Water Intake Calculator
Free, no signup, no email. Your daily target comes from the National Academies Adequate Intake for your age and sex, plus fluid lost to training and heat — and every number below shows where it came from.
The National Academies set adequate total water intake at 3.7 L (125 fl oz) a day for men and 2.7 L (91 fl oz) for women aged 19 and over. Food supplies about 19% of that, so from drinks alone the same report gives 3.0 L (3,000 mL / 101 fl oz, ~13 cups) for men and 2.2 L (2,200 mL / 74 fl oz, ~9 cups) for women — figures from the 2005 National Academies dietary reference intakes report.1 Training adds more: sweat rates run 0.5–4.0 L per hour according to the 2017 National Athletic Trainers' Association position statement on fluid replacement.3
Calculator
Adults only. Under 19, pregnant or breastfeeding?
Weight bounds the sweat estimate and feeds the clinical per-kg comparison line — it does not scale the reference value. Why
- Reference value, total water (age 32, female) 91 fl oz
- Minus ~19% that comes from food − 17 fl oz
Europe's figure is lower than the US one — different method, same question.
NICE gives 25–30 mL/kg/day for IV maintenance in hospital. This is where the "weight × 0.5 oz" rule comes from — not from any dietary reference value.
An estimate for healthy adults, not medical advice.
This is an estimate for healthy adults, not medical advice. Kidney disease, heart failure, diuretics, pregnancy complications and fever all change fluid needs in ways a general calculator cannot model — those belong with your clinician.
Under 19, pregnant or breastfeeding
The calculator above covers healthy adults aged 19 and over. For everyone else the National Academies publish reference values, but the adjustments around them — growth stage, trimester, milk output, medication — are a clinician's call, not a slider's. Here are the published numbers so you have them, without a personalised target attached.1
| Group | Total water / day | In fl oz |
|---|---|---|
| Girls 9–13 | 2.1 L | 71 fl oz |
| Boys 9–13 | 2.4 L | 81 fl oz |
| Girls 14–18 | 2.3 L | 78 fl oz |
| Boys 14–18 | 3.3 L | 112 fl oz |
| Pregnancy, 14–50 | 3.0 L | 101 fl oz |
| Lactation, 14–50 | 3.8 L | 129 fl oz |
These are total water figures, so they include what comes from food. For children the reference values also assume typical body size for the age band — a pediatrician is the right place to adjust them. Our terms put pregnancy and clinical conditions outside what this site advises on, and that applies to this tool too.
What already counts toward your target
Every beverage counts. In the US survey data behind the reference values, beverages of all kinds supplied about 81% of total water intake.1 Tick what you have had so far today.
On coffee: a 2014 crossover trial in habitual coffee drinkers compared 4 × 200 mL of coffee against the same volume of water and found no significant difference across hydration biomarkers, including total body water measured by deuterium dilution.4 Coffee counts.
Ate out today?
One fast-casual bowl can carry most of a day's sodium. The FDA Daily Value is less than 2,300 mg, and 20% DV or more in a single serving counts as high.5 These are dishes we have sourced numbers for.
Water intake by weight
If you specifically want the weight-based rule, here is where it actually comes from: NICE recommends 25–30 mL per kg of body weight per day of water for routine intravenous maintenance in hospital inpatients6 — roughly 0.38–0.46 fl oz per pound. No dietary reference value uses a per-kilogram coefficient at all. Read this chart as a sanity check next to your reference-value target above, not as a replacement for it.
| Weight | Weight (kg) | fl oz / day | mL / day | Liters / day |
|---|---|---|---|---|
| 110 lb | 50 kg | 42–51 oz | 1,250–1,500 mL | 1.3–1.5 L |
| 130 lb | 59 kg | 50–60 oz | 1,480–1,770 mL | 1.5–1.8 L |
| 150 lb | 68 kg | 57–69 oz | 1,700–2,040 mL | 1.7–2.0 L |
| 170 lb | 77 kg | 65–78 oz | 1,930–2,310 mL | 1.9–2.3 L |
| 190 lb | 86 kg | 73–87 oz | 2,150–2,580 mL | 2.1–2.6 L |
| 210 lb | 95 kg | 80–96 oz | 2,380–2,850 mL | 2.4–2.9 L |
| 230 lb | 104 kg | 88–105 oz | 2,600–3,120 mL | 2.6–3.1 L |
| 250 lb | 113 kg | 96–115 oz | 2,830–3,390 mL | 2.8–3.4 L |
How this is calculated
Most water calculators multiply your body weight by half an ounce and stop there. That rule is not wrong so much as unattributed: its actual provenance is hospital fluid prescribing — NICE clinical guideline CG174 gives 25–30 mL per kg per day for routine intravenous maintenance in inpatients6 — while the population reference values dietitians cite are set by age and sex, with no per-kilogram coefficient at all. So this page shows both, and names which is which.
