How is this estimate actually calculated?

It starts from a baseline of 30–38 ml of water per kilogram of body weight, scaled up with activity level (more exercise means more fluid loss through sweat), and adds a flat 500 ml for hot or humid climates where you lose more fluid even at rest. This mirrors the general approach used in most clinical hydration guidelines, scaled to your specific weight rather than a one-size-fits-all number.

Is the old "8 glasses a day" rule actually accurate?

It's a reasonable rough average for a sedentary adult of moderate size, but it ignores body weight, activity, and climate entirely — someone who's 50 kg and sedentary indoors needs meaningfully less than someone who's 95 kg and training outdoors in summer heat, which is exactly what this calculator adjusts for.

Does this count water from food and other drinks?

No — this estimates total fluid needs, and roughly 20% of daily water intake typically comes from food in a normal diet. If you're tracking plain water specifically, treat this number as a reasonable upper estimate rather than a number you must hit from water alone.

Does pregnancy or breastfeeding change how much water I need?

Yes, significantly — both substantially increase fluid needs, and this general calculator doesn't account for that. If you're pregnant or breastfeeding, ask your doctor or midwife for a target specific to your situation rather than relying on a generic formula.

WATER INTAKE CALCULATOR

Estimate your daily water target from body weight, activity level, and climate.

A general guideline based on body weight, activity, and climate — not medical advice. Needs vary by individual health conditions and medication, so check with a doctor for anything beyond everyday hydration.

Body weight

Climate

Activity level

Liters / day	2.3 L
Fluid ounces / day	78 fl oz
Cups / day	9.8 cups
8 oz glasses / day	10 glasses

How to calculate your daily water intake

1
Enter your body weight and pick kg or lb.
2
Select your activity level and whether you're in a hot/humid climate.
3
Read your estimated daily water target in liters, fluid ounces, cups, and 8 oz glasses.

Questions

How is this estimate actually calculated?: It starts from a baseline of 30–38 ml of water per kilogram of body weight, scaled up with activity level (more exercise means more fluid loss through sweat), and adds a flat 500 ml for hot or humid climates where you lose more fluid even at rest. This mirrors the general approach used in most clinical hydration guidelines, scaled to your specific weight rather than a one-size-fits-all number.
Is the old "8 glasses a day" rule actually accurate?: It's a reasonable rough average for a sedentary adult of moderate size, but it ignores body weight, activity, and climate entirely — someone who's 50 kg and sedentary indoors needs meaningfully less than someone who's 95 kg and training outdoors in summer heat, which is exactly what this calculator adjusts for.
Does this count water from food and other drinks?: No — this estimates total fluid needs, and roughly 20% of daily water intake typically comes from food in a normal diet. If you're tracking plain water specifically, treat this number as a reasonable upper estimate rather than a number you must hit from water alone.
Does pregnancy or breastfeeding change how much water I need?: Yes, significantly — both substantially increase fluid needs, and this general calculator doesn't account for that. If you're pregnant or breastfeeding, ask your doctor or midwife for a target specific to your situation rather than relying on a generic formula.