How Many Ounces of Water a Day Should You Drink?

If your bottle and your hydration app are marked in ounces, here is the number in that unit: about 83 fl oz a day for a 154 lb (70 kg) adult. The calculator scales it to your own weight and shows the liters alongside.

The official numbers, in ounces

The Institute of Medicine set its 2005 reference values in liters, but published the ounce conversions with them. Adults 19 and older:

	Total water	Beverages only
Men	131 fl oz (3.7 L)	101 fl oz (3.0 L)
Women	95 fl oz (2.7 L)	74 fl oz (2.2 L)

The right-hand column is the one to fill a bottle against. National survey data put about 20% of total water in food, so the headline 131 and 95 already include what arrives in soup, fruit and vegetables.

Two ounce figures get quoted as if they were the same thing, and they are not: 131 is total water and 101 is what you drink. If you have seen “men need 125 ounces of water” somewhere, that is the total-water number rounded, not a drinking target.

Ounces by body weight

Weight	Water per day (mL)	In ounces
110 lb (50 kg)	1,750 mL	~59 fl oz
132 lb (60 kg)	2,100 mL	~71 fl oz
154 lb (70 kg)	2,450 mL	~83 fl oz
176 lb (80 kg)	2,800 mL	~95 fl oz
198 lb (90 kg)	3,150 mL	~107 fl oz

The arithmetic runs in two steps: body weight in kg times 35 mL, then divided by 29.5735 mL, the size of a US fluid ounce. For 70 kg: 70 x 35 = 2,450 mL, and 2,450 divided by 29.5735 is about 83 fl oz.

Treat the result as a target range, not a figure to hit to the ounce. Bottles, glasses and pitchers all carry their own error, and so does the rule itself.

US ounce, imperial ounce

Two fluid ounces are in use. The US fluid ounce is 29.5735 mL; the imperial (UK) fluid ounce is 28.4131 mL. The gap is a little over 1 mL each, negligible for a glass and noticeable across a day’s target. Water bottles and hydration apps sold in the United States use the US ounce, and so does this page.

Note that the “cup” in these conversions is 8 fl oz, or 237 mL — the IOM’s own conversion factor is 1 cup equals 8 fluid ounces.

Bottle sizes and how many refills

Bottle	Capacity	Refills for 83 fl oz
16 oz	~473 mL	~5.2
24 oz	~710 mL	~3.5
32 oz	~946 mL	~2.6
40 oz	~1,183 mL	~2.1
64 oz	~1,893 mL	~1.3

Counting refills beats counting ounces. Pick a bottle, work out how many times it has to be emptied, and you have converted a fussy number into a countable event.

The 64 oz bottle is worth flagging: one of those is the whole eight-by-eight rule in a single container, which is exactly why it sells. For most adults over 154 lb, one is not the day.

When ounces are the wrong unit

If the containers around you are marked in liters or you prefer round metric numbers, work from how many liters of water a day instead. If you track by counting drinks, how many glasses of water a day does the same math in glasses. And for the reasoning behind all three, the daily water intake calculator is the parent page.

For the ounce-to-millilitre conversion in the one place it has to be exact — infant feeding — see oz to mL for baby formula.

This content is informational and does not replace medical advice.