Your details
1,800 ml of drinks per day
70 kg × 26 ml/kg = 1,820 ml, then × 1.00 sex × 1.00 age × 1.00 activity × 1.00 climate, plus 0 ml, rounded to the nearest 50 ml.
Fluid ounces are US fluid ounces (29.5735 ml). Glasses are 250 ml. “Per waking hour” spreads the total over 16 hours. Results change as you move the controls. Without JavaScript the page shows the 70 kg default.
What the result covers
The result is drinks only, not total water intake. Food supplies about 20 per cent of total intake, and the 26 ml per kilogram base already subtracts that share. Do not subtract anything yourself.
Public-health reference values are totals for drinks plus food, so they run higher:
| Source | Women | Men | Includes food |
|---|---|---|---|
| EFSA, 2010[1] | 2.0 L/day | 2.5 L/day | Yes |
| U.S. National Academies, 2004[2] | 2.7 L/day | 3.7 L/day | Yes, about 80% arrives as drinks |
Both are population averages for healthy adults, not personal targets. This calculator scales with body weight instead: a 55 kg adult and a 110 kg adult do not get the same target.
Which inputs matter most
Weight and activity dominate. Moving from sedentary to very active multiplies the target by 1.5: for a 75 kg adult, 1,950 ml becomes 2,950 ml.
- Climate: ×1.1 to ×1.3. Humid heat costs more water than dry heat at the same temperature. Sweat that drips instead of evaporating removes water but little heat, so you sweat more for the same cooling.
- Sex: ±5%. A proxy for body composition. Lean tissue is about 75 per cent water; fat tissue holds far less; men average a higher lean fraction at the same weight.
- Age: ±5%. Lean mass falls after about 65, and the thirst signal weakens. Weaker thirst is a reason to drink on a schedule, not a reason to drink more.
- Pregnancy and breastfeeding: +300 to +1,000 ml. The largest single additions. Milk is mostly water, and milk production is a direct daily loss.
Altitude and illness both raise water losses, but neither has a multiplier reliable enough to state as a number, so they are not inputs.
Why BMI is not an input
BMI is weight divided by height squared: it says whether a person is heavy for their height. Water need tracks total body mass: how much tissue there is to keep hydrated and how much water leaves it each day.
Example: a 1.60 m person at 64 kg and a 1.90 m person at 90 kg both have a BMI of 25. A BMI-based formula would give them the same target. This formula gives 1,650 ml and 2,350 ml. Height adds nothing once weight is known. The one signal BMI could add, body composition, is already covered by the sex and age factors.
Limitations
The result is a starting point, not a prescription, and it is not medical advice. Kidney disease, heart failure, diabetes, and some medicines change how much fluid is safe to drink. Sometimes the safe amount is less, not more. If you are in treatment for any of those, use your clinician's figure instead. Too much water is not harmless either. Litres of plain water, taken faster than the kidneys can clear them, dilute blood sodium. That is dangerous.
The practical check: pale straw urine several times a day, and no lingering thirst, means the number is about right. Dark urine, or thirst that keeps coming back, means raise it. Adjust the target you got here by 200 ml or so and watch what happens over a week.
Common questions
Does this number include the water in food?
No. It is the amount to drink. Food supplies roughly a fifth of total water intake. The 26 ml per kilogram base already subtracts that share.
Do coffee and tea count toward the total?
Yes. The Beverage Hydration Index trial compared thirteen drinks against still water over four hours. Skimmed milk, full-fat milk and an oral rehydration solution held water in the body better than water did. Tea, coffee, cola, orange juice and a sports drink were not measurably different from water.[3]
How many glasses is that?
Divide the millilitre result by 250. The calculator does this for you and draws the glasses. A 2,000 ml target is eight glasses. With the Imperial switch on, the panel counts 8 fl oz cups instead.
Can I get the result in fluid ounces?
Yes. Use the Metric/Imperial switch on the calculator. The result panel always shows both millilitres and US fluid ounces, whichever unit leads.
Should I spread it through the day?
Yes. Your body passes most of one large volume straight through. It absorbs and holds the same amount better in smaller servings.
Track it in the app
A target only helps if you can see your progress against it. The WaterChamp app for iPhone and Android works out the same goal from the same six inputs. It then counts what you drink toward that goal. It has 62 drinks built in, each with its own hydration, caffeine, alcohol and calorie values. A glass of wine and a glass of water do not count the same. It also shows a live hydration level through the day and tracks caffeine as your body clears it. It keeps home and lock screen widgets up to date. It writes your water to Apple Health and Health Connect.
[1] EFSA Panel on Dietetic Products, Nutrition and Allergies. “Scientific Opinion on Dietary Reference Values for water.” EFSA Journal, 2010;8(3):1459. efsa.onlinelibrary.wiley.com
[2] Institute of Medicine (now the National Academy of Medicine). Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. National Academies Press, 2004. nap.nationalacademies.org
[3] Maughan RJ, Watson P, Cordery PA, et al. “A randomized trial to assess the potential of different beverages to affect hydration status: development of a beverage hydration index.” American Journal of Clinical Nutrition, 2016;103(3):717-723. pubmed.ncbi.nlm.nih.gov/26702122
