Three ounces with one name
US kitchens use two different units called ounce, and British recipes add a third. Appendix C of NIST Handbook 44, the general tables of US units, lists them in separate tables: the fluid ounce under liquid volume, the ounce under avoirdupois mass.
| Unit | Measures | Defined as | Metric value |
|---|---|---|---|
| US fluid ounce (fl oz) | Volume | 1/128 of a US gallon of 231 cubic inches | 29.5735295625 ml |
| Avoirdupois ounce (oz) | Weight (mass) | 1/16 of a pound of 0.45359237 kg | 28.349523125 g |
| Imperial fluid ounce | Volume | 1/20 of a UK pint, 1/160 of a UK gallon of 4.54609 liters | 28.4130625 ml |
The US values follow from the definitions. With the inch fixed at exactly 2.54 cm, a cubic inch is 16.387064 ml, so the 231 cubic inch gallon is 3,785.411784 ml and a fluid ounce is that divided by 128. NIST's SP 811 conversion table rounds it to 29.57353 ml and the ounce to 28.34952 g. Handbook 44 also gives the cooking units: a cup is 8 fluid ounces, a tablespoon half a fluid ounce, a teaspoon a third of a tablespoon.
The UK definitions come from the Weights and Measures Act 1985. The imperial fluid ounce is about 3.9 percent smaller than the US one, but the UK pint holds 20 of them against 16 in a US pint, so a UK pint is 568.26 ml and a US pint 473.18 ml. The weight ounce is the same in both countries.
One more ounce exists: the troy ounce of 31.1034768 g, used for precious metals. It never appears in recipes, and Handbook 44 says that an ounce written without a system name is understood to be avoirdupois.
The first two are fixed and work for anything. The last two need a figure for the ingredient, because that is where volume and weight part ways.
Why 8 fl oz of flour is not 8 oz
A cup is 8 fluid ounces whatever you fill it with. What it weighs depends on how dense the ingredient is: how many grams fit into each milliliter. Water is close to 1 gram per milliliter. Flour is full of air between the particles, so the same cup holds about half that weight.
The cups to grams converter takes its weights from two sources. Dry baking ingredients come from King Arthur Baking's ingredient weight chart, which lists a cup of all-purpose flour at 4 1/4 ounces or 120 grams. Liquids, fats and salt come from USDA FoodData Central. Put into ounces:
- A cup of all-purpose flour weighs 120 g, which is 4.23 ounces. Eight fluid ounces of flour weigh about half of 8 ounces.
- A cup of water weighs 237 g, 8.36 ounces. Here the two numbers nearly agree.
- A cup of honey weighs 339 g, 11.96 ounces. Eight fluid ounces of honey weigh half again as much as 8 ounces.
Flour also shows that the volume itself is not fixed. King Arthur's guide to measuring flour says a cup holds around 120 grams when the flour is fluffed up and leveled, and can hold up to 160 grams when it is packed down. That is a third more flour from the same cup, which is why a scale gives more repeatable results than a cup.
Weight of 8 fluid ounces, ingredient by ingredient
Each row is one US cup, 8 fluid ounces, with the gram figure from the cups to grams data and its source. The last column turns it around: how many fluid ounces you need to get 8 ounces by weight.
| Ingredient | Grams per cup | Ounces by weight | g per ml | Fl oz for 8 oz | Source |
|---|---|---|---|---|---|
| Honey | 339 | 11.96 | 1.43 | 5.4 | USDA |
| Maple syrup | 315 | 11.11 | 1.33 | 5.8 | USDA |
| Table salt | 292 | 10.30 | 1.23 | 6.2 | USDA |
| Milk | 244 | 8.61 | 1.03 | 7.4 | USDA |
| Heavy cream | 238 | 8.40 | 1.01 | 7.6 | USDA |
| Water | 237 | 8.36 | 1.00 | 7.7 | USDA |
| Butter | 227 | 8.01 | 0.96 | 8.0 | USDA |
| Vegetable oil | 218 | 7.69 | 0.92 | 8.3 | USDA |
| Brown sugar (packed) | 213 | 7.51 | 0.90 | 8.5 | King Arthur |
| Granulated sugar | 198 | 6.98 | 0.84 | 9.2 | King Arthur |
| Chocolate chips | 170 | 6.00 | 0.72 | 10.7 | King Arthur |
| All-purpose flour | 120 | 4.23 | 0.51 | 15.1 | King Arthur |
| Powdered sugar | 113 | 3.99 | 0.48 | 16.1 | King Arthur |
| Rolled oats | 89 | 3.14 | 0.38 | 20.4 | King Arthur |
| Cocoa powder | 84 | 2.96 | 0.36 | 21.6 | King Arthur |
Three groups stand out. Syrups and salt are heavier than water, so fewer than 8 fluid ounces weigh 8 ounces. Milk, cream, water and butter sit right around 1 gram per milliliter, so a cup weighs close to 8 ounces. From oil down, everything is lighter than water, and the four fluffy ingredients at the bottom need about two to nearly three cups to make 8 ounces. Oil is the only liquid in the table that is lighter than water: 218 g a cup, 7.69 ounces.
