Water Intake Calculator

Hydration quietly governs energy and concentration, and how much water you need changes with how active you are and how hot it is, which makes a personalized target more useful than a one-size-fits-all rule. The estimate here works from your own numbers rather than a generic glass count.

The baseline is 0.5 ounces of water per pound of body weight. The activity setting then adds a flat amount: nothing for Sedentary, 12 ounces for Moderate, 24 for Active and 36 for Very Active. Finally, the climate setting scales the total up 20% for Hot or down 5% for Cold and leaves it unchanged for Temperate. The result is shown in ounces, liters, 8-ounce glasses and 16.9-ounce bottles, plus an hourly amount spread over 16 waking hours.

Use the result as a solid starting point, not a fixed prescription, since real needs shift from person to person. Some medical conditions, along with pregnancy and breastfeeding, change hydration requirements substantially and are worth raising with a healthcare professional. For most healthy people thirst is a workable guide: the National Academies' 2004 report on water intake found that the vast majority meet their daily hydration needs by letting thirst guide them, so treat the number as a reference rather than a quota.