How We Measure

Every number on HowManyOunces comes from one registry of unit sizes. US and metric units use the exact definitions in NIST Handbook 44, the US reference for weights and measures. Imperial units use the exact sizes in the UK Weights and Measures Act 1985, and the table below gives the definition of every unit.

The pound is defined as exactly 0.45359237 kilograms, and the ounce is one sixteenth of a pound, so 1 oz is exactly 28.349523125 grams. The US gallon is defined by volume, and the US fluid ounce is one 128th of it: exactly 29.5735295625 milliliters.

"Ounce" can mean weight or volume. On pages that pair ounces with a volume unit, such as cups or milliliters, "ounces" means US fluid ounces. Everywhere else it means the ounce of weight.

How much a cup of flour or sugar weighs comes only from USDA FoodData Central, the US Department of Agriculture's food database. Each ingredient uses one FoodData Central record, and the table below gives its ID and the portion we use.

We take the gram weight that USDA lists for a household portion, such as one cup, and work out the rest from it. We do not average sources or add our own kitchen tests. If USDA has no cup or spoon measure for an ingredient, we do not publish a weight for it.

Converters between weight and volume use water unless you choose an ingredient. USDA lists a liter of tap water at 1,000 grams, so 1 cup of water weighs 236.6 grams. Ingredient weights are shown to three significant figures, because the same cup can hold more or less depending on how it is filled.

Ingredient pages also list calories, protein, fat, carbohydrate, sugars and sodium for each measure. The values come from the same USDA FoodData Central records as the weights, where USDA gives them per 100 grams, and we scale them to the grams of each measure.

A dash in those tables means USDA lists no value for that nutrient. The figures are data about the food, not advice on what or how much to eat.

Conversions are worked out with exact fractions, and rounding happens only when a number is shown. Results show four significant figures, without trailing zeros, so 1 kg is shown as 2.205 lb. Results smaller than 0.001 switch to a smaller metric unit, such as milligrams.

Cups, tablespoons, teaspoons and fluid ounces are also shown as kitchen fractions. We use the nearest of 1/8, 1/4, 1/3, 3/8, 1/2, 5/8, 2/3, 3/4 and 7/8, next to the decimal: 1/3 cup is 5 1/3 tablespoons (5.33). When the fraction is more than 1% away from the true value, it gets a "≈" sign.

Where the exact value has a finite number of digits, it is shown in full on an "Exact" line. For example, 1 gal is exactly 3,785.411784 milliliters.

The US and the UK use different fluid ounces, pints, quarts and gallons. A US pint holds 16 US fluid ounces (473 ml), while a UK imperial pint holds 20 imperial fluid ounces (568 ml). The ounce and pound of weight are the same in both countries.

Canada, Australia and New Zealand use a metric cup of 250 ml in their official food guides (Health Canada, Eat For Health, New Zealand Ministry of Health). Health Canada pairs it with a tablespoon of 15 ml and a teaspoon of 5 ml, and Australian tablespoons are usually 20 ml.

UK law defines no kitchen cups or spoons, so on this site a UK cup or spoon means the metric one. US nutrition labels use a cup of 240 ml under 21 CFR 101.9, which is slightly larger than the US kitchen cup.

Answers always show the US and the UK value when they differ. The US, UK and Metric switch on each converter changes only the units that differ, and it remembers your choice.

If you think a number is wrong, please tell us. We check it against the source, fix it in the registry or the ingredient data, and run every automated test again before the fix goes live.

Each page shows the date its data was last reviewed, not the date the site was last built. Pages with facts that can change, such as bottle sizes or travel rules, name their sources and are checked again on a schedule.

Corrections to published numbers are listed here with the date, the page and what changed.