Q. How much baking soda is in baking powder?
The exact make-up of baking powder a combination of baking soda, one or two acid salts, and ground starch varies from brand to brand. But in general if you make your own single-acting baking powder, you combine 1/4 teaspoon of baking soda with 1/2 teaspoon of cream of tartar to substitute for
1 teaspoon of commercial baking soda.
Leaving out the starch, which is necessary in the commercial product to keep the soda and acid from reacting prematurely in moist air, that leaves you with a general ratio of 1 part baking soda to 2 parts acid.
Baking soda has four times the leavening capacity of baking powder, so you
don't want to alter the ratio of 1:4 either.