Weights for packed brown sugar, chocolate chips and flour depend on how you fill the cup. The figures in the table assume the method behind each source, so treat them as a good average rather than a promise for your cup.
When the two ounces almost match
For water and the watery liquids, mixing up the two ounces costs little. USDA FoodData Central lists 29.6 grams for a fluid ounce of water, which is 1.044 ounces by weight, about 4 percent more than an ounce. Milk is 1.08 ounces per fluid ounce. Butter is the neatest case: USDA lists 227 grams for a cup and 113 grams for a stick, so a cup weighs 8.01 ounces and a half-cup stick about 4.
Imperial units were built around water on purpose. Handbook 44 notes that the imperial gallon is defined as the volume of 10 avoirdupois pounds of water under specified conditions. A gallon holds 160 imperial fluid ounces and 10 pounds are 160 ounces, so an imperial fluid ounce of water weighs about one ounce. The US fluid ounce has no such link.
The nutrition-label cup: 240 ml
Look at a US nutrition facts panel and the serving might read "1 cup (240 mL)". That is not your measuring cup. For nutrition labeling, FDA rules say a teaspoon means 5 ml, a tablespoon 15 ml, a cup 240 ml and a fluid ounce 30 ml, and 1 ounce in weight means 28 g (21 CFR 101.9(b)(5)(viii)). The same rule asks labels to give serving sizes in cups, tablespoons or teaspoons wherever possible, except for beverages.
| Unit | US customary | Nutrition label | Difference |
|---|---|---|---|
| Teaspoon | 4.93 ml | 5 ml | +1.4% |
| Tablespoon | 14.79 ml | 15 ml | +1.4% |
| Fluid ounce | 29.57 ml | 30 ml | +1.4% |
| Cup | 236.59 ml | 240 ml | +1.4% |
| Ounce (weight) | 28.35 g | 28 g | −1.2% |
The label volumes are all 1.4 percent larger than the customary ones, because they are rounded up from the same ratios. The label ounce is rounded down instead, so it is 1.2 percent lighter than a real ounce. For cooking none of this matters: a label cup holds 3.4 ml more than a measuring cup, less than a teaspoon. It matters only when you compare a label's calories per serving with what you actually measured, and even then by little more than 1 percent.
The cooking measurement converter has the label cup, tablespoon, teaspoon and fluid ounce as their own units, next to the US customary, metric and UK ones.
Worked examples
A recipe asks for 8 oz of chocolate chips
Without "fl", this is a weight. 8 × 28.35 = 226.8 g. At 170 g per cup that is 1.33 cups, so about 1 1/3 cups if you have no scale.
A recipe says "1 cup (8 oz) flour"
The writer converted volume to weight as if flour were water. If you weigh 8 ounces you get 226.8 g, which is 1.89 cups by King Arthur's figure, nearly twice what the cup measure means. Follow the cup, or weigh 120 g, and you get the amount the writer meant.
A 12 oz jar of honey
A jar marked 12 oz without "fl" states a weight. 12 ounces are 340.2 g, and at 339 g per cup the jar holds 1.004 cups, just over 1 cup. Someone who reads 12 oz as 12 fluid ounces expects a cup and a half and comes up short.
A British recipe asks for 10 fl oz of milk
That is half a UK pint: 10 × 28.41 = 284.1 ml. In US measures it is 9.61 US fluid ounces, or 1.20 cups, a little over 1 cup plus 3 tablespoons. Pour 10 US fluid ounces instead and you add 11.6 ml more milk than the recipe meant.
