Jam: why 340 g per cup is wrong
King Arthur Baking Ingredient Weight Chart publishes 340 g (12 oz) per US cup of jam. This site rejects that figure and publishes 320 g (11.3 oz) instead, from USDA SR Legacy "Jams and preserves" (FDC 169641) (density 1.3526 g/mL, matching the composition of the substance). The reason:
1.437 g/mL would make jam denser than honey (King Arthur's own 21 g tablespoon of honey is 1.42 g/mL), but jam is about 30% water against honey's 17%. USDA's composition for jams and preserves (30.5% water, 68.9% carbohydrate) predicts about 1.35 g/mL, and USDA and every label checked give 20 g per tablespoon, 320 g per cup. A quarter cup at that density is 80 g, 2.8 oz; written as 3 oz it becomes 85 g, so King Arthur's figure looks like a weight rounded up to the next whole ounce.
The arithmetic
| Source | 1 cup | Density | Status |
|---|---|---|---|
| King Arthur Baking Ingredient Weight Chart | 340 g12 oz | 1.4371 g/mL | Rejected on physics |
| USDA SR Legacy "Jams and preserves" (FDC 169641) | 320 g11.3 oz | 1.3526 g/mL | Published here |
The published figure and its sources
The full sourcing for jam, including every accepted and rejected figure, the state or filling method behind each, and the physics or standards reasoning applied, is on the ingredient page.
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