How the target is calculated
The baseline is 35 ml of water per kilogram of body weight per day, a rule of thumb used in clinical dietetics for healthy adults (published guidance ranges from 30 to 40 ml/kg). On top of that, the calculator adds 350 ml per 30 minutes of exercise to cover typical sweat losses at moderate effort.
Worked example: a 70 kg person who trains 45 minutes gets 70 × 35 = 2,450 ml, plus about 525 ml for the session — roughly 3.0 liters. Remember this is total fluid, and food already supplies 20–30% of it for most people. The glasses you pour cover the rest.
About the 8-glasses rule
"Eight 8-ounce glasses" adds up to about 1.9 liters, and its origin is murkier than its fame. The usual citation is a 1945 US Food and Nutrition Board note suggesting about 2.5 liters a day — which, in the same sentence, said most of that already comes from food. The second half fell off somewhere in transmission, and a slogan was born.
The deeper problem is that one number cannot fit a 55 kg office worker in Oslo and a 95 kg builder in Dubai. Fluid needs scale with body size, sweat, and climate, which is what a weight-based formula at least tries to respect. Treat 8 glasses as a memorable average, and this calculator's output as a starting estimate — a planning number, not a medical requirement.
Urine color beats any formula
Your body publishes its own hydration report several times a day. Pale straw-colored urine means you are fine; darker amber means drink more; completely clear all day usually means more than you need. This feedback loop beats any formula, because it already accounts for your food, your climate, and this afternoon's workout. Use the number above to plan, and urine color plus thirst to adjust. (One caveat: B vitamins and some medications tint urine bright yellow regardless.)
When you need more than the formula says
Heat and humidity raise sweat losses well past the standard add-on, and so does altitude above roughly 2,500 m. Fever, vomiting, or diarrhea drain fluid quickly. Pregnancy and breastfeeding raise needs too — around 300 and 700 ml extra per day respectively in common guidance. In those situations, and especially with heart or kidney conditions where fluid can need capping rather than raising, get a target from a clinician instead of a calculator. For everyone else: if you sweat more (the calories burned calculator gives a sense of session effort), drink more.
