Water Intake Formulas Compared (and How Our Calculator Works)

Half your weight, two-thirds, or mL per kg: see how each water rule works and how far apart they land.

By Cedrick Reese, Daily Water Calculator

Short answer

This site doesn't use "half your body weight in ounces of water." It uses the metric rule of thumb behind it: your weight in kilograms times 32.5 mL for drinks, plus 0.6 liters for every hour of exercise, rounded to the nearest 50 mL. For a 160 lb adult who exercises 30 minutes, that's 2,650 mL (90 fl oz). Every figure below is labeled as either a rule of thumb or a published number, and none of it replaces advice from your own clinician.

The "half your body weight in ounces" rule

The most repeated rule online is simple: take your weight in pounds, divide by two, and drink that many ounces of water a day. A 160 lb person lands on 80 oz. A 2019 health system article from ThedaCare calls it a good rule of thumb, and WaterMinder's calculator page uses the same baseline. Neither page names an outside study behind it, and we couldn't find a published guideline that sets it. Treat it as folk wisdom that happens to land in a sensible range.

It also happens to have a metric twin. Half a pound of water per pound of body weight works out to about 32.6 mL per kg. That sits in the middle of the 30 to 35 mL per kg range people use when they work in kilograms.

What is the 2/3 rule?

The "2/3 rule" is a popular variant: drink two thirds of your body weight in pounds as ounces of water. We searched for a reliable description of where it comes from and didn't find one, so we can't tell you who proposed it or what it was tested against. What we can do is show what it produces. For 160 lb, two thirds is about 107 oz, or roughly 3.15 L. That's about 33 percent more than the half-weight rule, and it lands above the National Academies' figure for drinks even for men (more on that below). If you see it presented as an official standard, be skeptical.

35 ml per kg and where it stands

The metric version says to drink somewhere between 30 and 35 mL per kilogram of body weight per day. At 160 lb (72.6 kg) that's 2.18 L at 30 mL/kg and 2.54 L at 35 mL/kg.

We looked for a citable guideline that states this range for healthy adults and didn't find one we could read and confirm. So here is the honest status: it's a common rule of thumb, not a guideline. What we did instead was check it against a published number. The National Academies report adequate intakes of about 2.7 L (91 oz) of total water for women and 3.7 L (125 oz) for men, with about 80 percent of that coming from drinking water and other beverages. The remaining 20 percent comes from food.

How our calculator works

This is the exact method every calculator on the site uses, including the main calculator and the Water Intake by Weight Chart.

Base: body weight in kg times 32.5 mL. The range is 30 to 35 mL per kg, and 32.5 is the midpoint.
Exercise: hours of exercise times 0.6 L. The American College of Sports Medicine position stand gives 0.4 to 0.8 L per hour as a possible starting point for marathon runners who start well hydrated. We use the midpoint, 0.6 L, for any kind of exercise, which is our own simplification. The same position stand says sweat rates vary a lot between people and recommends weighing yourself before and after exercise to find yours.
Hot weather: use the top of both ranges, 35 mL per kg and 0.8 L per hour.
Rounding: round the total to the nearest 50 mL. Every other unit (fluid ounces, liters, cups, bottles) is converted from that rounded mL figure, so the numbers always agree with each other.
Food: the result is a target for drinks. Total water including food is the drinks target divided by 0.8, based on the 80 percent figure above.

Conversions use NIST's factors: a US fluid ounce is 29.57353 mL, a US cup is 236.5882 mL, and a US gallon is 3.785412 L. A pound is 0.4535924 kg.

The calculator is for adults and has no age or sex inputs. That's deliberate: neither changes the formula, so adding the fields would only imply precision that isn't there.

Worked example: 160 lb, 30 minutes of exercise, typical weather

160 lb is 72.6 kg.
72.6 kg times 32.5 mL is about 2,359 mL.
30 minutes is 0.5 h, and 0.5 h times 600 mL is 300 mL.
Total is about 2,659 mL, which rounds to 2,650 mL.
That equals 90 fl oz, 2.65 L, 11.2 cups, or 5.3 bottles of 500 mL.
The range uses the low end of both ranges (30 mL/kg and 0.4 L/h) and the high end of both (35 mL/kg and 0.8 L/h): 2,400 to 2,950 mL.
Total water including food would be 2,650 divided by 0.8, about 3.3 L.

