Baking Powder: US cups to milliliters

Baking powder is a finicky ingredient to convert because it is a blended powder, not a single substance. A typical can holds sodium bicarbonate (the base), one or two acid salts (monocalcium phosphate for fast action, sodium aluminum sulfate or sodium acid pyrophosphate for slow action), and cornstarch as a buffer to keep the acid and base from reacting in the can. Those four components have different particle sizes and different densities, and they settle at different rates during shipping and shelf storage. Open a can that has sat upright for six months and the heavier acid salts have migrated toward the bottom while the cornstarch fluffs up top. That is why the same teaspoon can weigh anywhere from 3.6 to 4.4 grams depending on whether you scoop from a freshly opened tin or one that has been knocking around in a cabinet since last Thanksgiving. The King Arthur Baking ingredient weight chart anchors the standard at 4 grams per teaspoon, which corresponds to a density of 0.8115 g/ml, and that is the value KitchenMath defaults to.

The single most common conversion mistake is treating baking powder as if it shared a density with baking soda. They look almost identical in the spoon, but baking soda is pure sodium bicarbonate at roughly 0.97 g/ml while baking powder is a diluted blend at 0.81 g/ml. A reader doing weight math at home will sometimes grab the baking soda figure (around 4.6 grams per teaspoon) and apply it to baking powder, which puts about fifteen percent too much leavener into the bowl. In a sensitive cake, that is the difference between a tender even crumb and a domed cake that rises fast, cracks across the top, and then collapses in the center as the over-stretched gluten gives out.

To measure baking powder correctly, fluff the contents of the can first. Slip a small whisk or the tines of a fork down into the powder and stir for a few seconds to break up any compaction at the bottom. Then dip your measuring spoon into the can, overfill it on purpose, and level the top with the straight edge of a knife or the flat side of the can's plastic insert. Do not tap the spoon to settle the powder, and do not pack it down with your thumb; both moves push the density well above 0.81 g/ml and over-leaven the recipe. If you are working with a kitchen scale, use 4 grams per teaspoon and skip the spoon dance entirely. That is the most reliable way to get a consistent result across cans, brands, and seasons.

Density matters most in tender chemically leavened cakes (yellow butter cake, pound cake, white layer cake), in pancake and waffle batters, and in any quick bread where the only lift comes from baking powder. These recipes have a narrow leavening window: too little and the crumb is dense and gummy, too much and the structure rises faster than the proteins can set, leaving a coarse open crumb with a soapy or metallic aftertaste from unreacted sodium aluminum sulfate. Cookies are more forgiving because they spread and set quickly, and yeasted breads using a touch of baking powder for tenderness barely register the difference. Biscuits and scones sit in the middle: a quarter teaspoon off in a single batch is usually invisible, but the same error scaled to a sheet-pan batch for a brunch crowd shows up as flat tops and a chemical edge.

For substitutions, baking powder can be reconstructed from its components when a can goes flat or runs out: combine one part baking soda with two parts cream of tartar by volume, which produces a single-acting equivalent that should be used immediately because it lacks the slow-acting acid salt. One teaspoon of standard double-acting baking powder (4 grams) corresponds to roughly one quarter teaspoon of baking soda plus one half teaspoon of cream of tartar plus one quarter teaspoon of cornstarch. Avoid swapping in self-rising flour as a one-to-one trade unless you also pull back on the recipe's salt and adjust the flour weight, since self-rising flour already carries about 1.5 teaspoons of baking powder per cup along with added salt.