Water Intake
A daily fluid estimate from weight and activity โ honestly framed.
About this calculator
Weight, exercise and climate in; a daily fluid estimate out, in litres, fluid ounces and 250 ml glasses. The formula uses roughly 33 ml per kilogram plus an allowance per half-hour of exercise and a bump for hot climates โ in line with common dietetic guidance, and presented as the estimate it is.
The honest hydration facts
Three things the hydration industry underplays. All fluids count โ tea, coffee (yes, despite the diuretic reputation; the fluid outweighs the effect), milk, soup, and the roughly 20% of intake that arrives as food. The "8 glasses" rule has no solid evidence โ it's a 1945 recommendation whose "mostly contained in food" second sentence got lost. Thirst works โ for healthy people in ordinary conditions, drinking to thirst plus keeping urine pale-yellow regulates hydration well. Where the estimate genuinely earns its keep: heat, long exercise, aeroplanes, people who forget to drink at a desk, and older adults, whose thirst signal genuinely dulls. Athletes weighing themselves before and after long sessions (each lost kilogram โ a litre to replace) get the most precise answer of all. Kidney and heart conditions rewrite these rules โ that's a doctor's conversation, not a calculator's. Track the habit itself with the habit tracker if remembering is the hard part.
Frequently asked questions
How much water should I drink a day?
A reasonable estimate is ~33 ml per kg of body weight โ about 2.3 L for 70 kg โ plus extra for exercise and heat. All fluids count toward it, and thirst plus pale-yellow urine is the better day-to-day regulator.
Do tea and coffee count toward hydration?
Yes. Caffeine's diuretic effect is far smaller than the fluid delivered โ regular drinkers barely register it. Alcohol is the exception that genuinely dehydrates.
Is the 8-glasses-a-day rule real?
It traces to a 1945 guideline whose caveat โ that most of the water is 'contained in prepared foods' โ got dropped in the retelling. As a hard target it has no strong evidence; as a rough order of magnitude it's fine.
Can you drink too much water?
Yes โ overdrinking beyond what kidneys can pass (roughly a litre an hour) can dilute blood sodium dangerously, mostly seen in endurance events. Drinking to thirst prevents it; forcing litres 'to be safe' is the risk.