How much water to drink a day
The most widely cited figures come from the Institute of Medicine, now part of the National Academies, which published dietary reference intakes for water in 2005. For adults aged 19 and over, the Adequate Intake for total water is 3.7 litres a day for men and 2.7 litres a day for women. Those numbers are the source of nearly every "how much water should I drink" answer on the internet, usually reproduced without the two caveats that matter most.
The first caveat is that these are figures for total water, not for drinks. The Institute of Medicine estimated that roughly 20 percent of total water intake arrives in food, with the remaining 80 percent from beverages of all kinds, including tea, coffee, milk and soft drinks. So the beverage figure is nearer 3.0 litres for men and 2.2 litres for women. Pages that tell you to drink 3.7 litres of water are quietly asking for about 20 percent more than the reference actually says.
The second caveat is what an Adequate Intake actually is. It is set when there is not enough evidence to establish a Recommended Dietary Allowance, and it describes the observed median intake of an apparently healthy population. It is a description of what people do, not a threshold you fall below at your peril. The Institute of Medicine was explicit that a wide range of intakes is compatible with normal hydration.
The published reference values
These are the Adequate Intake values for total water, reproduced from the 2005 dietary reference intakes. The beverage column applies the 80 percent share described above.
| Age and sex | Total water | Total (US oz) | From beverages |
|---|---|---|---|
| Men, 19 and over | 3.7 L | 125 oz | 3.0 L / 100 oz |
| Women, 19 and over | 2.7 L | 91 oz | 2.2 L / 73 oz |
| Males, 14 to 18 | 3.3 L | 112 oz | 2.6 L / 89 oz |
| Females, 14 to 18 | 2.3 L | 78 oz | 1.8 L / 62 oz |
| Males, 9 to 13 | 2.4 L | 81 oz | 1.9 L / 65 oz |
| Females, 9 to 13 | 2.1 L | 71 oz | 1.7 L / 57 oz |
Pregnancy and lactation carry their own reference values, 3.0 and 3.8 litres of total water respectively. This calculator does not cover them, because fluid intake during pregnancy and breastfeeding is a conversation to have with a midwife or doctor who knows your circumstances, not a checkbox on a web form.
The formula for how much water to drink
Search for a water intake formula and you will be given roughly 35 mL per kilogram of body weight, or its US cousin, "drink half your body weight in ounces." A 170 pound person gets 85 ounces from the second rule and about 2.7 litres, roughly 90 ounces, from the first. They are close enough to be the same rule in different clothes.
It is worth being straight about the status of this formula: it is a practical heuristic with no dietary reference intake behind it. No official body expresses fluid needs per kilogram of body weight for healthy adults. It survives because it is easy to remember and because it scales with body size, which the flat reference values do not. A 50 kilogram woman and a 100 kilogram woman get the same 2.7 litre reference figure, which is obviously a simplification.
That is why this calculator reports a range bracketed by both methods rather than picking a winner. If the two figures are close, you have a narrow range and a fairly clear answer. If they are far apart, which happens at the extremes of body weight, the width of the range is itself the useful information: it tells you the honest answer is uncertain.
Adjusting for exercise and heat
Sweat losses vary enormously between people, between sports, and between conditions. Published sweat rates for athletes span roughly 0.3 to 2.4 litres per hour, a range wide enough that any single multiplier is a rough approximation. This calculator adds 0.4 litres per 30 minutes of exercise, which sits in the middle of commonly used practical guidance, and a flat 0.5 litres for hot conditions or heavy sweating.
If you want a figure specific to you rather than to the middle of a distribution, there is a genuinely accurate method and it does not involve a calculator: weigh yourself before and after a training session, and each kilogram lost is approximately a litre of fluid to replace. That measurement is worth more than any estimate this page can produce, because it measures you rather than a population.
Hydration while fasting
This calculator lives on a fasting site for a reason. Around 20 percent of your daily water normally arrives in food, and a fasting window removes that source for part of the day. On a 16:8 schedule you are eating across 8 hours instead of 14 or 15, so the food contribution is compressed into a shorter period rather than eliminated. On longer fasts it disappears for the duration.
The practical implication is a scheduling one, which is the kind we are willing to state. Plain water does not break a fast under any common definition, so there is nothing stopping you drinking through the fasting window, and many people find that easier than trying to make up the difference during a short eating window. If you want to work out exactly when your windows fall, the fasting window calculator handles that, and the break-fast timing page covers what does and does not end a fast.
What we will not tell you is that drinking more water accelerates fat loss, suppresses appetite to a useful degree, or flushes toxins. Those claims turn up constantly on hydration pages and the evidence behind them is thin, mixed, or absent. If a page is confident about all three, that is a signal about the page rather than about water.
A better test than any calculator
Healthy kidneys regulate fluid balance continuously and thirst is a reasonably good signal in adults without medical conditions affecting it. Two everyday indicators beat any computed target: drink when thirsty, and check urine colour, where pale straw generally suggests adequate hydration and dark yellow suggests drinking more. Neither requires a calculator, and both respond to today rather than to a population average.
Thirst becomes a less reliable guide in older adults, and some medications and conditions alter both thirst and fluid handling. Those are exactly the situations where a number from a web page is least useful and a conversation with a clinician is most useful. This calculator exists to give you a reasonable reference point and to be clear about its limits, which is more than the tools currently ranking for this question tend to offer.
Where these numbers come from
Reference values are taken from the Institute of Medicine, Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate (2005), Chapter 4. The 80 percent beverage share and the note that a wide range of intakes is compatible with normal hydration both come from the same report. The body-weight heuristic is labelled as a heuristic throughout, because that is what it is.
The reference values and their source are stored as exported constants in the calculation engine rather than buried in page copy, so the provenance travels with the code. If the reference data ever changes, the model version changes with it.