Fluid Ounces to Milliliters

Fluid ounces are a volume, not a weight

This matters more in this direction than the other, because a fluid-ounce figure is often read off a package that also lists a weight. The two are different quantities:

• A fluid ounce is a volume. 8 fl oz of oil and 8 fl oz of honey fill the same measuring jug.
• An ounce of mass is about 28.35 grams. 8 oz of honey weighs the same as 8 oz of oil but takes up noticeably less room, because honey is denser.

There is no fixed millilitre value for an ounce of mass. If your figure came from a scale rather than a measuring jug, this page is not the right tool, and no volume converter can be.

Working it out by hand

Going this way you multiply, because the fluid ounce is the larger unit. The only thing worth knowing is where the multiplier comes from, and it is not a measurement anyone took. It falls out of three exact definitions.

The Imperial gallon is defined directly as 4.54609 litres, and a fluid ounce is one one-hundred-and-sixtieth of it, so 4,546.09 ÷ 160 = 28.4130625 mL.

A worked conversion

Take 8 fl oz, one US customary cup. Multiplying gives 8 × 29.5735295625 = 236.5882365 mL, shown as 236.59. Under the Imperial definition the same 8 fl oz is 8 × 28.4130625 = 227.30 mL. Note that this direction has no rounding of its own to worry about: multiplying a clean fluid-ounce figure by an exact constant lands on an exact millilitre value, and only the display shortens it.

Fluid ounces to milliliters reference table

Millilitre columns are rounded to two decimal places. The fluid-ounce figures on the left are the exact starting values.

The US pint, quart, and gallon rows line up exactly because those units are defined as 16, 32, and 128 US fluid ounces. The Imperial column simply restates the same count of ounces under the smaller Imperial definition, so it does not describe an Imperial pint except on the 20 fl oz row.

Why the label says 355 mL when the arithmetic says 354.88

This is the question that brings most people to a conversion in this direction, and the answer is duller than it looks. US packaging is sized in customary units and the metric figure beside it is that same volume rounded for print. A 12 fl oz can is 354.88235475 mL, printed as 355. A 16 fl oz bottle is 473.18 mL, printed as 473. A 32 fl oz carton is 946.35 mL, printed as 946.

Nothing has been re-sized and no second measurement was taken. The can holds what it holds; the two numbers on it are one volume expressed twice. It is worth knowing because the reverse inference does not hold: starting from the printed 355 mL and converting back gives 12.00 fl oz only because the rounding happens to be small, and for other sizes that round trip will not land as cleanly.

Edge cases

• Which pint you mean changes the answer by a fifth. A US liquid pint is 16 US fluid ounces, 473.18 mL. An Imperial pint is 20 Imperial fluid ounces, 568.26 mL. Neither is wrong; they are different units with the same name.
• The standard is a choice, not an assumption. US customary is the default, the toggle changes every figure on the page, and the result names which standard produced it.
• A blank field waits. Clearing the input leaves the result empty rather than treating the blank as zero and printing a confident 0.00 mL.
• Text that is not a number is refused. An entry like 12abc is rejected rather than read as 12. A converter built on a loose parser will silently do the latter.
• Negative volumes are refused. A volume below zero has no physical meaning, so it is declined rather than mirrored into the answer.
• This direction introduces no rounding of its own. Multiplying a whole number of fluid ounces by an exact constant lands on an exact millilitre figure. Only the two-decimal display shortens it, and the underlying value is unchanged.

Common mistakes

• Converting an ounce of weight. If the figure came from a scale, the millilitre answer depends on density and this page will overstate or understate it.
• Mixing the two pint definitions. A US pint is 16 fluid ounces and an Imperial pint is 20, so the two differ by far more than the roughly four percent between the ounces themselves.
• Dividing instead of multiplying. Fluid ounces are the larger unit, so for a positive input the millilitre figure comes out larger.
• Reading a rounded 355 mL figure as exactly 12 US fluid ounces. 12 fl oz is 354.88 mL, so 355 is the rounded form.

Your figures stay in the page

The values you enter are processed by this page in your browser. They are not sent to a server, not stored after you close the tab, and not visible to anyone else. There is no account and nothing to sign up for.

Sources

The multiplication itself needs no authority. What does is the three defining constants it rests on, and each is set by a body rather than chosen here. That is the whole of what these citations are for.

• NIST Special Publication 811, Appendix B.9, which gives the inch as exactly 2.54 cm. With the US liquid gallon of 231 cubic inches from NIST Handbook 44, Appendix C, and 128 fluid ounces to the gallon, the US fluid ounce works out to 29.5735295625 mL.
• Weights and Measures Act 1985, Schedule 1, which defines the gallon as 4.54609 cubic decimetres and the fluid ounce as one twentieth of a pint, giving 28.4130625 mL.

NIST also lists a rounded 2.957 353 E-05 cubic metres for the US fluid ounce, which it does not mark exact. That is the same definition shown to seven digits.

Related converters

• Milliliters to fluid ounces for the reverse, when the figure you have is metric.
• Volume converter for cups, pints, quarts, gallons, litres, and cubic units.
• How many mL in a liter for the metric side on its own.