Water Intake Calculator
With weight 75 kg, exercise per day 45 minutes, water intake comes to 3.17 L — daily fluid target. It is reached in 6 steps, the last of which is 3165 / 1000, and each one is printed on the page with its numbers filled in. The formula is the one published by National Academies, not an approximation fitted to it.
Daily fluid target from body weight and exercise, against the NASEM total-water reference intakes.
Formula and sources checked · How we check
Weight 75 kg, Exercise per day 45 minutes, Climate Temperate
3.17 L
Daily fluid target for the example below. Editing a field recomputes the calculator below; this figure holds the answer the page was loaded with.
It is written into the HTML rather than drawn by a script, so a search engine reading this page without running JavaScript still finds an answer.
- Baseline from body weight (35 ml/kg)
75 * 352,625 ml- Replacement for exercise (12 ml/min)
45 * 12540 ml- Total fluid
(2625 + 540) * 13,165 ml- Total in liters
3165 / 10003.165 L- In 8 oz cups
3165 / 23713.354 cups- Typically covered by food (about 20%)
3165 * 0.2633 ml
Worked example
75 kg with 45 minutes of exercise works out at 3.17 L a day, about 13 cups — but roughly a fifth of that arrives in food, so the drinking target is closer to 2.5 L. The NASEM reference for adult men is 3.7 L of total water and 2.7 L for women, which brackets this.
How to work it out yourself
- 1.Start from body mass at roughly 35 ml per kilogram. It is a starting point, not a requirement: thirst and urine colour are better daily signals than any equation.
- 2.Add for exercise at about 12 ml per minute of activity, and more in heat. Sweat rate varies by a factor of three between people, so treat this as a floor.
- 3.Around a fifth of intake comes from food, which the figure below already accounts for. What you drink is the remainder, not the whole number.
The formula
- Baseline from body weight (35 ml/kg)
75 * 35 - Replacement for exercise (12 ml/min)
45 * 12 - Total fluid
(2625 + 540) * 1 - Total in liters
3165 / 1000 - In 8 oz cups
3165 / 237 - Typically covered by food (about 20%)
3165 * 0.2
Source: National Academies — Dietary Reference Intakes for water and electrolytes, CDC — water and healthier drinks
Questions people actually ask
- Is the "eight glasses a day" rule real?
- It has no source in the literature. The NASEM reference intakes are 3.7 L total water for men and 2.7 L for women, and both include water from food and every other beverage — which is already more than eight glasses of plain water for most men.
- Do coffee and tea count?
- Yes. The diuretic effect of caffeine at normal intakes does not offset the fluid in the drink; controlled studies show coffee hydrating about as well as water. The NASEM figures explicitly include all beverages.
- How do I know if I am drinking enough?
- Urine colour is the practical check — pale straw is adequate, dark amber is not. Thirst lags actual need during heavy exercise, which is why the exercise allowance here is added rather than left to appetite.
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