Water Intake Calculator

How much water should you drink? Enough that this page is not the most hydrating part of your day.

Use this free water intake calculator to estimate how much water to drink per day based on bodyweight. Enter your weight in pounds and get a daily target in fluid ounces — a more personal answer than "eight glasses," which was never based on anything in particular.

Hydration affects energy, concentration, appetite, headaches, and athletic performance, which makes it the cheapest health intervention available. The calculation is simple; remembering to do it is the hard part.

What This Water Intake Calculator Does

• Calculates a daily water target in ounces from bodyweight
• Replaces the "8 glasses" myth with a number that scales with your body
• Converts easily to bottles, cups, or liters (see the notes below)
• Metric version available: the water intake (liters) calculator works from kilograms

How to Use the Water Intake Calculator

Inputs:

• Weight — bodyweight in pounds (kg × 2.205)

Steps:

1. Enter your weight in pounds
2. Click Calculate for your daily ounces
3. Divide by 16.9 for standard bottles, by 8 for cups, or by 33.8 for liters
4. Add 12–16 oz per hour of exercise, and more in heat or at altitude

How the Calculation Works

Daily Water (oz) = Weight (lb) × 0.67 Metric: Daily Water (L) = Weight (kg) × 0.033

• 0.67 oz per pound is the common two-thirds-of-bodyweight guideline; it lands close to the Institute of Medicine's total-water figures for average adults
• Conversions: 1 cup = 8 oz · 1 standard bottle = 16.9 oz · 1 liter = 33.8 oz
• This is total water, and food supplies roughly 20% of it — fruit, vegetables, soup, and yes, coffee all count
• Coffee and tea are net hydrating despite the diuretic reputation; alcohol is the one that actually works against you

Worked Example: 165 lb adult

1. 165 × 0.67 = 111 oz per day (about 3.3 liters)
2. In bottles: 111 ÷ 16.9 = 6.5 standard bottles
3. Food contributes roughly 20%, so about 89 oz needs to come from drinks — a little over 5 bottles
4. Add a 45-minute workout and it climbs by another 12–16 oz. The bottle on your desk is not decorative.

Tips (From People Who Have Made These Mistakes)

• Urine color beats arithmetic: pale straw means hydrated, dark amber means drink, completely clear means you are overdoing it
• Front-load the day. A glass on waking and one before each meal covers half the target without any effort
• Sweating heavily for over an hour? You are losing sodium too — plain water alone can leave you worse off than a sports drink
• Thirst lags behind need, especially over 60. Do not use it as your only signal

Frequently Asked Questions

Mini Roast Disclaimer

We roast the number, not you. Buying a 40-ounce insulated tumbler is not the same as drinking from it, and we both know it.

Reality Check

This is a bodyweight heuristic, not a medical target. Real needs move with climate, activity, altitude, pregnancy, breastfeeding, illness, and medication. Kidney disease, heart failure, and some medications require deliberately limited fluids — in those cases follow your doctor, not a multiplier.