Water Intake Calculator

How the water intake calculator works

There is no single number that suits everyone, so the calculator starts from body weight and then adds for sweat:

• Base: about 30–35 ml of fluid per kg of body weight per day
• Exercise: about 350 ml extra for every 30 minutes of activity
• Heat: extra on hot or humid days, when you sweat more even at rest

Example: a 60 kg adult who walks for 30 minutes a day needs roughly 1.8–2.1 litres for the base plus about 0.35 litres for the walk — around 2.2–2.5 litres of drinks a day, more during an Indian summer.

Typical daily drinking targets by weight

These figures cover drinks only. Food supplies roughly a fifth of daily water, more if you eat plenty of dal, curd, fruit and vegetables. For comparison, the European Food Safety Authority sets an adequate total water intake (food plus drinks) of about 2.0 litres a day for women and 2.5 litres for men in a temperate climate.

Hot climates, Indian summers and fasting days

In high heat and humidity you can lose well over a litre of sweat in a few hours of outdoor work or travel. Sip through the day rather than drinking a lot at once, and include salty fluids such as chaas, nimbu pani with a pinch of salt or homemade soups when you sweat heavily. On fasting days, drink more in the hours you can eat and drink. If you have diarrhoea or vomiting, oral rehydration solution (ORS) replaces salts as well as water.

Women, men, older adults, athletes and pregnancy

• Women and men: men usually need more simply because they tend to be heavier; the calculation is by weight, so the same formula works for both.
• Pregnancy and breastfeeding: EFSA advises about 300 ml a day extra in pregnancy and around 600–700 ml extra while breastfeeding.
• Older adults often feel less thirsty and may drink less to avoid night-time toilet trips. A glass with every meal and medicine dose helps; spread drinks earlier in the day.
• Athletes and active workers: weigh yourself before and after a long session. Each kilo lost is about a litre of sweat; sports bodies suggest replacing it with roughly 1.25–1.5 litres over the next few hours.
• Children need less in total but are more vulnerable in heat — offer water regularly rather than waiting for them to ask.

Signs you need more — and the limits of the estimate

Dark yellow urine, needing the toilet rarely, headaches, tiredness and dizziness can all mean you are behind on fluids; pale straw-coloured urine usually means you are fine. The calculator cannot account for fever, illness, medicines such as diuretics, or very high-altitude travel. Drinking far more than you need is also possible: several litres within a few hours, especially during long endurance events, can dilute blood sodium (hyponatraemia), which is dangerous.

When to talk to a doctor

• you have heart failure or kidney disease, or have been told to limit fluids — follow that advice over any calculator
• you are always thirsty and passing a lot of urine, which can be a sign of diabetes
• you feel confused, faint, or have passed no urine for 8 hours or more
• a child or older person has diarrhoea or vomiting and cannot keep fluids down

Related tools: the calorie calculator, the protein calculator and the healthy juice and tea finder for drinks without added sugar.

Common questions

Sources: EFSA: Dietary reference values for water (2010) · National Academies: Dietary Reference Intakes for water and electrolytes (2005) · ACSM position stand: Exercise and fluid replacement (2007) · NHS: Water, drinks and your health · WHO: Diarrhoeal disease and oral rehydration

Checked against the sources listed. This is general health information, not a diagnosis or personal medical advice — please speak to your doctor about your own situation.