Ounce to Grams

What one ounce equals in grams, and where the number comes from

The US ounce used on food, packages and postal scales is called the avoirdupois ounce, and it is a fixed number rather than a measurement you take. One avoirdupois pound was set at exactly 453.59237 grams in the 1959 international yard and pound agreement, and a pound holds 16 ounces. Divide the pound by 16 and the ounce comes out clean:

1 oz = 453.59237 / 16 = 28.349523125 g

The NIST Guide to the SI, Appendix B.9 lists the same factor as 2.834 952 E+01 grams per avoirdupois ounce and prints it in boldface, which the guide reserves for exact factors. The pound sits in the same table as 4.535 924 E-01 kilograms. The NIST Handbook 44, Appendix B spells out the yard and pound side of the agreement, including one avoirdupois pound equal to 0.453 592 37 kilogram.

Because the factor is exact, converting ounces to grams is a single multiplication with nothing to look up:

• grams = ounces x 28.349523125
• 1 gram = 0.0352739619 ounces
• 1 kilogram = 35.27396195 ounces

Put in a common kitchen amount and the arithmetic is short. Four ounces times the factor gives 113.3981 g, and twelve ounces gives 340.1943 g. Three and a half ounces, the weight that matters for an international letter, comes to 99.2233 g. The converter above runs the same multiplication and adds the troy reading, the pounds plus ounces split, and a price per gram when you type a price.

Scaling a recipe that is written in ounces

Most US recipes still measure by the ounce, and most metric recipes measure by the gram, so anybody cooking from both ends needs this number. The same ounce amount shows up in a lot of common packages, and each one has a metric twin.

One stick of butter is the anchor most people already own, and 4 oz lands at 113.3981 g, which is why metric butter wrappers in Europe print 125 g for a standard block of a different size. Check the wrapper rather than the assumption: a 250 g block is not a 4 oz stick.

Two rules keep a scaled recipe on target. First, weigh rather than convert twice, since every extra multiplication adds a rounding step. Second, match the precision of your scale. A scale that reads 1 g steps cannot support four decimal places of grams, and 0.04 oz is the smallest ounce amount it can show. Two decimal places in ounces is the useful limit for home work.

Shipping thresholds: ounces on the scale, grams on the customs form

Postal services and customs offices in different countries use different units on the same package, so a letter that is legal by one rule can be over a limit under another. The USPS First-Class Mail International page states that letters travel up to 3.5 ounces, large envelopes up to 15.994 ounces, and that a 1 ounce letter or postcard can go anywhere in the world with one Global Forever stamp, listed at $1.75. Convert those thresholds into grams and the numbers line up with the forms.

That 15.994 ounce limit is a good lesson in why people quote odd numbers. The Postal Service rounds each piece to three decimal places of ounces, so an item weighing 15.995 ounces would be recorded as 16 ounces and pushed into the next price bracket. The ceiling sits just below a full pound to keep flat pricing intact.

Customs forms for international shipments ask for weight in grams or kilograms, and the same conversion handles that field. A 453.4223 g parcel enters the form as 0.453 kg, and a 99.2233 g letter enters as 0.099 kg. Convert in grams first, then move the decimal, rather than trying to write pounds and ounces into a metric box.

Troy ounces for gold, silver and jewelry

Precious metals are sold by the troy ounce, and it is not the same weight as the food ounce. A troy ounce is 31.1034768 g, listed in the same NIST table as 3.110 348 E+01 grams, and it is 9.7 percent heavier than the avoirdupois ounce because the troy pound holds 12 ounces instead of 16.

The difference comes from the grain, the small unit both systems use. One grain is 0.06479891 g, an avoirdupois ounce holds 437.5 grains, and a troy ounce holds 480. Same grain, different count, and that 42.5 grain gap produces the 9.7 percent difference.

