Is 64 oz of Water a Day Enough?
Where the 8 glasses rule came from, who 64 oz fits, and who it does not.
Short answer
For a lot of healthy adults, 64 oz is a reasonable amount, but it's a round number, not a rule. It matches our calculator's baseline for a person of about 128 lb (58 kg) who doesn't exercise. If you weigh more, work out, or live somewhere hot, your own target will usually be higher. If you're on a doctor's fluid limit, it could be too much.
Run the Daily Water Intake Calculator to get a number based on your weight and activity.
What 64 oz actually looks like
Sixty-four fluid ounces is half a US gallon. A US gallon is 3,785.41 mL according to NIST's conversion tables, so half is about 1,893 mL. In 16.9 oz (500 mL) bottles, that comes to roughly 3.8 bottles a day. If you want to see the bottle math for your own target, the water bottles per day calculator does it for you, and how many bottles of water are in a gallon covers the gallon version.
Where the "8 glasses" rule came from
The 64 oz figure is just "eight 8-ounce glasses" multiplied out. A physiologist at Dartmouth, Heinz Valtin, went looking for the evidence behind that advice in a review published in the American Journal of Physiology. His abstract is blunt: "No scientific studies were found in support of 8 x 8."
What he found instead was that surveys of thousands of apparently healthy adults suggest people do fine on less. He also noted that caffeinated drinks can count toward the daily total, and said moderate beer probably counts to a lesser extent. Later studies are mixed, so this site doesn't count alcohol. In a press release from Dartmouth, Valtin's best guess was that the rule grew out of a National Research Council recommendation of about 1 mL of water per calorie of food, followed by a sentence saying most of that water is already in prepared foods. People may have read the first half and missed the second.
Valtin didn't say more water is useless. His conclusion applies to healthy adults in a temperate climate with a mostly sedentary life. He also wrote that larger intakes can be advisable for some diseases and during vigorous work and exercise, especially in hot weather.
What the National Academies say
The National Academies set adequate total water intake, from drinks and food together. About 80% of that comes from beverages, and about 20% from food. They also say the vast majority of healthy people meet their needs by letting thirst be their guide.
| Group | Total water | From drinks |
|---|---|---|
| Women | 2.7 L (91 oz) | 2.2 L (74 oz) |
| Men | 3.7 L (125 oz) | 3.0 L (101 oz) |
The drinks column is the National Academies' published figure for water from beverages, which is about 80% of the total. Against those numbers, 64 oz sits below the average drinks share for both groups. These are averages for adequate intake, not minimums.
Does 64 oz fit your size?
Our calculator multiplies body weight in kilograms by 32.5 mL (the range is 30 to 35 mL per kg), then adds 0.6 L for each hour of exercise (the middle of a 0.4 to 0.8 L per hour range that the ACSM position stand gives for marathon runners). For a 58.2 kg person, 32.5 mL per kg gives about 1,892 mL, which is 64 oz. The 30 to 35 mL range puts that same person between about 59 and 69 oz.
| Body weight | Target (mL) | Target (oz) | Compared with 64 oz |
|---|---|---|---|
| 120 lb | 1,750 | 59 | About 5 oz less |
| 128 lb | 1,900 | 64 | Same |
| 140 lb | 2,050 | 69 | About 5 oz more |
| 160 lb | 2,350 | 79 | About 15 oz more |
| 180 lb | 2,650 | 90 | About 26 oz more |
| 200 lb | 2,950 | 100 | About 36 oz more |
Thirty minutes of exercise adds about 300 mL, or roughly 10 oz. So a 160 lb person who works out most days is looking at something closer to 90 oz than 64.
Who might need more or less than 64 oz
More. Mayo Clinic lists exercise that makes you sweat, hot or humid weather, and high altitude as reasons to drink extra. Illness with fever, vomiting, or diarrhea also raises fluid needs, and Mayo says pregnant or breastfeeding women may need more fluids. The CDC says needs vary by age, sex, pregnancy status, activity level and breastfeeding status.
Less, or a set limit. Some medical conditions come with a fluid cap, and it can sit well under 64 oz:
- The American College of Cardiology notes that U.S. and European guidelines recommend limiting people with heart failure to 1,500 mL a day (about 51 oz). A 2025 trial (FRESH-UP) in stable, mild to moderate heart failure didn't show a clear benefit from the cap, so your care team's current advice is what counts.
- The National Kidney Foundation says that for most people on dialysis, a fluid limit means about three or four 8-ounce cups a day (24 to 32 oz), set by their dialysis team.
- A liver specialists' teaching article describes restricting fluid to 1 to 1.5 liters (about 34 to 51 oz) for chronic hypervolemic hyponatremia in cirrhosis.
