Why '8 Glasses a Day' Isn't Based on Your Body At All

A memorable number, not a calculated one

"Drink eight glasses of water a day" is easy to remember precisely because it's a flat, one-size-fits-all number — it doesn't move whether you weigh 50kg or 100kg, whether you're sedentary or training for hours a day, which is a strange property for a hydration guideline to have, since fluid needs obviously scale with body size and activity.

A formula that actually scales

A more individualized approach ties water intake directly to body weight: roughly 30–35mL per kilogram at rest, rising to 40–45mL per kilogram for intense exercise or hot climates. Because it's weight-based, the recommended amount automatically scales up or down for different body sizes, the way an actual physiological need would.

Why activity level shifts the number so much

Sweat losses during exercise or heat exposure can be substantial, which is why the high end of the range (40–45mL/kg) is meaningfully higher than the resting baseline — someone training hard in heat can need noticeably more fluid than the same person on a sedentary day, even at identical body weight.

What this doesn't cover

This formula assumes ordinary health conditions. It isn't adjusted for pregnancy, breastfeeding, fever, or kidney, heart or liver conditions that affect fluid balance — anyone managing one of those should follow medical guidance rather than a general weight-based formula.

Get your number

Our Water Intake Calculator scales the 30–45mL/kg range to your own body weight and activity level.