The methods side by side for a 160 lb adult

Here is every method run on the same person: 160 lb, or 72.6 kg. The last column matters as much as the numbers.

Daily drinks target for a 160 lb adult by method
Method	Calculation	Liters	Fluid oz	What it is
Half your body weight in ounces	160 ÷ 2	2.37	80	Rule of thumb
2/3 rule	160 × 2/3	3.15	107	Rule of thumb, origin unverified
30 mL per kg	72.6 × 30 mL	2.18	74	Rule of thumb
35 mL per kg	72.6 × 35 mL	2.54	86	Rule of thumb
National Academies, women (drinks)	Published drinks figure (about 80% of 2.7 L)	2.2	74	Published adequate intake, not scaled to weight
National Academies, men (drinks)	Published drinks figure (about 80% of 3.7 L)	3.0	101	Published adequate intake, not scaled to weight
This site, no exercise	72.6 × 32.5 mL, rounded	2.35	79	Rule of thumb, cross-checked
This site, 30 minutes of exercise	2,359 + 300 mL, rounded	2.65	90	Rule of thumb, cross-checked

The same numbers as a chart: liters per day for a 160 lb adult

Half your body weight in ounces2.37 L
2/3 rule3.15 L
30 mL per kg2.18 L
35 mL per kg2.54 L
National Academies, women (drinks)2.2 L
National Academies, men (drinks)3.0 L
This site, no exercise2.35 L
This site, 30 minutes of exercise2.65 L

The National Academies figures are published averages, not scaled to weight. The bars start at zero and the scale ends at 3.5 L.

Two things stand out. First, the weight-based rules cluster between roughly 2.2 and 2.5 L, with the 2/3 rule as the outlier. Second, our no-exercise number of 2.35 L sits between the National Academies figures for women and men, which is reassuring for a person of average build. Set the site's formula at 32.5 mL per kg and it lands within about 50 mL of the women's drink figure (2.2 L) at about 147 lb, and the men's (3.0 L) at about 201 lb. Those two weights are a sanity check, not a claim that the National Academies derived anything from body weight.

How other calculators differ

Online calculators don't agree because they aren't measuring the same thing. Two examples, described as their pages read when we checked:

WaterMinder starts with pounds divided by two for ounces, then adds 12 oz for every 30 minutes of sweating. For heat it gives two different rules on the same page, add 16 to 32 oz or raise needs by 25 to 50 percent, and it doesn't reconcile them. For our 160 lb example with 30 minutes of exercise, the exercise rule gives 80 + 12 = 92 oz (2.72 L), against our 90 oz (2.65 L). Close, since the starting points are close.
Omni Calculator asks whether you are a child or an adult and your sex, then looks up the National Academies figure. Weight and activity aren't inputs. That makes it a faithful reading of a published number, but two adults of very different size get the same answer.

We chose the weight-and-exercise approach because it responds to the two things most people actually vary on, while staying checkable against a published figure.

When the formula does not apply

The calculator is built for healthy adults, and a population-level formula can't account for individual medical circumstances. It shouldn't be treated as a target in any of these situations:

Pregnancy or breastfeeding. Mayo Clinic says people who are pregnant or breastfeeding may need more fluids. The pregnancy water intake calculator shows the National Academies figures for pregnancy and breastfeeding.
Kidney, heart or liver conditions, or a doctor-set fluid limit. Some conditions call for restricting fluid rather than increasing it. A general formula has no way to know that.
Illness. Mayo notes that fever, vomiting and diarrhea cause fluid loss, which changes the picture.
Children under 18. The calculator is adults only.
Long endurance exercise. The 0.4 to 0.8 L per hour range is the ACSM starting point for marathon runners, so treat it as a rough guide for long runs and a weaker fit for other kinds of exercise. The ACSM position stand recommends customized plans because sweat rates and sweat electrolyte content vary considerably between people, and says you can estimate your own sweat rate by weighing yourself before and after exercise.