Step 1 — reference value. Your Adequate Intake for total water comes straight from the 2005 National Academies table: 3.7 L for men 19+, 2.7 L for women. The values for ages 9–18, pregnancy and lactation are published in the reference table above rather than computed here.1
Step 2 — subtract food. Food supplied about 19% of total water intake in the same survey data, so roughly 81% of the reference value is what you drink. That is why the report lists 3.0 L of beverages for men and 2.2 L for women against totals of 3.7 and 2.7 L — and why running our multiplier over the reference values reproduces the report's own published beverage figures exactly.1 Note this is an observed proportion from survey data, not a normative split.
Step 3 — session replacement, kept separate. The 2017 NATA position statement reports exercise sweat rates spanning 0.5–4.0 L per hour.3 We split that published range into four bands by intensity and shift it upward in heat. Those bands are our estimate, not a figure any reference body published. Two consequences we build in: the session estimate is shown as fluid to replace around training rather than folded into a 24-hour target, and it is capped at 2% of your body mass — the loss threshold NATA tells athletes to stay inside, and the point past which you should be measuring rather than estimating. To measure: weigh yourself before and after, then sweat loss (L) = weight before (kg) − weight after (kg) + fluid drunk (L) − urine (L), divided by hours.3
Step 4 — pace it. At rest the kidneys excrete roughly 0.7–1.0 L per hour.1 The National Academies set no tolerable upper intake level for water in healthy people; what the report does note is that excess hypotonic fluid alongside low sodium intake can produce excess body water, hyponatremia and cellular edema. NICE makes the same point from the prescribing side, flagging that more than 2.5 L a day of intravenous maintenance raises hyponatraemia risk.6 Hence a pacing guide rather than a daily ceiling.
What we deliberately do not do: scale the reference value by your body weight. The National Academies did not publish a per-kg coefficient, so inventing one and presenting it as a reference-derived number would be fake precision. Your weight bounds the sweat estimate and drives the clinical comparison line — nothing else.
Correction, 26 July 2026: this page first shipped with the per-kilogram line at 30–40 mL/kg attributed to the ESPEN geriatrics guideline. That document contains no per-kilogram fluid coefficient. The figure now reads 25–30 mL/kg and cites NICE CG174, which does. The ESPEN guideline remains cited for what it actually states.7
FAQ
How much water should I drink a day?
The National Academies set an Adequate Intake for total water of 3.7 liters (3,700 mL / 125 fl oz) a day for men aged 19 and older and 2.7 liters (2,700 mL / 91 fl oz) for women. Those figures include water from food. Counting drinks alone, the same report puts typical intake at 3.0 liters (3,000 mL / 101 fl oz, about 13 cups) for men and 2.2 liters (2,200 mL / 74 fl oz, about 9 cups) for women, because food supplies roughly 19% of total water. Exercise and heat push the number up; the calculator on this page adds that on top.
How do I calculate how much water I should drink based on my weight?
There is no per-kilogram coefficient in any dietary reference value. The National Academies and EFSA both set intake by age and sex, not by body weight. The per-kilogram rule that consumer calculators use traces back to hospital fluid guidance: NICE recommends 25–30 mL per kg of body weight per day of water for routine intravenous maintenance in inpatients, which works out to roughly 0.38–0.46 fl oz per pound. That is a prescribing figure for people on a drip, not a drinking target for a healthy adult, so this calculator shows it as a labelled comparison line rather than as your answer.
Why does every water calculator give a different number?
Because the reference bodies themselves disagree. The National Academies set adequate total water intake at 3.7 L for men and 2.7 L for women. EFSA, using a different method that combines observed intakes with desirable urine osmolality, sets 2.5 L and 2.0 L for the same groups under moderate temperature and activity. Most calculators pick one basis silently, or use the weight rule with no reference at all. This page shows all of them and names the source for each.
Is 64 oz of water a day enough?
For an average adult woman, 64 fl oz of drinks is below the 74 fl oz that the National Academies report as typical beverage intake, and well below the 91 fl oz total water Adequate Intake once food is counted. For a man the gap is larger. That said, 64 oz is a floor, not a failure — thirst, urine color and climate matter more than hitting a round number, and the 8×8 rule has no reference-body basis behind it.
