Picture a table at a school science fair. Two identical cups sit on it, one full of water and one full of peanut butter, with a kitchen scale in between. Which cup is heavier? Plenty of kids would bet on the water. They’d lose.
So how many grams in a cup of peanut butter? About 258, per USDA. A cup of water, at 1 gram per mL, is about 236.6 g. That 21 g gap comes down to density, and density is the best way to make sense of every cup conversion you’ll meet. Here it is with real examples.
Density in one minute
Density is just weight per volume, grams in each millilitre. A US cup holds 236.6 mL. Multiply the density by that and you get the cup weight. In other words, a cup isn’t a weight at all, it’s a space. For peanut butter, USDA’s record, FDC 172470, works out to 1.0905 g per mL, and 1.0905 times 236.6 gives you roughly 258 g.
King Arthur’s chart implies a density of 1.1412, from 135 g per half cup. That’s why its cup comes out at 270 g. Both numbers are above 1, so on either source peanut butter outweighs water. ExactCups answers how many grams in a cup of peanut butter with the USDA figure and shows the King Arthur one beside it.
Lining it up with the neighbours
Butter, a little under water
Butter is roughly 80 percent milk fat and 16 percent water, and fat weighs less than water, so butter comes in around 0.96 g per mL. A cup is 227 g, about 31 g lighter than a cup of peanut butter. Hold one in each hand and the peanut butter would feel the heavier one.
Cream cheese, close to water
Cream cheese is about 53.3 percent water and 34.9 percent fat, with a density of roughly 0.98 g per mL from USDA. That makes a cup 232 g, a hair under the water cup at 236.6 g. It sits between milk, which is denser than water, and butter, which is lighter.
The light end of the shelf
Cocoa powder is the opposite extreme. USDA gives it 0.3635 g per mL, so a cup is only 86 g. Chocolate chips land at 0.7101 g per mL, because the gaps between the chips are part of what the cup holds, and a cup comes to 168 g. Neither gets anywhere near the weight of the water cup.
That’s the real reason you can’t use one conversion for everything. A cup is a fixed volume, and each ingredient fills it with a different amount of stuff. Water makes 236.6 g and peanut butter 258 g, while cocoa manages a skinny 86 g. A recipe that mixes them, like a no bake bar, needs different numbers.
What the 5 percent tolerance means
ExactCups sorts ingredients into seven categories and gives each a tolerance. Peanut butter sits in fats and oils, at 5 percent, which means two sources can differ by up to that much before the site stops printing a single number. USDA and King Arthur are about 4.5 percent apart, so they just squeak through. That’s the sort of detail most sites never show.
Flour doesn’t get that luxury. All purpose flour’s three sources sit about 18 percent apart, so the site shows the spread, 120 g spooned up to 141.7 g dipped. Peanut butter’s two sources agree well enough for one figure, which is why you get a clean 258 g and not a shrug. The page was last checked in mid September 2026, and confidence stays medium.
Doing the sum on a Saturday
Say you’re making peanut butter cookies for a science fair bake table and the recipe wants a cup and a half. Half a cup is 129 g, so a cup and a half is 387 g. Weigh it into the bowl, and you’re done. A plain cup measure and a stubborn spoon would take far longer and make a mess.
Now try the same job from a different angle. If your recipe follows King Arthur’s chart, it assumes 270 g a cup, so 405 g for a cup and a half. Using USDA’s number, you’d be 18 g short. That’s about a tablespoon. Not a disaster, though it’s worth knowing which source your recipe quietly trusts. The scale doesn’t care which source you trust, so write the gram number on the recipe card and move on.
Ounces, grams and the weight trap
A weight ounce is 28.3 g, so 1 oz of peanut butter is about 0.11 of a cup. Four ounces is 113.4 g, or 0.44 of a cup. Eight ounces is 226.8 g and lands at 0.88 of a cup, and a pound is 453.6 g, about 1.76 cups. Going backwards, 100 g is about 0.39 of a cup.
Here’s the trap. Eight fluid ounces make a cup, so people read 8 oz as a cup. Peanut butter’s cup is 9.1 oz by weight, so an 8 oz portion leaves you short by about 31 g. Butter is almost exactly on the mark at 227 g, which is why the habit survives.
Why jars don’t settle the question
Because jars have no cup markings, a cup of peanut butter is a real measurement of how the stuff fills a vessel. Peanut butter is a plastic solid, which means it deforms under pressure. A firm press can squeeze a little more into the cup, and that’s probably what King Arthur’s 270 g reflects.
Brand labels lean the other way. The big smooth brands, Jif and Skippy among them, declare 32 g per 2 tablespoons, which matches USDA’s figure to the nearest gram. That nudges the site towards the lower number.
A note on natural, chunky and reduced fat jars
All of this assumes a smooth jar that’s been stirred. Natural peanut butter separates, and the oil has to be mixed back in. Chunky and crunchy are separate products on the site, even though USDA lists chunk style at the same 258 g, because chunks make a cup less repeatable.
Reduced fat jars usually add maltodextrin and other bulking agents, which raise the density above the smooth figure. So for those, expect a heavier cup and use a scale. The 258 g isn’t meant for them.
Quick takeaway
A cup of peanut butter is 258 g, which is heavier than a cup of water at 236.6 g. Butter is lighter, cocoa far lighter. Keep the water cup in your head as a yardstick, and you can sanity check almost any cup number you meet.