, milk, squash and the water in food all count.
- Fluid needs change with illness, medication, pregnancy, breastfeeding and several medical conditions.
Healthy kidneys regulate fluid balance efficiently, and thirst reliably prompts drinking before dehydration becomes significant. Calculated targets are most useful for people who routinely ignore thirst, work in heat, or have been advised to drink a specific amount.
How this calculator works
One multiplication and an addition:
Daily fluid = ((weight in kg × ml per kg) + exercise allowance) ÷ 1000The rate is an editable field because published figures differ, and because clinicians sometimes set a specific target — people with kidney stones are often advised to drink considerably more, and those with heart or kidney failure may be advised to restrict fluid.
Worked example: 75 kg with exercise
Using the default figures — 75 kg at 30 ml per kg, plus a 500 ml exercise allowance:
- Base: 75 × 30 = 2,250 ml
- Plus exercise: 2,750 ml
- Daily fluid: 2.75 litres
That is total intake from everything, not 2.75 litres of water on top of your normal drinks. NHS guidance suggests six to eight glasses a day, which lands in a similar range once food and other drinks are counted. Treating a calculated figure as additional to what you already drink is the most common way people end up drinking considerably more than they need.
Common mistakes
- Treating the target as water only. All drinks and the water in food count towards it.
- Adding it on top of normal drinking. It is the total, not a supplement.
- Ignoring a fluid restriction. If a clinician has set a limit, it overrides any general calculation.
- Forcing fluid when not thirsty. Drinking to a number rather than to thirst is unnecessary for most people.
- Using the same figure in very hot weather. Losses rise substantially and so should intake.
Frequently asked questions
Do tea and coffee count?
Yes. NHS guidance is explicit that tea and coffee count towards fluid intake. The mild diuretic effect of caffeine at ordinary intakes does not outweigh the fluid in the drink itself. Milk, squash, soft drinks and the water in fruit, vegetables and cooked food all contribute too.
How do I know if I am drinking enough?
Urine colour is the simplest practical guide: pale straw suggests adequate hydration, darker suggests drinking more. Feeling thirsty, having a dry mouth, headaches or passing urine infrequently all point the same way. For most healthy adults, drinking when thirsty is sufficient.
Can you drink too much?
Yes, though it is uncommon. Drinking very large volumes quickly can dilute blood sodium, a condition called hyponatraemia, which is serious. It has occurred in endurance athletes drinking heavily during long events and in some other circumstances. Spreading fluid across the day rather than consuming large amounts at once avoids it.
Should older adults drink more?
Older adults are more prone to dehydration because the thirst response weakens with age and some medications increase fluid loss. Drinking to a routine rather than waiting for thirst is often sensible. The Water Intake Calculator applies an age adjustment; this simpler tool does not.
Is what I enter stored?
No. Weight and activity figures are processed in your browser and never transmitted or retained.
Related tools
References
- NHS — guidance on fluid intake, dehydration and what counts as a drink
- GOV.UK — Scientific Advisory Committee on Nutrition advice on water and hydration
Sources are checked at publication and can change — how I choose and check references.