Free tools
Water Intake Calculator
How much water you actually need — based on body weight, activity, and climate. Not based on the unsourced "8 glasses a day" rule.
Climate
Myth check
There's no scientific basis for "8 glasses a day". The original 1945 recommendation noted most fluid comes from food — but the food part got dropped from later popular versions. Your body is excellent at signalling thirst; that's still the most reliable cue.
Method
Baseline 35 ml/kg bodyweight (in line with EFSA: ~2 L for women, ~2.5 L for men). Adjusted for activity (+0.3-1.0 L) and hot climates (+0.5 L). Includes all dietary fluids.
Estimates only
Increase further if pregnant, breastfeeding, ill with fever, or doing prolonged endurance work in heat. Pale-yellow urine is the simplest practical check.