Converting Pounds and Ounces to Grams in Recipes

The two numbers to remember

Almost every conversion between US customary or imperial weights and metric weights rests on two exact definitions. One avoirdupois ounce is 28.349523125 grams, and one pound is 453.59237 grams. A pound contains 16 ounces, which is why the figures fit together: 16 × 28.349523125 = 453.59237.

Cooking never needs nine decimal places. Rounded sensibly, an ounce is about 28.35 g and a pound is about 453.6 g. Because both factors are exact, the only error in a conversion is the rounding you choose to apply, and that choice is yours.

Converting ounces to grams

Multiply the number of ounces by 28.35 and round the result. Eight ounces of chocolate is 8 × 28.35 = 226.8 g, which most cooks would treat as 227 g. Twelve ounces is 340.2 g, usually written as 340 g. To go the other way, divide grams by 28.35: 100 g is about 3.53 oz.

Recipes often include fractions, such as 5½ oz. Turn the fraction into a decimal first and convert the whole value: 5.5 × 28.35 = 155.9 g, so about 156 g. That is simpler and less error-prone than converting the whole number and the fraction separately and adding the pieces.

A short worked example shows the whole process. A pastry recipe lists 6 oz of sugar, 9 oz of flour and 4½ oz of butter. In grams that is 170 g, 255 g and 128 g (6 × 28.35 = 170.1, 9 × 28.35 = 255.2 and 4.5 × 28.35 = 127.6). Each figure is rounded only once, at the end.

Pounds, and pounds plus ounces

Pounds work the same way with the larger factor. Multiply by 453.6: 2 lb is 907.2 g, and 2½ lb is about 1,134 g, or 1.13 kg. A quantity written as “1 lb 6 oz” is a mixed amount, so the easiest route is to convert everything to ounces first: 16 + 6 = 22 oz, and 22 × 28.35 = 623.7 g, about 624 g.

You can also convert each part and add the results: 453.6 g + 170.1 g = 623.7 g. Both routes agree. Choose whichever is easier to do in your head or on a phone, and keep the full precision until the final step.

A quick mental rule

For forgiving dishes such as stews, soups and roasts, treating 1 oz as 28 g and 1 lb as 450 g is close enough. Two anchor values are worth remembering: a quarter pound (4 oz) is about 113 g, and half a pound (8 oz) is about 227 g. In the US, butter is sold in 4 oz sticks, so a stick is roughly 113 g, and a full cup of butter at 227 g is 8 oz.

The popular shortcut of 30 g per ounce is looser than it looks. Sixteen ounces would come out as 480 g, about 6% more than a true pound at 453.6 g. On a pound of flour that is a 26 g difference, enough to change the texture of bread or cake. Using 28 g per ounce gives 448 g for a pound, only about 1% low.

How much to round

Match the precision to the ingredient. Flour, sugar, butter, meat and vegetables are fine to the nearest gram, or even the nearest 5 g for large amounts. Small quantities of salt, yeast, baking powder or spices matter more in proportion, so keep a decimal place before rounding: ¼ oz is 7.09 g, which is best treated as 7 g rather than 5 g or 10 g.

Round at the end, not in the middle. If several quantities need to be added or scaled, work with the unrounded values and round only the figures you will actually weigh out. Rounding each ingredient early can add up to a noticeable drift across a long ingredient list.

Labels, cans and fluid ounces

Packaged food often prints both units, and the metric figure is itself a rounded conversion. A 14.5 oz can works out to 411 g, a 15 oz can to 425 g, and a 16 oz pack to 454 g. Small differences between your calculation and the printed value are normal, and the printed value is the one to follow if a recipe refers to a can or pack.

Be careful not to mix up ounces with fluid ounces. An ounce measures weight, while a US fluid ounce measures volume. A cup holds 8 fluid ounces, but it does not weigh 8 ounces: a US cup of water weighs about 237 g, or 8.36 oz, and a cup of honey weighs about 336 g, or 11.85 oz. Charts differ by a few grams, so treat such figures as typical rather than exact.

Letting a converter do the work

A cooking converter such as this one has a weight tab that handles grams, kilograms, ounces, pounds and milligrams, shows the result as you type and lists the same amount in every unit. It is useful for checking the mental arithmetic above, particularly for mixed amounts like 1 lb 6 oz, which you first enter as 22 oz.

Whichever method you use, the principle is the same: multiply ounces by 28.35, multiply pounds by 453.6, and round sensibly at the end.