The US measures are 8 fl oz to a cup, 16 fl oz to a pint, 32 fl oz to a quart and 128 fl oz to a gallon. The UK measures are 20 fl oz to a pint, 40 fl oz to a quart and 160 fl oz to a gallon. 1 fl oz UK is equal to 0.9607584 fl oz US.

Depending upon where you are: In UK: 28.4 ml ≈ 1 fl oz → 15 ml ≈ 15 ÷ 28.41 fl oz ≈ 0.53 fl oz In USA: 29.57 ml ≈ 1 fl oz → 15 ml ≈ 15 ÷ 29.57 fl oz ≈ 0.51 fl oz