A fluid ounce (fl oz) measures volume, the space a liquid takes up. NIST lists the US fluid ounce as 29.57353 milliliters. It sits inside the US liquid system: 8 fluid ounces make a cup, 16 make a pint, 32 a quart and 128 a gallon. The ounce used on a kitchen scale is a different unit, a measure of weight.
Conversion
mL = fl oz × 29.57353
fl oz = mL ÷ 29.57353
For US nutrition labeling, regulation rounds the fluid ounce to 30 mL (21 CFR 101.9(b)(5)(viii)).
Example
A 12 fl oz can holds 12 × 29.57353 = 354.9 mL. In cups, 12 ÷ 8 = 1.5 cups. A 16.9 fl oz bottle holds 16.9 × 29.57353 = 499.8 mL, or about half a liter.
Use ml to oz and oz to ml for other values, and the guide to ounces in a cup for cooking measures.
Fluid ounce vs ounce
A fluid ounce is volume; an ounce is weight. They are only roughly equal for water-like liquids. The British imperial fluid ounce, about 28.41 mL, is also slightly smaller than the US one; see the ounces in a gallon guide.