Daily Water Intake Calculator

Daily water intake needs vary with body weight, physical activity, climate, and health. The old “8 glasses a day” rule is an oversimplification. A 55 kg office worker and a 95 kg roofer in August do not have remotely the same requirement.

The baseline this calculator uses: Baseline daily water = Body weight (kg) × 33 mL That is roughly half a fluid ounce per pound, which lines up with the National Academies adequate-intake figures once you subtract the water you get from food.

Activity and climate are then applied as multipliers on top of that baseline:

Cold surprises people. You lose water fast in cold air because it holds almost no moisture, so every breath you exhale is more humid than the air you took in, and dry indoor heating does the same thing all winter.

Worked example: A 170 lb (77 kg) person who trains 3 to 5 days a week and lives somewhere hot and humid:

• Baseline: 77 × 33 mL = 2.5 L (86 fl oz)
• Moderately active: × 1.30 gives 3.3 L
• Hot and humid: × 1.15 gives 3.8 L per day (129 fl oz, about 15 glasses)

Official totals, from all sources including food:

Those totals are higher than what this calculator returns, for two reasons. About 20% of your daily water arrives in food, and the panel below adds that back in. Watermelon is roughly 92% water, cucumber 96%, and a bowl of soup counts for most of its volume. The rest of the gap is that an adequate intake figure is deliberately set high enough to cover almost everyone, including people considerably larger than you, so it is a population floor rather than a personal target. Pregnancy adds about 300 mL a day to the figure below, and breastfeeding adds roughly 700 mL.

How to tell whether you have it right: urine that is pale straw, six to eight trips to the bathroom a day, and no mid-afternoon headache. Dark amber means you are behind. Completely colorless all day usually means you are overdoing it.

Above about 10 L a day, plain water starts working against you by diluting blood sodium. If you are drinking that much because of heat or long endurance work, some of it needs electrolytes rather than more water.