Fluid ounces and ounces: choose volume or mass before converting
Use the correct converter for fl oz and oz, and include density when changing between volume and mass.
Method
An ordinary ounce, written oz, measures mass. A fluid ounce, written fl oz, measures volume. The shared word does not make them interchangeable. Also identify the volume system: US and imperial fluid ounces are different. A precious-metal troy ounce is another mass unit and differs from the ordinary ounce. Read the full label before choosing a converter.
Worked example
For ordinary mass, 8 oz × 28.349523125 = 226.796185 G. For US volume, 8 US fl oz × 29.5735295625 = 236.5882365 mL. These numbers measure different quantities. If a liquid has density 0.9 G/mL, that volume has mass 236.5882365 × 0.9 = 212.92941285 G. The density assumption changes the volume-to-mass result; it does not change either unit conversion. Round only the final displayed result to match the precision of the original measurement.
Checks and limits
Use Weight for oz to G and Volume for US fl oz to mL. Use Density when a mass must be inferred from a volume, with density and volume units that agree. Do not reuse the density of water for oil, flour or another substance. Temperature, product composition and packing can matter, and a dry measuring cup may not provide a precise mass. The converter preserves the meaning of its chosen unit; it cannot identify a product’s density or correct an ambiguous package label.
Related calculators
Source: NIST — conversion factors