The base formula
The standard starting point: 35ml of water per kilogram of body weight per day. A 70kg person needs about 2,450ml — roughly 2.5 litres — before accounting for exercise or weather.
This is the European Food Safety Authority baseline. Indian dietitians commonly recommend 2.5–3.5 litres for most adults given the climate.
Why Indians need more water than the “8 glasses” rule
The “8 glasses a day” (2 litres) advice comes from temperate climates. In most Indian cities, especially from March to June, ambient temperatures of 35–42°C increase sweat loss significantly. Someone doing outdoor work in Chennai in May needs closer to 4–5 litres than 2.
The calculator adds 250–750ml for climate on top of the body-weight base.
Exercise adjustment
You lose roughly 0.5–1 litre of water per hour of moderate exercise. The calculator adds 350ml for light activity (30 min), 700ml for an hour, and 1,050ml for 90+ minutes.
Drink 500ml in the 2 hours before exercise, 200ml every 20 minutes during, and another 500ml after.
Signs you’re not drinking enough
Urine darker than pale yellow is the clearest signal. Headache in the afternoon, fatigue by 3pm, and difficulty concentrating are common dehydration symptoms that people attribute to other causes.
A 1–2% drop in body water reduces cognitive performance measurably. In Indian summer heat, that threshold hits faster than most people realise.
Related: BMI Calculator · TDEE Calculator · Body Fat Calculator