Water Intake Calculator

Weight, sweat and the climate factor

Body weight, minutes of exercise or heavy physical work a day, and the climate you are in this month. The climate box is the one to get right, because it is the one that changes most: the same person needs a different amount in Shimla in January and in Nagpur in May.

• Mild is a hill station, or Delhi and Lucknow between November and February.
• Warm is Bengaluru or Pune for most of the year, or the north in October and March.
• Hot or humid is everything else: the plains from April to October, Chennai and Mumbai more or less always. Humidity counts as much as temperature, because sweat that will not evaporate still leaves the body.

There is no clinical consensus formula for water. Anyone selling you a precise litre figure is overselling. This page uses the common bodyweight heuristic and says so:

base     = weight in kg × 35 ml
exercise = minutes × 12 ml
total    = (base + exercise) × climate factor

Climate factors are 1.0 mild, 1.1 warm, 1.2 hot or humid. About a fifth of the total comes from food, so the “to actually drink” line is 80 percent of the headline.

Worked through for 70 kg in May

The default is a 70 kg adult, half an hour of activity, hot weather.

base     = 70 × 35  = 2,450 ml
exercise = 30 × 12  =   360 ml
total    = 2,810 × 1.2 = 3,372 ml

Total water per day	3.37 litres
To actually drink	2.70 litres
Glasses of 250 ml	13.5
In US fluid ounces	114

So about 2.7 litres from drinks, which is a one-litre bottle at the desk finished twice, plus chai, buttermilk and whatever comes with dinner. The other 0.67 litres is in the dal, the curd, the fruit and the rice.

The same person in mild weather drops to 2.81 litres total. With no exercise at all in the heat, 2.94. A 60 kg woman with no exercise in the hot months comes to 2.52 litres total, about 2 litres to drink. These are planning figures, not prescriptions, and thirst will correct most errors for a healthy adult.

Eight glasses, and what ICMR-NIN says instead

The eight glasses rule has no study behind it. It goes back to a 1945 American Food and Nutrition Board note recommending about 2.5 litres a day, which added in the next sentence that most of that is already in the food. The second sentence got lost, and the first became a rule. Eight glasses of 250 ml is 2 litres, which is about right for a 55 kg person in mild weather and about a litre short for an 80 kg man in a Delhi June.

The Indian reference, the ICMR-NIN Nutrient Requirements for Indians 2020, puts total water for adults at roughly 2.5 to 3 litres a day including what food supplies, more with heat and heavy work. That is a range for a reference body, and the calculator’s 3.37 litres for a 70 kg man in hot weather with some exercise sits just above the top of it, which is where you would expect it to.

Two Indian habits make the food share bigger than the Western 20 percent figure assumes. A plate of dal, sabzi, curd and rice is over half water by weight. And buttermilk, nimbu pani, coconut water and cut fruit are counted as food by most people while being almost entirely water. If your day includes those, the “to actually drink” line is on the high side for you.

Kidney stones and the hot belt

Ask any urologist in Jaipur, Ahmedabad, Ludhiana or Nagpur what fills the waiting room from April to July, and it is stones. The Indian stone belt runs through Rajasthan, Gujarat, Punjab, Haryana, Delhi and parts of Maharashtra and the north-east. Heavy sweating, low intake, hard water in places, and a diet heavy in salt and oxalate-rich greens; the urine gets concentrated and minerals crystallise.

The standard advice after a first stone is to drink enough to pass 2 to 2.5 litres of pale urine a day. In a hot month that means 3 litres or more going in, which is exactly what this calculator lands on for the default person. Lemon water helps a little through citrate; beer, whatever the uncle at the wedding says, does not. Outdoor workers in these states, delivery riders and traffic police included, are the group I would most like to see with a bottle in hand, and they are the group least likely to have one.

Signs you have it wrong

Urine colour is the only test you need. Pale straw is right. Dark yellow by afternoon means drink more. Completely clear all day means you are past the point of any benefit.

The 4 pm headache in an air-conditioned office is very often dehydration. AC air is dry, you have not sweated, so you have not felt thirsty, and you have had three chais and no water. Chai counts, incidentally; the caffeine’s diuretic effect is smaller than the cup. It just is not enough.

Older parents drink too little. Thirst weakens with age, and many older people cut water deliberately to avoid getting up at night. Confusion, a fall or a UTI in a parent in May is worth a question about how much they are actually drinking.

Overdoing it. Several litres in an hour or two dilutes blood sodium and can be fatal. This has happened to marathon runners and in drinking challenges. Spread it across the day. For anything over an hour in the heat, a cricket match or a construction shift, plain water is not enough either; sweat takes salt with it, and ORS, nimbu pani with a pinch of salt, or coconut water replaces what a plain bottle does not.

The wrong weight unit. 70 typed in the pounds box gives a figure for a 32 kg child. Check the units toggle if the number seems small.

The figure scales with body weight, so if you are unsure what your weight should be in the first place, the BMI calculator on the Indian cut-offs is where to look, and the calorie calculator covers the other half of what goes in each day. If the activity minutes are the input you are unsure about, the BMR calculator uses the same honesty test for activity level.