The National Academies (then the Institute of Medicine) set an Adequate Intake for total water of about 3.7 liters a day for men and 2.7 liters for women. That includes water from all drinks and food. Their report notes that about 80% of total water typically comes from drinking water and beverages, including caffeinated ones, and about 20% from food. So the drinking part is roughly 3.0 L (about 100 fl oz) for men and 2.2 L (about 73 fl oz) for women.
The panel also said that most healthy people meet their needs by letting thirst guide them, and it deliberately gave no "glasses per day" rule. People who are very active or in hot climates may need more. This calculator lets you add a planning allowance for that.
How to use the water intake calculator
- Choose sex (the Adequate Intakes are for adults 19 and older).
- Enter how many minutes a day you exercise or work in heat.
- Adjust the extra fluid per hour if you know your sweat rate (weigh yourself before and after exercise). The 0.5 L default is only our planning figure.
- Read the daily drinking target in liters, fluid ounces and cups.
Formula
1 US fluid ounce = 29.5735 mL and 1 US cup = 236.588 mL (NIST SP 811).
Worked example
A woman who exercises 30 minutes a day, using the 0.5 L per hour planning allowance.
- Total water AI: 2.7 L; from drinks: 0.8 × 2.7 = 2.16 L.
- Activity: 30 ÷ 60 × 0.5 = 0.25 L.
- Drinking target ≈ 2.16 + 0.25 = 2.4 L a day, about 81 fl oz or 10.2 cups.
A man with no exercise allowance: 0.8 × 3.7 = 2.96 L ≈ 100 fl oz.
Signs you are drinking enough
For healthy adults, the National Academies concluded that normal drinking at meals and in response to thirst usually maintains hydration. The same report notes that people who are very physically active or who live in hot climates may need more water. Illness, pregnancy and breastfeeding also change needs. People with heart, kidney or liver conditions may be told to limit fluids, so follow your clinician's advice.
Convert between liters, ounces and cups with the volume converter. For energy rather than fluid needs, see the tdee calculator.
Turning liters into cups and bottles
2.4 liters is about 81 U.S. fluid ounces, 10 cups (8 fl oz each) or roughly five 16.9 oz (500 mL) bottles. Divide the liters by 0.2366 for cups, or by 0.5 for half-liter bottles. Spreading drinks across the day, with meals and around exercise, makes a target easier to reach. To convert other amounts, use the volume converter.
Caveats and assumptions
- Adequate Intakes are population medians for healthy adults, not personal prescriptions.
- The activity allowance is our planning default; individual sweat rates vary several-fold.
- Not for children, pregnancy or breastfeeding, or anyone on a medical fluid restriction. Not medical advice.
Frequently asked questions
How much water should I drink a day?
The National Academies’ Adequate Intake is 3.7 L of total water for men and 2.7 L for women. About 80% usually comes from drinks: roughly 3.0 L (100 oz) and 2.2 L (73 oz).
Is 8 glasses of water a day right?
It is a popular rule of thumb, not an official recommendation. The National Academies panel chose not to give a glasses-per-day rule because food and all beverages count.
Do coffee and tea count toward water intake?
Yes. The National Academies’ report counts caffeinated beverages toward total water intake.
How many ounces is 2.7 liters?
About 91 US fluid ounces.
Sources
- National Academies — Report Sets Dietary Intake Levels for Water, Salt, and Potassium — 2.7 L / 3.7 L, ~80% from beverages, thirst as a guide (confirmed through search results)
- National Academies — Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate (2005), summary — AI for total water set from median intakes
- NIST SP 811 Appendix B.9 — Conversion factors — fluid ounce and cup in milliliters
This calculator gives general estimates for healthy adults and is not medical advice, diagnosis or treatment. Equations are averages and individuals differ. Talk to a qualified health professional before changing your diet, exercise or medication, and during pregnancy. See our disclaimer and how we build and check calculators.