Fluid Ounce vs Ounce
A US fluid ounce is 29.5735 millilitres of volume, while an ounce is 28.3495 grams of mass; there is no fixed conversion between them.
By Piero Nanni · Last checked
Why the two are unrelated
Converting between them requires knowing the density of the substance, which the units themselves do not carry. Water is the case where they nearly coincide: one US fluid ounce of water weighs about 29.6 g, close enough to one ounce of mass that the two got conflated.
For anything else the relationship falls apart. One fluid ounce of flour weighs about 15 g, of honey about 42 g, of cooking oil about 27 g. Treating a weight in ounces as a volume in fluid ounces can be off by a factor of three.
| Substance | Approximate mass | In ounces of mass |
|---|---|---|
| Water | 29.6 g | 1.04 oz |
| Milk | 30.5 g | 1.08 oz |
| Cooking oil | 27.2 g | 0.96 oz |
| Honey | 42.0 g | 1.48 oz |
| All-purpose flour, spooned | 15.5 g | 0.55 oz |
Spotting which one a package means
US packaging is usually explicit: liquids are labelled "fl oz" and solids "oz" or "net wt oz". A 12 oz bag of coffee is 340 g of coffee, while a 12 fl oz can of soda is 355 mL of liquid. The numbers look parallel and describe entirely different quantities.
The clearest tell is what the product is. If it pours, the ounce on the label is almost certainly fluid. If it is a solid or a powder, it is mass.
The imperial fluid ounce is different again
The UK imperial fluid ounce is 28.4131 mL against the US 29.5735 mL, so it is about 4 percent smaller. It was originally defined as the volume of one ounce of water, which is why it lands so close to the mass unit.
That definition is also why the numbers nearly match in imperial but not in US customary: the imperial system built the connection in deliberately, and the US system inherited a gallon that predates it.
Frequently Asked Questions
How many ounces are in a fluid ounce?
There is no fixed answer. It depends on density. For water, one US fluid ounce weighs about 1.04 ounces.
How many mL is a fluid ounce?
29.5735 mL in the US, 28.4131 mL imperial.
Is a 12 oz bag of coffee the same as 12 fl oz?
No. That is 12 ounces of mass, 340 g. 12 fluid ounces would be 355 mL of volume.
Why do fluid ounces and ounces nearly match for water?
The imperial fluid ounce was defined as the volume of one ounce of water, and the US fluid ounce happens to land close to the same value.