How this instrument works
The US fluid ounce traces back to the wine gallon that English law recognised for centuries and that America kept using after 1776, long after Britain replaced it with a larger imperial gallon in 1824. That old gallon holds 231 cubic inches by statute, and splitting it into 128 equal parts gives one US fluid ounce — a volume unit with no independent definition of its own, only a fraction of a container.
Metric precision arrived once the inch was pinned down. Since the 1959 international yard and pound agreement, an inch has equalled exactly 25.4 millimetres, so a cubic inch works out to 16.387064 cubic centimetres with nothing approximate about it. Multiply that by 231 and divide by 128 and 29.5735295625 millilitres appears — a terminating decimal because every input to the calculation terminates, even though the number itself looks anything but tidy.
The millilitre side of this pairing has its own repair story. Until 1964 a litre was defined as the space filled by one kilogram of pure water at its densest, which in practice ran to 1.000028 cubic decimetres rather than exactly one. That year the General Conference on Weights and Measures dropped the water reference and pinned the litre to exactly one cubic decimetre, so a millilitre and a cubic centimetre have matched perfectly ever since. Today the ounce rules US recipe cards and drink menus while millilitres cover pharmacy vials, wine bottling and any bar stocked with European or Australian spirits.
- Enter your volume into the US fluid ounces (floz) field — it opens at 8, one standard American measuring-cup fill.
- Read the answer straight off the Millilitres (ml) field beneath it; the figure updates with every keystroke.
- Working from a metric bottle instead? Divide its millilitre figure by 29.5735295625 to land back on fluid ounces.
- Small pours matter behind a bar, so the input accepts thousandths — try 1.5 for a standard spirit measure.
Worked example — 8 fl oz of syrup behind a bar
A cocktail bar in London imports a bottle of American-made grenadine labelled in fluid ounces, but every jigger and graduated pitcher behind the counter is stamped in millilitres. The recipe card calls for 8 fl oz of syrup for a batch of drinks. Enter 8 into US fluid ounces (floz) and Millilitres (ml) reads 236.5882365 — the bartender pours to that mark on a metric jug and the batch comes out right.
Checking the arithmetic by hand confirms it: 8 × 29.5735295625 = 236.5882365 exactly, with nothing rounded away at any step. Scaled down, a single 1.5 fl oz spirit measure comes to 44.36029434375 ml, close enough to the 44 ml pour lines etched into standard European jiggers that most bartenders never notice the sliver of difference.
Questions
Is a US fluid ounce exactly 29.5735295625 ml?
Yes, exactly, and it is a matter of legal definition rather than laboratory measurement. The US gallon has held at 231 cubic inches for centuries, one fluid ounce is a strict 1/128 of that gallon, and an inch has been exactly 25.4 mm since the 1959 international yard and pound agreement. Multiply those definitions through and 29.5735295625 millilitres appears with zero uncertainty — the only imprecision anywhere is in how carefully a person pours.
Why is the factor such an ugly number instead of something rounder?
Because nobody designed the US gallon around the metric system — the two simply collided later. Splitting 231 cubic inches into 128 equal fluid ounces, then converting each cubic inch to 16.387064 cubic centimetres, produces 29.5735295625 as an unavoidable byproduct. It happens to terminate cleanly at twelve decimal places only because every input to the calculation was itself an exact decimal; a slightly different gallon size would have left this figure running forever.
Does a UK or Australian fluid ounce give the same answer?
No — expect roughly a 4% gap. Britain abandoned the old wine gallon in 1824 for a larger imperial gallon of 4.54609 litres, divided into 160 fluid ounces rather than 128, which makes one imperial fluid ounce 28.4130625 ml, noticeably smaller than the US figure. Older British, Irish, Canadian and Australian recipes and pharmacy references mean the imperial ounce; anything printed in the United States means the 29.5735295625 ml version this page converts.
Is a fluid ounce the same as an ounce of weight?
Only by coincidence for water-like liquids, and not reliably even then. A fluid ounce measures volume — 29.5735295625 ml — while an avoirdupois ounce measures mass, 28.349523125 g, a completely separate system. Cool water fills a fluid ounce at close to 29.6 g, near enough that home cooks sometimes conflate the two, but honey, oil and syrup all weigh noticeably more or less per fluid ounce, so a recipe that mixes up the two ounces will throw off proportions fast.
Why do some labels round a fluid ounce to 30 ml?
US labelling rules allow rounded metric equivalents on packaging for readability, so 30 ml stands in for 29.5735295625 ml on many bottles and single-dose cups, a difference of under one and a half percent. That rounding is a printing convenience, not a redefinition — anywhere precision actually matters, such as compounding pharmacy work or calibrating a filling line, the full 29.5735295625 ml figure is what governs, and this page always returns that exact value.
How did the millilitre itself come to equal a cubic centimetre?
Not from the start. Between 1901 and 1964 a litre was legally the volume occupied by one kilogram of pure water at its point of maximum density, which measured out to 1.000028 cubic decimetres rather than exactly one — a tiny gap traceable to how precisely that mass of water could actually be weighed. The 12th General Conference on Weights and Measures scrapped the water-based wording in 1964 and fixed the litre at exactly one cubic decimetre, which is why a millilitre and a cubic centimetre have been perfectly interchangeable ever since.