Water intake calculator

There is no single daily requirement for water. Reference intakes come from what healthy people drink, and they differ. The National Academies (NASEM, 2005) set 3.7 L of total water a day for men and 2.7 L for women, about 81% of it from drinks (3.0 L and 2.2 L). EFSA (2010) sets 2.5 L and 2.0 L of total water for moderate temperatures and activity. Total water includes the water in food. Most healthy people meet their needs by drinking to thirst.

The estimate from drinks has three parts.

Baseline = 19 mL × weight (kg),
           kept between EFSA and NASEM
19 mL/kg = 35 mL/kg × 1,410 / 2,600,
           D-A-CH’s split from drinks,
           to a whole mL
Exercise = 10 mL × minutes a day
Heat     = 1,000 mL when hot, else 0
Drinks   = baseline + exercise + heat

Baseline. The D-A-CH reference values (Germany, Austria and Switzerland) give 35 mL of total water per kg of body weight for adults aged 19 to 50. Their total includes the water the body makes as it burns food, which the NASEM and EFSA totals leave out. They put the part from drinks at 1,410 of 2,600 mL a day for ages 25 to 50 (54%), and 1,470 of 2,700 mL for 19 to 24. So the baseline from drinks is 35 × 1,410 / 2,600 = 18.98 mL, rounded to 19 mL per kg.

The per-kg rule is not built for every body, so the baseline stays between EFSA’s figure (80% of its total) and NASEM’s for your sex: 2.0 to 3.0 L for men and 1.6 to 2.2 L for women. 19 mL per kg falls in that range for men from 105.3 to 157.8 kg and for women from 84.3 to 115.7 kg. At lighter weights the baseline is EFSA’s figure, and at heavier ones NASEM’s. D-A-CH’s value from age 51 is lower, 30 mL per kg; the calculator does not ask your age and uses the value for 19 to 50 for every adult. Where the baseline is EFSA’s figure, the lower value would not change it.

Exercise. Sweat rates run from about 0.3 to 2.4 L an hour, depending on the activity, the heat and the person. For marathon runners who do not know their own rate, ACSM suggests drinking 0.4 to 0.8 L an hour; the calculator uses the middle, 0.6 L an hour. To learn your own sweat rate, weigh yourself before and after an hour of exercise: each kilogram lost, plus what you drank, is about a liter of sweat.

Heat. The NASEM report puts daily needs at about 3 L for sedentary people in temperate climates and about 4 L in hot ones, and far more with hard work in the heat. Choosing hot adds that 1 L.

For example, a 70 kg woman who exercises 30 minutes a day in a temperate climate gets 19 × 70 = 1,330 mL. That is below EFSA’s 1,600 mL, so her baseline is 1,600 mL. With 300 mL for exercise, the total is 1.9 L, or 7.6 glasses of 250 mL. A 110 kg man who exercises 60 minutes gets 19 × 110 = 2,090 mL plus 600 mL, 2,690 mL: 2.7 L, or 10.8 glasses. Pounds are converted exactly (1 lb = 0.45359237 kg), and fluid ounces are US fluid ounces (1 fl oz = 29.5735 mL). The parts are shown to 10 mL (0.1 fl oz), the total is their sum rounded to 0.1 L (a whole fl oz), and each difference from a reference intake is taken between the numbers as shown.

In Pendum Hydration, the daily goal starts at 2,500 mL and is yours to set; the app does not personalize it. Coffee and tea count toward the goal. Alcohol does not.