How to use the water intake calculator
- Choose US or metric units and enter your body weight.
- Enter your average minutes of exercise per day that make you sweat, or 0 if none.
- Read the rule-of-thumb target in fluid ounces, cups, liters and 500 mL bottles.
- Compare it with the National Academies reference totals in the table, which count water from food as well as drinks.
Formula
Base target = body weight in pounds x 0.5 fluid ounces
Exercise add-on = exercise minutes / 30 x 12 fluid ounces
Daily target = base + add-on
1 US fluid ounce = 29.5735295625 mL
The half ounce per pound and the 12 ounces per 30 minutes are common rules of thumb, not measured physiological requirements. They are used here because they are simple and conservative, and they are labeled that way on the page.
Worked example
A 150 lb person who does not exercise gets 150 x 0.5 = 75 fl oz, which is 2.22 liters or about 9.4 cups.
If the same person exercises 30 minutes a day, the add-on is 12 fl oz and the target becomes 87 fl oz, or 2.57 liters.
For comparison, the National Academies adequate intake for total water is 3.7 liters for adult men and 2.7 liters for adult women. Those totals include water from food, which is why the drinking target here is lower.
What the reference numbers mean
The National Academies of Sciences, Engineering, and Medicine set adequate intake levels for total water of about 3.7 liters per day for adult men and 2.7 liters for adult women. These cover water from beverages and from food, and they assume healthy, sedentary people in temperate climates. They are based on typical intakes, not on a measured requirement, and people who are very active or in hot conditions may need more.
Why this page uses a rule of thumb
There is no single correct number of cups for everyone. A weight-based rule scales the target with body size, and an exercise add-on reflects sweat loss. Thirst, urine color, climate, diet, medications and health conditions also matter. Use the result as a planning number and let thirst and your clinician guide the details.
When to be careful
Some people are told to limit fluids, for example because of heart, kidney or liver conditions or certain medications. If that applies to you, follow your doctor's instructions instead of this page. Drinking far more than you need, quickly, can also be harmful, so the tool warns when a target passes one gallon. For converting between cups, ounces and milliliters, use the cooking measurement converter.
Assumptions and limits
- The target counts fluids you drink. It does not try to separate water from coffee, tea, milk or the water in food.
- Climate, altitude, illness, pregnancy, breastfeeding and medications change needs and are not modeled.
- The 0.5 ounce per pound and 12 ounces per 30 minutes rules are rough conventions, not clinical guidance.
- Weight must be between 66 and 990 lb (30 to 450 kg) and exercise from 0 to 600 minutes.
- This is general information, not medical advice. Nothing you enter leaves your browser.
Frequently asked questions
How much water should I drink a day?
There is no single answer. A common rule of thumb is half an ounce per pound of body weight plus extra for exercise, which this page calculates. Reference totals from the National Academies are higher because they include water from food.
Does coffee or tea count toward my water intake?
For most people, yes, they contribute to total fluid intake, though plain water is a simple default. This tool counts any fluid you drink and does not subtract caffeine.
Why is my target lower than 3.7 or 2.7 liters?
Those figures are total water including food, and food supplies a meaningful share. The drinking target here is a rule of thumb for fluids only, so it normally comes out lower.
Should I drink more in hot weather?
Heat and sweating raise water losses, so needs can go up. This calculator only adds water for the exercise minutes you enter, so adjust upward for hot days and listen to thirst.
Can drinking too much water be a problem?
Yes. Drinking very large amounts quickly can dilute the sodium in the blood. Spread fluids through the day, and ask a doctor if you have a medical condition that affects fluid balance.