“Drink 8 glasses of water a day” is a memorable rule of thumb, but it ignores body size and activity level entirely. Our water intake calculator uses a more individualized formula based on body weight, with an adjustment for exercise.
The formula
Base intake (oz) = body weight (lbs) × 0.5
Exercise addition (oz) = (exercise minutes / 30) × 12
Total = base + exercise additionThis “half your body weight in ounces” rule is a widely cited general guideline — a 150 lb person’s baseline works out to 75 oz, roughly 9 cups.
Example
150 lbs, 30 minutes of daily exercise:
- Base: 150 × 0.5 = 75 oz
- Exercise addition: (30/30) × 12 = 12 oz
- Total: 87 oz (2.57 liters)
What this doesn’t account for
This is a general wellness estimate, not medical advice. It doesn’t factor in climate (hot/humid conditions increase fluid needs substantially), altitude, pregnancy/breastfeeding, or existing health conditions. People with kidney disease, heart failure, or other conditions affecting fluid balance should follow their doctor’s specific guidance rather than a generic formula — those conditions can require notably different, often lower, intake targets.
Don’t forget food
Roughly 20% of daily hydration typically comes from food in a normal diet (especially water-rich fruits and vegetables), so your direct beverage intake can reasonably run somewhat below the raw total this calculator shows.