How Much Water You Need Per Day
The figures most people quote from the National Academies, about 3.7 litres a day for men and 2.7 for women, already include the water in your food and every drink you have. Food supplied roughly a fifth of total water in the intake data behind them, so a man aiming at 3.7 litres of plain water on top of his meals and his coffee has misread the number by several hundred millilitres. Those totals come from the National Academies report on total water intake.
Total water includes food
These are population reference values, and no part of the report sets a compulsory daily quota of plain water. The food share varies substantially between diets, and tea, coffee, milk and other drinks all count toward your total. A drink can therefore count toward your litres and still be a poor choice for you, which is where alcohol lands whatever it adds to the sum. Any fluid restriction you have been given takes priority over a generic intake figure.
What this calculator estimates
The site’s rule is 35 ml per kilogram of body weight plus 350 ml for each 30 minutes of exercise. It is a planning heuristic. The National Academies report validates neither the 35 nor the fixed exercise allowance. Start with the water-intake calculator, read its assumptions, and fill your bottles afterwards.
At 80 kg with 30 minutes of exercise, the heuristic gives 2,800 + 350 = 3,150 ml of total water. Assume a 20% food contribution. That leaves 2,520 ml to drink. Divide by a 750 ml bottle and you get 3.36 bottles, which is a unit conversion and not an instruction to drink 3.36 bottles of plain water on top of everything else.
The food-share control changes an assumption. It measures nothing about the water in your own meals. The cup on this tool is 240 ml. A US fluid ounce is about 29.57 ml.
Activity and heat change the estimate
Sweat is personal, and a fixed allowance has no way to know your rate; an easy indoor session and a long event in hot weather are different problems. The sweat-rate guide explains a rough way to measure your own losses and where that method runs out; the electrolyte guide covers when sodium starts to matter.
Do not chase a number at any cost
Thirst, your usual urine pattern and changes from your normal weight all give you context. Urine colour on its own diagnoses nothing, because your food, your vitamins, your medicines and any illness all change it, and aiming for clear urine is aiming at the wrong thing.
Drinking too little is harmful, and so is drinking far too much. Do not force fluid down to complete a calculator target. Anyone with kidney or heart conditions, a prescribed fluid limit, or who is pregnant or breastfeeding needs individual guidance. Confusion, collapse or severe illness during exercise needs urgent medical assessment.
Use your own bottle size
Choose your bottle size in the water calculator to convert the drinks portion into containers you own. It takes preset sizes and custom mL or US fl oz entries, keeps the food-share assumption visible, and shows each step of the conversion in the report.
Scope and correction note, 13 September 2026: This guide provides general adult education. It has been revised to distinguish estimates from measurements and to remove unsupported guarantees. It has not received independent clinical review. See the editorial policy and medical disclaimer.