Quick answer
For every 1 teaspoon baking powder, combine 1/4 teaspoon baking soda + 1/2 teaspoon cream of tartar + 1/4 teaspoon cornstarch or tapioca starch.
If you are mixing the substitute directly into batter and baking immediately, the starch is less important; its main job in a dry mixture is to help absorb moisture and keep the acid and base separated.
Conversion table
| Baking powder needed | Baking soda | Cream of tartar | Starch |
|---|---|---|---|
| 1 tsp | 1/4 tsp | 1/2 tsp | 1/4 tsp |
| 2 tsp | 1/2 tsp | 1 tsp | 1/2 tsp |
| 1 tbsp (3 tsp) | 3/4 tsp | 1 1/2 tsp | 3/4 tsp |
What changes in the recipe?
Most supermarket baking powder is double-acting: it releases some carbon dioxide when moistened and more as the batter heats. Cream of tartar plus baking soda reacts primarily after it becomes wet, so the batter should not sit around for a long time before baking.
Can I use baking soda alone?
Not reliably. Baking soda needs enough acid to neutralize it. A recipe containing buttermilk, yogurt, natural cocoa, molasses, lemon juice, or another acidic ingredient may already have acid available, but changing the leavener without recalculating the formula can affect flavor, browning, spread, and rise.
For an emergency substitution without reformulating the recipe, the cream-of-tartar mixture above is the more predictable starting point.
When not to improvise
Use extra caution in very light cakes, recipes with large amounts of chemical leavener, or recipes where the acid/base balance is central to texture and color. In those cases, using the specified baking powder or following a recipe developed for the alternative is more dependable.
Sources & related guides
This ratio is based on King Arthur Baking's tested guidance for replacing baking powder with baking soda, cream of tartar, and starch.