One of English's more annoying coincidences: the word "ounce" refers to two completely different things. A weight ounce (oz) is about 28 grams. A fluid ounce (fl oz) is about 30 milliliters. They are not the same, they don't convert simply, and mixing them up can break a recipe, a dose, or a shipment. This guide sorts them out for good.
The two ounces, defined
Ounce (weight): 1/16 of a pound = 28.35 grams. Used for solids, produce, meat, flour, sugar, and anything you put on a scale. Sometimes abbreviated "oz" or "oz avdp" (avoirdupois).
Fluid ounce (volume): 1/128 of a US gallon = 29.57 milliliters. Used for liquids. Abbreviated "fl oz" in careful usage, often just "oz" when context is obvious (e.g., "12 oz can of soda").
The two only correspond for water, at which 1 fl oz of water weighs approximately 1 oz. For every other substance, the two numbers diverge.
Why they're different
Weight measures mass; volume measures how much space something occupies. For a given volume, denser substances (honey, mercury, lead) weigh more; less dense substances (oil, foam, flour) weigh less.
Examples:
- 1 fl oz of water ≈ 1 oz (29.57 mL × 1.00 g/mL = 29.57 g ≈ 1 oz weight)
- 1 fl oz of honey ≈ 1.44 oz weight (honey is denser)
- 1 fl oz of olive oil ≈ 0.92 oz weight (oil is less dense than water)
- 1 fl oz of flour ≈ 0.5 oz weight (lots of air between particles)
Where the confusion bites
Recipes. A recipe calling for "8 oz of flour" — is that 8 fluid ounces (1 cup by volume) or 8 ounces weight (227 g, roughly 1.9 cups)? The answer is almost always weight, but the notation is ambiguous. Modern cookbooks increasingly specify "8 oz (227 g)" to remove doubt.
Nutrition labels. US packages list the weight in ounces on the front and the volume in fluid ounces for liquids. A "16 oz" can of soup is 16 fluid ounces. A "16 oz" jar of peanut butter is 16 ounces by weight.
Cocktail recipes. "2 oz of whiskey" always means 2 fluid ounces (60 mL). Bartending is firmly volume-based.
Shipping. Package weight is in ounces (or pounds). Never fluid ounces.
Medicine doses. Liquid medicines use milliliters or fluid ounces. Solid medicines (tablets) use milligrams.
Quick identification rules
When you see "oz" in a recipe or product description, ask:
- Is it a liquid or a solid? Liquids → fluid ounces. Solids → weight.
- Is the quantity on a scale or in a measuring cup? Scale → weight. Measuring cup → volume.
- Does the source use "fl oz"? Yes → definitely volume.
- Does the label also show mL/L? That's your clue — it's volume.
Conversions each stays within
Weight ounces to metric: 1 oz = 28.35 g; 16 oz = 1 lb = 454 g; 1 kg = 35.27 oz.
Fluid ounces to metric: 1 fl oz = 29.57 mL; 8 fl oz = 1 cup; 128 fl oz = 1 US gallon; 1 L = 33.8 fl oz.
The UK fluid ounce is different
Another trap: the UK (imperial) fluid ounce is 28.41 mL, while the US fluid ounce is 29.57 mL. And an imperial pint is 20 fl oz (568 mL), not 16 fl oz like the US pint (473 mL). So a "16 oz pint" in the US holds less than a "20 oz pint" in the UK — and when a British pub says "a pint," they mean more beer than an American bar does. Yes, really.
Baking: why modern recipes use grams
To avoid all this confusion, most serious baking recipes now specify weights in grams (not ounces of either type). "250 g flour" is unambiguous. "1 cup flour" could be 115 g, 120 g, or 130 g depending on how it's packed. Scale-first baking has become standard among professional pastry chefs worldwide.
Density table for common liquids
- Water: 1.00 g/mL (baseline)
- Milk: 1.03 g/mL (slightly denser)
- Honey: 1.42 g/mL
- Maple syrup: 1.33 g/mL
- Olive oil: 0.92 g/mL (floats on water)
- Vodka (40% ABV): 0.95 g/mL
These let you convert fluid ounces to weight ounces for a given substance when the recipe demands it.
The takeaway
If it's a solid, "oz" means weight ounces (28 g each). If it's a liquid, "oz" almost always means fluid ounces (30 mL each). When the context is ambiguous, look for "fl oz" explicitly. And when precision matters, use grams for weight and milliliters for volume — the metric world has no equivalent confusion.
Our volume converter and weight converter handle each type separately, so you never cross streams. Two simple tools; one category of confusion eliminated. Your recipes, packages, and cocktails will thank you.