Out of one but have the other? Convert between baking powder and baking soda using the standard 4:1 substitution ratio — and see exactly when you need to add an acid to make the swap actually work.
| Baking soda | Baking powder | Plus acid needed for the soda |
|---|---|---|
| 1/4 tsp | 1 tsp | 1/2 tsp cream of tartar |
| 1/2 tsp | 2 tsp | 1 tsp cream of tartar |
| 3/4 tsp | 3 tsp (1 tbsp) | 1.5 tsp cream of tartar |
| 1 tsp | 4 tsp | 2 tsp cream of tartar |
Baking soda (sodium bicarbonate) and baking powder both release carbon dioxide gas to make batter rise, but they aren't the same product at different strengths — they're built differently. Baking soda is a single pure compound that needs an acid already present in the batter (buttermilk, yogurt, honey, molasses, brown sugar, natural cocoa, or lemon juice, among others) to trigger its reaction. Baking powder is baking soda pre-mixed with its own powdered acid (commonly cream of tartar or a sodium aluminum compound) plus a starch filler to keep the two dry ingredients from reacting early in the can, so it can leaven a batter with no other acid required. Because of the acid and filler, baking powder is considerably more dilute: it takes roughly four times as much baking powder as baking soda to produce the same amount of lift, which is the ratio this converter is built around (1/4 teaspoon baking soda ≈ 1 teaspoon baking powder).
The direction of the substitution matters for a different reason too. Going from baking powder to baking soda (using less soda in place of more powder) is the trickier direction: the soda alone provides plenty of lift, but only if there's enough acid around to react with it. If your recipe doesn't already contain a strongly acidic ingredient, you'll need to add one — commonly 1/2 teaspoon of cream of tartar for every 1 teaspoon of baking powder you're replacing — or the cake will under-rise and can pick up a distinctly bitter, metallic aftertaste from unreacted soda. Going the other direction, from baking soda to baking powder, is more forgiving: baking powder brings its own acid, so you don't need to add anything extra, though using several times more of it than the original soda amount can occasionally leave a faint bitter or salty edge if the recipe was tightly balanced.
One thing this converter can't fix: baking soda also neutralizes acidity in a batter, which affects browning and can soften a crumb in ways baking powder alone won't replicate exactly. For a small one-off substitution in cookies, pancakes or quick breads, the difference is rarely noticeable; for a recipe that leans heavily on that acid-base reaction (like a very dark chocolate cake using natural cocoa), expect a slightly different result even with the ratio corrected.