One US fluid ounce is exactly 29.5735295625 millilitres. The imperial fluid ounce is a different volume — 28.413 ml — and unlike the weight ounces, the American one is the larger of the two.
About the units
The US fluid ounce is 1/128 of a US gallon, which is defined as exactly 231 cubic inches. The imperial fluid ounce is 1/160 of an imperial gallon, and the imperial gallon was originally defined as the volume of ten pounds of water. Those two independent definitions produce values four per cent apart, and the direction is the reverse of what most people expect: the US fluid ounce is bigger, but the US pint contains 16 of them against the imperial pint's 20, so the imperial pint is the larger by twenty per cent. That crossover is why a pint of beer in London is noticeably more than a pint in New York. A third value exists on American nutrition labels, where the fluid ounce is rounded to exactly 30 ml.
The exact factor
The exact derivation is 3785.411784 ml per US gallon ÷ 128 = 29.5735295625 ml. The practical approximation of 30 ml is high by 1.4 per cent and is the value American food labelling uses officially. Reference points: 8 fl oz is one US cup at 236.6 ml, 16 fl oz is a US pint at 473.2 ml, and 32 fl oz is a US quart at 946.4 ml — just under a litre, which is why American and metric liquid packaging sizes look almost but not quite matched on a shelf.
Where you meet this conversion
Beverage packaging is where most people meet it: a 12 fl oz can is 355 ml and a 20 fl oz bottle is 591 ml. Cocktail recipes use fluid ounces throughout, and a jigger is 1.5 fl oz or 44 ml. Cosmetics and household liquids carry both figures by law in most markets. In cooking, the fluid ounce appears mainly in American recipes for liquids, where the cup is used for larger volumes and the tablespoon for smaller, leaving the fluid ounce as an intermediate that shows up more often on packaging than in instructions.