Cups to Grams: Why a Cup of Flour Isn’t a Cup of Sugar

Search for "1 cup in grams" and you'll get a different answer for every ingredient: 125 g of flour, 200 g of sugar, 227 g of butter, 339 g of honey. None of them is wrong. A cup measures space, a gram measures weight, and how much an ingredient weighs depends on how tightly it fills that space.

This guide explains where the numbers come from, why the way you fill the cup matters as much as what's in it, and when it's worth getting the scale out.

A cup is a volume

In US recipes a cup is a fixed volume: 8 US fluid ounces, or 236.6 mL (236.5882365 mL exactly, by the US definitions NIST publishes). Two other cups are worth knowing about:

The nutrition-label cup on US food packaging is rounded to 240 mL, under the FDA's labelling rules. It's 1.4% bigger than a recipe cup, which doesn't matter in the kitchen.
The metric cup used in some other countries' recipes is 250 mL, about 5.7% bigger than a US cup. Over three cups of flour that's roughly 20 g, which can matter for bread.

A gram is a unit of mass. To turn one into the other you need the ingredient's density: how many grams fit in each millilitre. Milk, which is mostly water, comes to about 1 g per mL. Most dry ingredients are much lighter, because a cup of them is partly air.

What a cup weighs, ingredient by ingredient

The US Department of Agriculture weighs standard household measures for its food composition database, FoodData Central. Here's a level US cup of common ingredients, with the density that implies (grams ÷ 236.6 mL):

Ingredient	One cup	Grams per mL
Rolled oats	81 g	0.34
Cocoa powder	86 g	0.36
Whole wheat flour	120 g	0.51
Powdered sugar, unsifted	120 g	0.51
All-purpose flour	125 g	0.53
Bread flour	137 g	0.58
Uncooked white rice	185 g	0.78
Granulated sugar	200 g	0.85
Olive oil	216 g	0.91
Brown sugar, packed	220 g	0.93
Butter	227 g	0.96
Whole milk	244 g	1.03
Table salt	292 g	1.23
Honey	339 g	1.43

A cup of honey weighs more than four cups of oats. The pattern is about how much air the cup holds:

Flour, cocoa and oats are fine, light particles or flakes with lots of space between them.
Sugar and salt are small, hard crystals that pack closely, so far less of the cup is air.
Butter, oil and milk are close to the density of water, because there's no air at all (milk is mostly water).
Honey is denser than water. It's mostly sugars dissolved in a little water, so a cup holds a lot of mass.

That's also why "1 cup = 240 g" conversion charts are only right for water-like liquids. Use them for flour and you'd add nearly twice as much as the recipe meant.

Spooning, scooping and sifting

For dry ingredients, how you fill the cup changes the weight, and not by a little.

USDA researchers weighed hundreds of measured cups of flour, filled in different ways, and published the averages in Average Weight of a Measured Cup of Various Foods (Home Economics Research Report No. 41, first issued in 1969 and reissued in 1977). Their cup was 237 mL. For all-purpose flour:

How the cup was filled	Average weight	Cups weighed
Dipped (scooped from the bag) and levelled	143 g	300
Spooned in lightly and levelled	126 g	510
Sifted, then spooned in and levelled	116 g	500

The same flour, in the same cup, ranged from 116 g to 143 g. A scooped cup was 23% heavier than a sifted one and 13% heavier than a spooned one. The report measured everything by spooning lightly unless it said otherwise, and today's USDA figure for all-purpose flour, 125 g, sits right next to that spooned average.

Sugar shows the same effect in other ways. In the same report, powdered (confectioners') sugar weighed 113 g a cup unsifted and 95 g sifted. Packed brown sugar averaged 211 g, but with a standard deviation of 22.5 g across 118 cups, three times the spread of granulated sugar (7.6 g across 81 cups), because everyone presses it down a little differently.

What this means in practice:

If a recipe says "1 cup flour" and was written for volume, spoon the flour into the cup and level it with a straight edge, unless the recipe says otherwise. Don't press or tap the cup.
"1 cup sifted flour" and "1 cup flour, sifted" are different. The first means sift, then measure (lighter). The second means measure, then sift (the full, heavier cup).
Brown sugar is measured packed unless the recipe says otherwise.

A worked example

Say a cookie recipe calls for 2¼ cups all-purpose flour, 1 cup granulated sugar, ¾ cup packed brown sugar and 1 cup butter. Using the USDA weights:

Ingredient	Cups	× grams per cup	Grams
All-purpose flour	2¼	125	281 g
Granulated sugar	1	200	200 g
Brown sugar, packed	¾	220	165 g
Butter	1	227	227 g

Now suppose you scooped the flour straight from the bag. At 143 g a cup, 2¼ cups is 322 g, about 41 g more than intended, roughly an extra third of a cup in a recipe built around 2¼. Weighing 281 g removes the question.

You can run each line in the converter with the ingredient already picked:

When to weigh, and when a cup is fine

NIST, the US standards agency, puts it plainly in its kitchen measurement tips: measuring ingredients by mass with a kitchen scale "is a best practice to improve recipe accuracy". Measuring cups still have their place, though.

Weigh:

flour in bread, cakes and pastry, where the ratio of flour to liquid and fat decides the texture;
any recipe written in grams, because the author weighed it, so converting to cups puts back the uncertainty they removed;
sticky ingredients like honey, syrup or peanut butter. Put the bowl on the scale, zero it and pour, and there's no cup to scrape.

Cups are fine for:

liquids: water, milk and oil fill a cup the same way every time, so a measuring jug is as good as a scale;
forgiving dishes: rice, oats for porridge, soups and stews;
small amounts: a teaspoon of salt or baking soda weighs only a few grams, and a scale that reads in whole grams is coarse at that size. Use measuring spoons for those.

If you're turning a metric recipe into cups, go the other way: 500 g of flour is 4 cups, spooned and levelled.

Try it

The cups to grams converter has all fifteen ingredients from the table above, each with its USDA FoodData Central reference so you can check the figure. Pick the ingredient, type the cups and the grams come out, with a table for ¼, ⅓ and ½ cups. For plain volume, there's cups to mL.

Sources

USDA Agricultural Research Service. FoodData Central, SR Legacy (April 2018): household cup weights, e.g. all-purpose flour FDC 168936 and granulated sugar FDC 169655. Every ingredient's FDC ID is listed in the converter.
Fulton, L., Matthews, E., & Davis, C. (1977). Average Weight of a Measured Cup of Various Foods. Home Economics Research Report No. 41, USDA Agricultural Research Service. Internet Archive
NIST. Metric kitchen: culinary measurement tips. nist.gov
NIST Handbook 44, Appendix C: General tables of units of measurement (the US cup and fluid ounce). nist.gov
21 CFR 101.9(b)(5)(viii): the 240 mL cup used on US nutrition labels. eCFR