Two fluid ounces, neither an ounce of weight
The US fluid ounce is 1/128 of a US gallon, exactly 29.5735295625 ml. The imperial (UK) fluid ounce is 1/160 of an imperial gallon, 28.4130625 ml — about 4% smaller. Neither has anything to do with the ounce on a kitchen scale, which is 28.35 g; for water they happen to be close, for oil or honey they are not. US nutrition labels round the fluid ounce to 30 ml, which is why a “12 fl oz” can says 355 ml.
US fluid ounces = millilitres × 0.0338140227 | millilitres = US fluid ounces ÷ 0.0338140227
Rough conversion in your head
Divide millilitres by 30 for a quick US fluid-ounce figure: 250 ml → 8.3 fl oz (true 8.45). Multiply fluid ounces by 30 the other way: 16 fl oz → 480 ml (true 473). A US cup is 8 fl oz, a pint 16 and a quart 32, so 500 ml is just over a US pint but under a UK pint (568 ml).