Fluid Ounces: US vs Imperial, and Why They Differ

Same name, two sizes

A US fluid ounce is exactly 29.5735295625 millilitres. An imperial fluid ounce, used historically in the United Kingdom and some other Commonwealth countries, is exactly 28.4130625 millilitres. The US ounce is therefore larger, by 1.16 mL, or about 4.1 per cent.

In everyday terms the difference is small. Pour out a single ounce of either and you would struggle to notice. The trouble starts when the ounce is the building block of larger units, or when a recipe, a bottle or a medicine label does not say which system it uses.

Do not confuse the fluid ounce with the ounce of weight. They share a name and a symbol (oz), but one measures space and the other measures mass, and neither converts to the other without knowing the substance.

Where the two systems come from

The US gallon traces back to the English wine gallon. It is defined as exactly 231 cubic inches, or 3.785411784 litres, and the US fluid ounce is one 128th of it.

Britain took a different route. In 1824 Parliament introduced the imperial gallon, originally defined by the volume of ten pounds of water under specified conditions. It was later refined to exactly 4.54609 litres. The imperial gallon is divided into 160 fluid ounces, and a fluid ounce of water was meant to weigh about one avoirdupois ounce. That tidy link between volume and weight is one reason the system was built the way it was.

Both countries kept the name “ounce” but divided differently: 128 to the gallon in the United States, 160 in the imperial system.

Pints, quarts and gallons are different too

The ounce difference is modest. The difference in the larger units is not, because the pint is built differently as well:

• US pint: 16 US fluid ounces = 473.176473 mL
• Imperial pint: 20 imperial fluid ounces = 568.26125 mL
• US gallon: 3.785411784 L
• Imperial gallon: 4.54609 L

The imperial pint is about 20 per cent larger than the US pint (568.26 ÷ 473.18 = 1.20), and so is the imperial gallon. A pint of beer in a British pub is about 95 mL more than an American pint, roughly 3.2 US fluid ounces.

Notice also that a pint holds 16 ounces in one system and 20 in the other. “A pint is 16 ounces” is true only for the US pint. The imperial pint is 20 of its own, slightly smaller ounces.

Worked conversions

Imperial to US ounces. A recipe from an older British cookbook calls for 10 fluid ounces of cream. That is 10 × 28.4130625 = 284.13 mL, which is 284.13 ÷ 29.5735 = 9.61 US fluid ounces. Rounding to 9½ or 10 US ounces will usually be fine in cooking.

US to imperial ounces. A 12-ounce US can holds 12 × 29.5735 = 354.88 mL, which is 354.88 ÷ 28.4131 = 12.49 imperial fluid ounces.

Same count, different amount. Eight US ounces is 236.59 mL; eight imperial ounces is 227.30 mL. The shortfall is 9.28 mL, about two thirds of a US tablespoon.

A whole pint. A British recipe asking for 1 imperial pint of milk needs 568.26 mL. That is 19.22 US fluid ounces, or 2.40 US cups — noticeably more than the 2 cups a US reader would assume from the word “pint”.

Factor to remember: multiply imperial fluid ounces by 0.9608 to get US fluid ounces, and US fluid ounces by 1.0408 to get imperial.

Where each is used today

The US fluid ounce is still the everyday unit for drinks, cooking and many package sizes in the United States. US nutrition labels use a separate convention in which a fluid ounce for serving sizes is rounded to 30 mL.

In the United Kingdom, metric units have largely taken over on packaging, and British law has limited the use of the fluid ounce in trade, though the pint survives for draught beer and cider and the older ounce is still found in cookbooks, patterns and family recipes. If a source is old, British or Commonwealth, and the unit is spelled out as an ounce, the imperial size is a reasonable guess, but check for a millilitre figure to be sure. The same caution applies to the pint, since the word alone does not tell you whether it means 473 mL or 568 mL.

Converting with a tool

The volume tab of a converter such as this one includes the US fluid ounce, the imperial pint and the imperial gallon, but not an imperial fluid ounce. You can still get there with a small step: one imperial pint is 20 imperial fluid ounces, so 10 imperial fluid ounces is 0.5 imperial pint. Enter 0.5, convert from imperial pint to US fluid ounce and you get 9.61.

Alternatively, multiply by 28.4130625 yourself to get millilitres and convert from there. Either route gives the same answer.

For medicines, chemicals and anything else where the amount is critical, do not convert from a vague “ounce”. Use the metric amount printed on the original label or instruction, and ask a professional if it is missing.