Water Intake Calculator

The calculation

The tool applies one multiplier per activity level:

Daily water (mL) = body weight (kg) × factor

The result is shown in liters (two decimals) and in 200 mL glasses (rounded); with a weight in pounds you also get U.S. fluid ounces (one decimal) and 8-oz cups (rounded). The "per pound" column is the same factor converted to U.S. fluid ounces per pound, handy for a quick mental check: a 200-lb lightly active person needs about 200 × 0.51 ≈ 102 oz.

What the official guideline actually says

The National Academies of Sciences, Engineering, and Medicine (then the Institute of Medicine) set Adequate Intakes, not requirements, in its 2004 report on water and electrolytes. For adults 19–30:

Three details are commonly misreported. First, the 3.7 and 2.7 L figures include the water in food, so the drinking target is lower. Second, "beverages" includes milk, juice, coffee and tea, not just plain water. Third, the report states that most healthy people meet their needs by letting thirst guide them, and that very active people continually exposed to heat can need six liters or more.

No Tolerable Upper Intake Level was set, because healthy kidneys excrete excess water efficiently. The report does warn that acute water toxicity has occurred when people drank large volumes faster than the kidneys' maximum excretion rate of about 0.7 to 1.0 L per hour.

Calculator result vs the guideline

For an average-weight adult the two approaches land close together. A 150-lb (68 kg) lightly active woman gets 2.25 L from this calculator, near the 2.2 L of beverages in the guideline. A 180-lb (81.6 kg) lightly active man gets 2.69 L, a little below the 3.0 L beverage figure. The weight-based method diverges most for people well above or below average weight: a 120-lb person gets 1.80 L and a 220-lb person 3.29 L at the same activity level. Use whichever framing you find more useful; neither is a medical prescription.

Liters, ounces and cups

One liter is 33.8 U.S. fluid ounces. A standard 16.9-oz bottle is 500 mL. In the UK, an imperial pint is larger than a U.S. pint (568 mL vs 473 mL), so "glasses" and "pints" are not interchangeable across countries.

When you need more, and when to be careful

CDC lists hot climates, higher physical activity, fever, diarrhea and vomiting as situations that raise fluid needs. Pregnancy and breastfeeding also raise them. In those cases the "Intense exercise" setting is a better starting point than "Lightly active", and thirst and urine color are better day-to-day guides than any fixed number.

Fluid can also be a problem in the other direction. People with heart failure, advanced kidney disease and some other conditions are often told to restrict fluids; if you have been given a fluid limit, follow it rather than this estimate.

Daily water by weight in pounds

Here are the results for common U.S. body weights (1 lb = 0.4536 kg); liters come straight from the calculator and ounces are liters × 33.8.

Each extra 10 lb adds about 0.15 L (5 oz) at the lightest setting and about 0.2 L (7 oz) at the most intense. Moving up one activity level adds 5 mL per kg, roughly 0.34 L (11–12 oz) for a 150-lb adult. These are drinking targets on a typical day; heat, illness and long endurance sessions push real needs higher, and the National Academies note that most healthy people stay hydrated by drinking when thirsty.

Does coffee count? What about food?

The guideline's beverage figure includes all drinks. CDC points out that sodas, sweetened coffee and tea, sports drinks and energy drinks add calories with little nutritional value, and that plain water has none, so water is the default for most of the target. Water-rich foods such as fruit, vegetables and soup supply the remaining share. That is why a weight-based fluid number like the one here is usually treated as a drinking target, with food water on top of it.