Why grams to cups conversion is not easy?
Conversions between grams and ounces is straightforward. One ounce is always 28.35 grams no matter what. It is similarly easy to convert from cups to milliliters; one cup always equals to 236.6 ml. These conversions never try to hide and deceive you, they are open and direct. That's because we convert units of the same measurement: weight to weight and capacity to capacity. Strictly speaking, ounces and kilograms are units of mass, not weight, but for the sake of cooking recipe's ingredients it does not matter.
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Things become complicated when you need to convert from weight to capacity. This time nothing is straightforward. One cup of water weighs 0.522 lbs., but if you fill the same cup with cake flour, it will weigh almost 3 times less, only 0.20 lbs. Why is it so? Why the same amount of various ingredients weighs so different?
What is density and how is it involved?
Density is a property of each substance. If one ingredient is "denser" than another, it will weigh more if you take the same capacity. Density of water is higher than the one of flour, but less than the density of corn syrup.
Each substance or ingredient has its density. The density is the hidden factor you need to know when converting between weight and capacity units, e.g. from grams to ounces. If you know the density of an ingredient, you have everything you need to make the conversion.
Density vs. specific gravity
Density is measured in grams per milliliters, pounds per gallon, etc. To make weight to capacity conversions it is often easier to use specific gravity instead of density. Specific gravity shows the density of an ingredient comparing to water.
Pure water has specific gravity of 1. Cake flour is lighter than water, it has specific gravity of 0.38. Corn syrup is heavier than water, its specific gravity is 1.48.
Knowing the specific gravity of your ingredient, recipe ingredient conversion from volume to weight becomes straightforward. You make the conversion for water and then multiply the result by specific gravity of the ingredient.
Let’s make the calculation for corn syrup. One cup of water weighs 0.522 lbs. We multiply the value by 1.48 (specific gravity of our syrup) and get 0.773 lbs.
How do I convert grams to cups for my recipe ingredient?
If you search the internet, you can find a lot of tables containing densities of various substances. A great source of information is USDA Food Composition Database that contains data for thousands of foods and ingredients. Having the data, you can make the calculations as described above.
There is a much easier way though. You might want to try our Cooking Ingredients Converter. You simply pick your ingredient from the list, enter its weight or capacity and get it instantly converted to the unit you need. No software to download, no registration required, everything works online, and it is free. Don’t forget to bookmark the converter to easily get back when you'll need a recipe conversion again.
A disclaimer on conversion precision that must be here.
Foods and ingredients are mostly natural products and thus may differ in form, size, humidity, etc. All of this affects the density and thus the conversion between weight and capacity. Any density data for a particular product as well as any online conversion between weight and volume only provides an average data for the product of the kind and may not exactly match the results for the product you have.