Use 1/3 teaspoon of baking soda for every 1 teaspoon of baking powder called for in the recipe.
Baking Powder Substitute With Cream of Tartar:
To make 1 teaspoon of baking powder substitute, add 1/3 teaspoon of baking soda and 2/3 teaspoon of cream of tartar in a small bowl. Mix until thoroughly combined.
Notes
To increase the cream of tartar baking powder substitute:
For 2 teaspoons baking powder: 1/3 teaspoon baking soda plus 1 1/3 teaspoon cream of tartar
For 1 tablespoon baking powder: 1 teaspoon baking soda plus 2 teaspoons cream of tartar
To substitute baking powder for baking soda:Using baking powder instead of baking soda is trickier. That’s because you would need to use 3 times as much baking powder to get the same rising effect; you could end up with a bitter, chemical taste if you add too much leavening.
If you need 1 teaspoon of baking soda, then go for 1 tablespoon baking powder.
If the recipe calls for more than 1 teaspoon baking soda, you may want to wait for another time.