How much water should you drink? The body-weight math

"Drink eight 8-ounce glasses a day" is the most repeated hydration rule there is, and it's the same number for everyone regardless of body size — a 55 kg adult and a 110 kg adult get identical advice despite needing very different amounts of fluid to function. A per-kilogram estimate scales the target to the person instead of handing out one flat number.

The method

A commonly used rough guide is about 33 milliliters of fluid per kilogram of body weight per day. In liters, that's:

daily water (L) = body weight (kg) × 0.033

To convert that liter figure into US cups, multiply by 4.2267 (one liter is about 4.2267 US cups, since one US cup is roughly 236.6 ml and 1000 ÷ 236.6 ≈ 4.2267):

daily water (cups) = daily water (L) × 4.2267

A real example: an 82 kg adult

82 × 0.033 = 2.706 liters a day. In cups: 2.706 × 4.2267 = 11.4375, so about 11.44 cups. That's the full-day total fluid target, not a single sitting — spread across meals, drinks and plain water through the day.

A real example: a 65 kg adult

65 × 0.033 = 2.145 liters a day. In cups: 2.145 × 4.2267 = 9.0663, so about 9.07 cups. The gap between this and the 82 kg example — 2.706 L versus 2.145 L, a difference of 2.706 − 2.145 = 0.561 liters — is exactly why a flat "eight glasses" rule misses body-size variation: a single multiplier scales with weight, a fixed number of glasses doesn't.

A real example: converting an existing target back to liters

If a doctor or dietitian gives a target in ounces instead — say 90 fl oz — convert to liters by dividing by roughly 33.814 fl oz per liter: 90 ÷ 33.814 = 2.662 liters. That lands close to the 82 kg example above, which is a useful sanity check when comparing a guideline from one source against this tool's output in a different unit.

What "about 33 ml per kg" doesn't account for

This multiplier is a starting baseline, not a precise physiological requirement. Actual need shifts with heat, humidity, exercise, altitude, pregnancy or breastfeeding, fever, and some medical conditions and medications — all of which can raise or lower the real figure well outside a single formula. It also assumes "water" loosely as total fluid intake; food and other beverages contribute too, so this isn't a tally of only what comes from a tap or bottle.

When to ignore the number entirely

Anyone with a kidney, heart, or liver condition, or anyone on fluid-restriction medical advice, should follow their clinician's guidance over a general formula — this estimate is informational only and not a substitute for medical advice tailored to an individual health situation.

Run your own numbers

The free Water Intake Calculator takes your body weight and returns the daily target in both liters and cups instantly, entirely on your device.