If any of those apply to you, or you're pregnant, breastfeeding, or planning water intake for a child, treat our number as general information and ask your own doctor what applies to you.
A simple check. Mayo Clinic says you're probably drinking enough if you're rarely thirsty and your urine is colorless or light yellow. Dark urine, headaches, or lower physical performance can point the other way.
How to find your own number
Start with the Daily Water Intake Calculator. Enter your weight and how much you exercise, and you get a target in both mL and oz. It estimates drinks, so the water in food comes on top.
Keep reading
Frequently asked questions
Is 64 oz of water a day enough for a woman?
It can be. The National Academies' adequate total water for women is 2.7 L (91 oz), and the part from drinks is 2.2 L, or about 74 oz. The Academies say most healthy people do well going by thirst, and your weight and activity matter too, so check the calculator.
How many fl oz of water should you drink a day?
There's no single figure. Using the National Academies' averages, drinks come to about 74 oz for women and 101 oz for men. Our calculator gives a personal number: from about 59 oz for a 120 lb adult to about 100 oz for a 200 lb adult, before exercise.
Do coffee and tea count toward 64 oz?
Valtin's review found published studies showing that caffeinated drinks can count toward the daily total. He said moderate beer probably counts to a lesser extent, but this site doesn't count alcohol.
Is 64 oz the same as half a gallon?
Yes. A US gallon is 128 fl oz, so 64 oz is exactly half. That's 1,892.7 mL, or about 3.8 standard 16.9 oz bottles.
Can 64 oz be too much?
For most healthy people it isn't a problem. For someone with heart failure, kidney failure on dialysis, or cirrhosis with low blood sodium, a clinician may set a lower limit. In that case, follow the limit your care team gave you.
Sources
- Valtin H. "Drink at least eight glasses of water a day." Really? Is there scientific evidence for "8 x 8"? American Journal of Physiology, Regulatory, Integrative and Comparative Physiology, volume 283, issue 5, pages R993 to R1004. Abstract via PubMed: https://pubmed.ncbi.nlm.nih.gov/12376390/ (DOI 10.1152/ajpregu.00365.2002)
- Dartmouth Geisel School of Medicine press release on Valtin's review: https://geiselmed.dartmouth.edu/news/2002_h2/08aug2002_water.shtml
- National Academies of Sciences, Engineering, and Medicine, news release on dietary intake levels for water, salt, and potassium, Feb. 11, 2004: https://www.nationalacademies.org/news/report-sets-dietary-intake-levels-for-water-salt-and-potassium-to-maintain-health-and-reduce-chronic-disease-risk
- Mayo Clinic Staff, "Water: How much should you drink every day?": https://www.mayoclinic.org/healthy-lifestyle/nutrition-and-healthy-eating/in-depth/water/art-20044256
- NIST Special Publication 811, Appendix B.8, conversion factors: https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b8
- CDC, "About Water and Healthier Drinks": https://www.cdc.gov/healthy-weight-growth/water-healthy-drinks/index.html
- American College of Cardiology press release on the FRESH-UP trial, "Limiting Fluid Intake May Not be Needed for Some People with Heart Failure": https://www.acc.org/about-acc/press-releases/2025/03/30/12/24/limiting-fluid-intake-may-not-be-needed-for-some-people-with-heart-failure
- National Kidney Foundation, "Water, Water Everywhere, and Not a Drop to Drink": https://www.kidney.org/news-stories/water-water-everywhere-and-not-drop-to-drink
- AASLD Liver Fellow Network, "Low Sodium High Stakes: Back to Basics of Hyponatremia in Cirrhosis": https://www.aasld.org/liver-fellow-network/core-series/back-basics/back-basics-low-sodium-high-stakes-back-basics
- National Academies Press. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate, 2005. Water chapter: https://www.nationalacademies.org/read/10925/chapter/6
- Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ, Stachenfeld NS. American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc 39(2):377-390, via PubMed (DOI 10.1249/mss.0b013e31802ca597): https://pubmed.ncbi.nlm.nih.gov/17277604/
How this page was built. This guide was drafted with AI assistance for Daily Water Calculator, and the 64 oz comparison was run through the calculator code, so the table matches the tool. The history of the 8 glasses idea comes from Valtin's review abstract and a Dartmouth press release about it. The fluid-limit figures come from the cardiology, kidney and liver sources listed below, and they're examples of what a care team may set, not advice. Last reviewed: .
General information for healthy adults, not medical advice. If you are pregnant or breastfeeding, or you have kidney, heart or liver conditions or a fluid limit, ask a clinician first. Terms and medical disclaimer.