Price per gram follows from the same multiplication. A troy ounce priced at $2,000 works out to $64.30 a gram, because 2000 divided by 31.1034768 gives that figure. A jeweler quoting by the gram and a dealer quoting by the troy ounce are quoting the same metal, and the troy number should be about 9.7 percent larger than the gram number divided back out.

Pick the right ounce before you convert. The selector in the converter above switches the factor, so 1 in the box means 28.3495 g for food and 31.1035 g for gold. Using the food factor on a gold quote understates the metal by close to a tenth.

How food labels round the same conversion

Labels rarely print the exact number, and that is not a mistake. The FDA guidance on the Nutrition Facts label states that when a household serving size is 1 ounce, manufacturers should use 28 grams as the rounded metric equivalent, and that 1 ounce technically weighs 28.35 grams. The rule behind that sits in 21 CFR 101.9(b)(5), which also says the gram or milliliter equivalent is rounded to the nearest whole number, with finer steps only for amounts under 5 g.

Rounding to the nearest gram gives the numbers printed on shelf:

Each gap is under half a gram, less than the resolution of any kitchen scale, so a label that reads 28 g for a 1 ounce serving is correct within the rules. The label and the exact factor are not in conflict, they are answering the same question at different levels of detail.

One trap: some people treat 1 ounce as 30 grams because 28.35 rounds up in their head to a tidy 30. For a single serving that is 1.65 g of error, which is about 6 percent. For a 12 ounce bag of chocolate chips the gap grows to 19.81 g, and a recipe built on that shortcut drifts every time you double it.

Ounces to grams table for common amounts

Here is the same multiplication written out for the ounce amounts that show up most often, rounded to four decimals so you can see the exact figure next to the practical one.

Double the ounces and you double the grams, with no rounding in between. Six ounces is 170.0971 g and twelve ounces is 340.1943 g, so a recipe that calls for 6 ounces of chocolate chips needs 340.1943 g when you double it. Metric packages are the reverse of that: a 340 g bag is 11.9931 ounces, which is why the bag in the metric aisle and the bag on the US shelf are close but not identical sizes.

Weight ounces and fluid ounces are not the same thing

A fluid ounce measures volume, and a gram measures mass, so the two need the density of whatever you are pouring. One US fluid ounce is 29.5735 mL. Water runs close to 1 g per mL, so a fluid ounce of water weighs about 29.6 g and a cup of water comes to 236.6 g. Honey is denser, so the same cup of honey weighs about 336 g, and flour is lighter, so a cup of all purpose flour weighs roughly 120 g.

That gap is where recipes go wrong. A package that says 8 oz of chocolate means weight, 226.7962 g. A recipe that says 8 fluid ounces of cream means volume, 236.6 mL, and cream is heavier than water, so it needs about 242 g on the scale. The two numbers sit close enough to swap, and the cake tells you they were not the same.

The rule of thumb: if the label uses a capital O in a recipe, it means weight, and the 28.349523125 factor applies. If it says fluid ounces, or fl oz, you need a density for that ingredient before any gram figure means anything.

How many decimals you need

The exact factor has nine decimal places because the pound does. Once you are in a kitchen, most of those digits are noise. A scale that reads to 1 g supports about three significant figures, and 0.04 oz is the smallest ounce step it can show. A postal scale that reads 0.1 oz supports two decimal places, and a jewelry scale that reads 0.01 g leaves ounce precision close to useless.

Three habits prevent drift. Decide the precision before you start. Round once, at the end, rather than rounding the ounces and then the grams. And keep the two ounce families straight, because swapping troy for avoirdupois on a gold order changes 31.1035 g into 28.3495 g, a gap of about 2.75 g of metal on every troy ounce.

The shortcut of multiplying by 28 lands within 1.2 percent, which is fine for a shopping list and wrong for a lab notebook or an invoice. Twelve ounces times 28 gives 336 g, and the true figure is 340.1943 g, a difference of 4.19 g that grows every time you double the recipe. Multiply by 28.35 instead and 12 ounces comes to 340.2 g, within 0.006 g of the exact value.