Ounces vs. Fluid Ounces: Why Mixing Them Gives Wrong Comparisons

Two honey containers sit side by side. Jar A says “12 oz” — a weight — and costs $5.49. Squeeze bottle B says “12 fl oz” — a volume — and costs $5.99. They look like the same amount, so A seems about 8% cheaper.

But honey is much heavier than water: roughly 1.4 times as dense (it varies a little by honey). Twelve fluid ounces of honey weigh about 17.5 oz. Compared properly:

Bottle B is about 25% cheaper per ounce of honey — the opposite of what the labels suggested.

You don’t need to know a product’s density by heart. The nutrition label often gives the serving size in both volume and weight, for example “1 tbsp (21 g)”. Use it to convert:

1. A tablespoon is about 14.8 mL, so 21 g ÷ 14.8 mL ≈ 1.42 g per mL.
2. Bottle B holds 12 fl oz × 29.57 mL ≈ 355 mL.
3. 355 mL × 1.42 g/mL ≈ 504 g, which is 504 ÷ 28.35 ≈ 17.8 oz.
4. Now enter the bottle as 504 g (or 17.8 oz) and compare it with the 12 oz jar.

Try it in the calculator below: if you leave one product in oz and the other in fl oz, it shows both prices but warns you instead of naming a winner.

An ounce (oz) is 1/16 of a pound — a weight, about 28.35 g. A US fluid ounce (fl oz) is 1/128 of a US gallon — a volume, about 29.57 mL. They share a name for historical reasons, but they measure different things, just like grams and milliliters do.

A fluid ounce of water weighs about 1.04 oz, so for water, milk, juice and soda the two numbers are close enough that the mix-up rarely matters much. The trouble starts with products that are much heavier or lighter than water: honey, syrups, nut butters and molasses are heavier; oils, whipped products and puffed snacks are lighter.

• Honey, maple syrup and pancake syrup — sold by weight in some packages and by volume in others.
• Sauces and dressings — usually fl oz, but some brands use oz.
• Yogurt and sour cream — usually oz (weight), while drinkable yogurt uses fl oz.
• Ice cream — sold by volume (quarts, pints, fl oz), even though it is a solid.
• Cooking oil — fl oz or liters; spray oils sometimes in oz.

• Always read the unit after the number: “oz” and “fl oz” look almost the same on a busy label.
• Compare products that use the same kind of unit; if they don’t, convert using the nutrition label as in example 2.
• Metric labels make this clearer: grams are weight, milliliters are volume. Many US labels also print the metric amount in brackets — use it.
• For drinks and liquids, use the price per fluid ounce calculator; for solid food, the price per ounce calculator.

More conversion factors are in the metric vs. US units cheat sheet.

The weight of one fluid ounce depends on what is in it. These approximate values show why water is the only case where ounces and fluid ounces nearly match:

For anything that isn’t close to water, a weight label and a volume label describe very different amounts. When in doubt, use the nutrition label to convert, as in example 2.

• Comparing a jar of peanut butter (oz) with a bottle of syrup (fl oz) as if both were ounces.
• Reading “16 oz” on a drink sign as weight — on menus and signs, drink sizes in “oz” almost always mean fluid ounces.
• Using the water rule (1 fl oz ≈ 1 oz) for heavy products like honey or molasses.