Is 100 oz of water a day too much?
For most healthy adults, no. 100 fl oz is close to the 101 fl oz of beverages the National Academies report as typical for men, and the National Academies set no tolerable upper intake level for water in healthy people. What is worth watching is the rate rather than the total: the kidneys excrete roughly 0.7 to 1.0 liters per hour at rest, and cases of water intoxication are reported when intake greatly exceeds that clearance, particularly alongside low sodium intake. As a practical ceiling, staying under about 24 fl oz (0.7 L) in any single hour keeps you inside that clearance. Clearance also falls during hard exercise, when thirst and body-weight change are better guides than any fixed number.
Does coffee count toward my water intake?
Yes. The National Academies count all beverages toward total water, and beverages of all kinds supplied about 81% of total water intake in the US survey data behind the reference values. A 2014 crossover trial in habitual coffee drinkers found no significant difference between 4 × 200 mL of coffee and the same volume of water across hydration biomarkers, including total body water measured by deuterium dilution.
Sources
- 1. Institute of Medicine (2005). Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. National Academies Press.
Adequate Intake for total water: 3.7 L/day men and 2.7 L/day women aged 19+; 3.0 L/day in pregnancy, 3.8 L/day in lactation. Beverages supplied ~81% of total water intake, food ~19%. Maximal renal excretion rate ~0.7–1.0 L/hour.
https://www.nationalacademies.org/read/10925/chapter/2 - 2. EFSA Panel on Dietetic Products, Nutrition and Allergies (2010). Scientific Opinion on Dietary Reference Values for water. EFSA Journal 8(3):1459.
Adequate Intake for total water: 2.5 L/day men, 2.0 L/day women, under moderate ambient temperature and physical activity level 1.6.
https://www.efsa.europa.eu/en/efsajournal/pub/1459 - 3. McDermott BP et al. (2017). National Athletic Trainers’ Association Position Statement: Fluid Replacement for the Physically Active. J Athl Train 52(9):877–895.
Sweat rates during exercise range 0.5–4.0 L/h. Keep body-mass loss under 2%. When recovery is shorter than 4 hours, replace up to 150% of the fluid deficit.
https://pmc.ncbi.nlm.nih.gov/articles/PMC5634236/ - 4. Killer SC, Blannin AK, Jeukendrup AE (2014). No Evidence of Dehydration with Moderate Daily Coffee Intake. PLoS ONE 9(1):e84154.
Habitual coffee drinkers consuming 4 × 200 mL coffee showed no significant difference from water across hydration biomarkers, including total body water measured by deuterium dilution.
https://pmc.ncbi.nlm.nih.gov/articles/PMC3886980/ - 5. U.S. Food and Drug Administration. Daily Value on the Nutrition and Supplement Facts Labels.
Daily Value for sodium is less than 2,300 mg per day. 5% DV or less per serving is low; 20% DV or more per serving is high.
https://www.fda.gov/food/nutrition-facts-label/daily-value-nutrition-and-supplement-facts-labels - 6. National Institute for Health and Care Excellence (2013, updated 2017). Intravenous fluid therapy in adults in hospital. NICE clinical guideline CG174.
Recommendation 1.4.1 gives 25–30 mL/kg/day of water for routine maintenance in hospital inpatients aged 16 and over. Recommendation 1.4.4 notes that prescribing more than 2.5 litres per day increases the risk of hyponatraemia. This is intravenous maintenance guidance for inpatients, not a drinking target for healthy adults — it is shown here only because it is where the per-kilogram rule used by consumer calculators actually comes from.
https://www.nice.org.uk/guidance/cg174/chapter/recommendations - 7. Volkert D et al. (2022). ESPEN practical guideline: Clinical nutrition and hydration in geriatrics. Clinical Nutrition 41(4):958–989.
Older women should be offered at least 1.6 L of drinks per day and older men at least 2.0 L, absent a clinical reason to differ. Note that this guideline sets no per-kilogram fluid coefficient — earlier drafts of this page attributed a 30–40 mL/kg figure to it, which the document does not contain. Corrected 26 July 2026.
https://www.espen.org/files/ESPEN-Guidelines/ESPEN_practical_guideline_Clinical_nutrition_and_hydration_in_geriatrics.pdf
Written and maintained by Alec Zakhary, who is a product manager, not a dietitian. Methodology is research aggregation from primary sources — see our editorial policy and source citation policy. Last reviewed 26 July 2026.