Ground Mustard

One US cup of ground mustard weighs 101.3 grams. Going the other way, 100 grams is about 0.99 cups.

Also called mustard powder, dry mustard, mustard flour, ground mustard seed, English mustard powder.

Ground mustard packs at about 101 g per US cup. USDA and reference charts agree within 6 percent, and unlike most ground spices, ground mustard is actually lighter per cup than the whole seeds it comes from.

Ground mustard, also sold as mustard powder or dry mustard, is the finely milled seed of one or more species of the mustard plant. Yellow mustard (Sinapis alba) gives the milder, brighter powder typical of American mustard preparations. Brown mustard (Brassica juncea) gives a sharper, more pungent powder used in Dijon and Chinese-style mustards. Commercial products like Colman's English Mustard blend both. The powder on its own carries no heat, because mustard's characteristic pungency requires the enzyme myrosinase in the powder to react with cold water to release allyl isothiocyanate. Dry powder is stable in the jar; the moment it hits water in a recipe, the reaction begins and heat develops over the next 10 to 15 minutes.

Sources agree well on the density. USDA FoodData Central's SR Legacy record for Spices, mustard seed, ground gives 107 g per metric cup, which works out to 101 g per US cup at a density of 0.4280 g/mL. USDA Branded Foods reports mustard powder (UPC 076606598289) at 101 g per metric cup (96 g per US cup, density 0.4040 g/mL). Chef's Resources publishes 3.43 oz per cup (97 g, density 0.4108 g/mL). The three sources span 6.25 percent — within the 10 percent spice tolerance and consistent enough for a single figure to represent the ingredient accurately. The USDA SR figure sits at the high end of the observed range, while the commercial products (USDA Branded and CR) cluster slightly lower.

Unlike ground coriander, ground mustard is lighter per cup than whole mustard seeds. Chef's Resources gives whole mustard seeds at 6.40 oz per cup (181 g) versus ground at 3.43 oz per cup (97 g) — a nearly two-to-one ratio in favor of whole. The physical reason is seed geometry. Mustard seeds are small, hard, roughly spherical particles about 1 to 2 mm across with solid dense interiors (roughly 1 g per mL for the seed body itself), and they pack efficiently in a measuring cup because small spheres nest tightly with little space between them. When those seeds are milled into a fluffy powder, the resulting particles are irregular, cohesive, and prone to bridging across the sides of the cup, trapping air pockets that the whole seeds did not have. The same pattern shows up with other solid-seed spices like ground cumin (roughly 96 g/cup ground versus 109 g/cup whole in the same reference tables). It reverses only for seeds like coriander whose whole form contains internal hollows around the germ — grinding coriander collapses those cavities and the powder ends up denser than whole seed. Mustard belongs to the more common pattern.

Ground mustard sits in the medium-density spice cluster alongside ground coriander (96 g/cup), ground cardamom (93 g/cup), pumpkin pie spice (90 g/cup), and paprika (109 g/cup). It is well below the dense-spice cluster of cinnamon (128 g/cup) and cloves.

King Arthur's Ingredient Weight Chart does not list ground mustard or mustard powder in any form. The chart is silent on this ingredient, so there is no correction to make here; the site simply publishes what USDA and the reference chart already agree on.

Ground mustard is used in dry rubs and spice pastes where its pungency will develop on contact with liquid, in cheese sauces (mac and cheese, Mornay), in salad dressings and vinaigrettes both as an emulsifier and a flavor, in pickle brines, and as the raw material for prepared mustards. A teaspoon of ground mustard mixed with cold water and rested for ten minutes produces a very hot condiment; the same teaspoon added at the end of hot cooking loses much of its heat because prolonged heat destroys the enzyme reaction that produces the pungency. For recipes that specify prepared mustard by tablespoon, a rough substitution is one teaspoon of dry mustard plus one teaspoon of vinegar or water per tablespoon of prepared mustard, though the flavor will be sharper because homemade lacks the salt, sugar, and turmeric of commercial products.