Mayo says there is no simple, one-size answer, and that needs depend on factors like health, age, activity level, location and weather. A calculator gives a starting estimate. Your health professional can say what is right for you.

Keep reading

Frequently asked questions

Is "half your body weight in ounces" an official recommendation?

No. It's a widely shared rule of thumb. The pages we read that use it, including ThedaCare's, call it a rule of thumb and don't name a study behind it. It converts to about 32.6 mL per kg, which is why it lines up with the 30 to 35 mL per kg convention.

Does the 2/3 rule give more water than the half-weight rule?

Yes, by a third. For 160 lb, half body weight is 80 oz (2.37 L) and two thirds is about 107 oz (3.15 L). We couldn't verify where the 2/3 rule comes from, so we don't use it.

Why does the calculator not ask for age or sex?

Because they don't change the formula. The National Academies publish different totals for women and men. At 147 lb and 201 lb our weight-based method lands within about 50 mL of their drink figures, 2.2 L for women and 3.0 L for men. Those aren't average weights, since CDC data put the average adult at about 172 lb for women and 199 lb for men. Adding the inputs would suggest a precision the evidence doesn't offer.

Do coffee, tea and food count?

The National Academies figure that about 80 percent of total water comes from drinking water and beverages, with the rest from food. Our result is a drinks target, so beverages other than plain water count toward it. If you want total water including food, divide the drinks target by 0.8.

Can I drink too much while exercising?

Yes. The 2015 consensus statement on exercise-associated hyponatremia ties it to drinking more than you lose, and the ACSM position stand flags losing more than 2 percent of body weight as a performance risk. That's why the exercise add-on is a range and a starting point, not a mandate to drink on a schedule.

Why is my hot-weather number higher?

In hot weather the calculator uses the top of both ranges: 35 mL per kg and 0.8 L per hour. The National Academies note that very active people and people in hot climates may need more water than their reference amounts.

Sources

National Academies of Sciences, Engineering, and Medicine, "Report Sets Dietary Intake Levels for Water, Salt, and Potassium to Maintain Health and Reduce Chronic Disease Risk" (news release, 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, "Water: How much should you drink every day?": https://www.mayoclinic.org/healthy-lifestyle/nutrition-and-healthy-eating/in-depth/water/art-20044256
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/
NIST Special Publication 811, Appendix B.8, Factors for units listed by kind of quantity or field of science (volume): https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b8
ThedaCare, "How much water should you be drinking?": https://thedacare.org/news-and-events/healthy-living/how-much-water-should-you-be-drinking/
WaterMinder, "Water intake for 120 lbs": https://waterminder.com/calculator/water-intake-for-120-lbs.html
Omni Calculator, "Water Intake Calculator": https://www.omnicalculator.com/health/water-intake
National Academies Press. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate, 2005. Water chapter: https://www.nationalacademies.org/read/10925/chapter/6
Centers for Disease Control and Prevention, National Center for Health Statistics, "Body Measurements" (mean weight of adults 20 and over, August 2021 to August 2023): https://www.cdc.gov/nchs/fastats/body-measurements.htm
Hew-Butler T, Rosner MH, Fowkes-Godek S, et al. Statement of the 3rd International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. British Journal of Sports Medicine 49(22):1432 to 1446. Citation details via PubMed: https://pubmed.ncbi.nlm.nih.gov/26227507/

How this page was built. This guide was drafted with AI assistance for Daily Water Calculator, and every method was run on the same 160 lb example in code, so the table values are computed, not copied. Pages from other sites, such as WaterMinder and Omni Calculator, are described as they read when we checked. The National Academies, Mayo Clinic and NIST statements come from the linked pages, and the ACSM statement comes from its PubMed abstract. Where we found no source for a rule, this page says so. Last reviewed: October 16, 2026.

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.