Nutrition label against your cup
A milk label serving of 240 ml weighs about 248 g at USDA's 244 g per customary cup. Your measuring cup of milk weighs 244 g. The 3.5 g gap is the 240 ml cup at work: a cup you measure holds about 1.4 percent less than one label serving, calories included.
Which converter for which job
- Volume to volume, like cups to milliliters or UK to US fluid ounces: the cooking measurement converter for kitchen units, or the volume converter for everything else.
- Weight to weight, like ounces to grams or pounds to kilograms: the weight converter.
- Volume to weight, like cups of flour to grams: the cups to grams converter, which needs the ingredient, because no single factor turns fluid ounces into ounces.
Frequently asked questions
Is a fluid ounce the same as an ounce?
No. A US fluid ounce is a volume, 29.57 milliliters. An ounce on a scale is a weight, 28.35 grams. They only line up for liquids about as dense as water: a fluid ounce of water weighs about 1.04 ounces by USDA figures, while a fluid ounce of all-purpose flour weighs about 0.53 ounces.
How many ounces are in a cup?
A US cup is exactly 8 fluid ounces of volume. Its weight depends on what is in it: about 8.36 ounces of water, 4.23 ounces of all-purpose flour, or 11.96 ounces of honey, using the cup weights in the cups to grams converter.
Is a cup of water 8 ounces by weight?
Close, but a little more. USDA FoodData Central lists 237 grams for a cup of water, which is 8.36 ounces. Some baking charts round it to 8 ounces: King Arthur Baking's chart lists a cup of water as 8 ounces or 227 grams. For bread dough, where water is measured against flour, weigh it if the recipe gives grams.
When a recipe just says "oz", which ounce is it?
NIST Handbook 44 says that an ounce or pound written without "avoirdupois" is understood to be the avoirdupois unit, the ordinary weight ounce. A fluid ounce should carry the word "fluid" or "fl". So "8 oz cream cheese" is a weight and "8 fl oz cream" is a volume. Recipes are not always that careful, so if the ingredient is dry and the number is in "oz", reach for the scale.
How do I convert fluid ounces to ounces?
Only with a figure for the ingredient. Multiply the fluid ounces by the grams per cup, divide by 8 for grams per fluid ounce, then divide by 28.35 for ounces. For 6 fl oz of milk at USDA's 244 g per cup: 6 × 244 ÷ 8 = 183.0 g, which is 6.46 oz. The cups to grams converter does this for over a hundred ingredients.
Why does the nutrition label say 1 cup is 240 ml?
For nutrition labeling, FDA rules define a cup as 240 ml, a tablespoon as 15 ml, a teaspoon as 5 ml and a fluid ounce as 30 ml (21 CFR 101.9(b)(5)(viii)). Your measuring cup holds 236.59 ml, so the label cup is 1.4 percent bigger. The difference is less than a teaspoon per cup, too small to matter when you cook.
Sources
- NIST Handbook 44 (2026), Appendix C: General Tables of Units of Measurement (liquid volume, avoirdupois mass, British units)
- NIST Special Publication 811, Appendix B.8: Factors for units listed alphabetically (ounce, fluid ounce, cup)
- 21 CFR 101.9, Nutrition labeling of food, paragraph (b)(5) (revised as of April 1, 2024), govinfo.gov
- UK Weights and Measures Act 1985, Schedule 1: definitions of the gallon, pint, fluid ounce, pound and ounce
- King Arthur Baking, Ingredient weight chart
- King Arthur Baking, Measuring flour: how to do it right (2023)
- USDA FoodData Central, SR Legacy 169640: Honey
- USDA FoodData Central, SR Legacy 169661: Syrups, maple
- USDA FoodData Central, SR Legacy 173468: Salt, table
- USDA FoodData Central, SR Legacy 171265: Milk, whole, 3.25% milkfat, with added vitamin D
- USDA FoodData Central, SR Legacy 170859: Cream, fluid, heavy whipping
- USDA FoodData Central, SR Legacy 174158: Water, bottled, generic
- USDA FoodData Central, SR Legacy 173410: Butter, salted
- USDA FoodData Central, SR Legacy 172336: Oil, canola