The coffee myth, settled
“Coffee dehydrates you” is one of the most repeated hydration myths, and it doesn't hold up. Caffeine is a weak diuretic in people who rarely have it, but regular drinkers develop tolerance within days. In controlled studies, moderate coffee intake produced the same hydration markers as water. That's why this calculator credits coffee at about 95% of its volume and tea at nearly 100%. Our article on whether coffee counts as water walks through the research.
Where caffeine starts to cost you
Above roughly 300 mg of caffeine in a day — three or four cups of coffee — a mild diuretic effect becomes measurable. The calculator subtracts about a milliliter of water for every milligram over that threshold. It's a small correction; the bigger issues with heavy caffeine use are sleep and jitters, not hydration.
Alcohol is different
Alcohol suppresses vasopressin, the hormone that tells your kidneys to hold onto water. Each standard drink (about 10 g of alcohol) produces roughly 100 ml more urine than the drink itself supplied, so a pint of beer still leaves you slightly ahead while a shot of spirits leaves you behind. The extra-water figure assumes you replace that loss; the classic one-water-per-drink habit does it with room to spare and slows your drinking down. If you're wondering about water before bed after a night out, a glass is sensible — a liter is not.
Logging it honestly
The practical takeaway: count your coffees and teas, count alcohol at zero or slightly negative, and don't over-correct either way. Realwater applies a hydration value to each beverage type automatically, so a cappuccino and a glass of water each move the sphere by what they're actually worth. More myths worth retiring are in our hydration myths roundup.
