Hydration Calculator

About Hydration Calculator

The old "eight glasses a day" guideline is a generic rule of thumb that doesn't account for the biggest factor in actual hydration needs: body size. A larger person has more body mass and generally needs more water than a smaller person, which is why a weight-based calculation gives a more personalized starting estimate than a flat, one-size-fits-all number.

This tool uses a commonly cited baseline of roughly 35 milliliters of water per kilogram of body weight — a widely referenced general hydration guideline — then adds additional water for exercise, since physical activity increases fluid loss through sweat that needs replacing beyond the baseline amount.

The exercise addition is calculated at roughly 350ml for every 30 minutes of activity, a reasonable general estimate for moderate-intensity exercise, though actual sweat loss varies significantly based on exercise intensity, individual sweat rate, and ambient temperature — someone training hard in hot weather loses considerably more fluid than someone doing light activity in a cool room.

This is a general estimation tool, not personalized medical guidance — actual hydration needs vary with climate, altitude, overall health, certain medical conditions (including some that require fluid restriction), and pregnancy or breastfeeding status. For anyone with a specific medical condition affecting fluid needs, guidance from a doctor takes priority over a general calculator like this one.

How it works

1. Enter your body weight. The primary factor in the baseline calculation.
2. Enter your daily exercise minutes. Additional water is added to account for fluid lost through sweat.
3. View your recommended daily intake. Shown in liters, milliliters, and approximate glasses.