Butter

One US cup of butter weighs 227 grams. Going the other way, 100 grams is about 0.44 cups.

Also called salted butter, unsalted butter, sweet cream butter.

Sources agree to within 0.4%, the closest agreement in the dataset so far. Salted and unsalted are identical by weight: USDA records 227 g per cup for both FDC 173410 (salted) and FDC 173430 (unsalted), so one entry covers both. Butter is the one ingredient where cup measurement is genuinely reliable, and the reason is visible in the data itself: USDA gives a stick as 113 g and a cup as 227 g, which is twice the stick.

The cup figure reflects the packaging convention rather than a measurement of how butter fills a cup. A US butter wrapper is printed with cup and tablespoon markings, so the cook cuts a pre-portioned block instead of filling a vessel, and the packing variability that affects flour and brown sugar does not arise. Whipped butter is a separate product, not a state of this one: USDA records it at 151 g per cup (FDC 173411), roughly a third lighter because air is beaten into it.

USDA also publishes a pat of butter at 5 g, defined as a 1-inch square a third of an inch high. It is not included as a yield here because King Arthur's chart does not list it and no second source was found, so rule 8 applies. The pat is a real US measure, used for restaurant portions and in older cookbooks, and is worth adding if a second source turns up.

Two sources only, so this entry does not meet rule 2 and is held at medium confidence until an independent third is found.