What is a substitute for baking powder?

If I don't have any baking powder, what should I use?

Baking powder substitute:
For each 1 teaspoon baking powder called for in a recipe, use:

1/4 teaspoon baking soda plus 1/2 teaspoon cream of tartar;

OR

1/4 teaspoon baking soda plus 1/2 cup buttermilk, sour cream or yogurt (to replace 1/2 cup liquid called for in recipe).

Substitutions: 1/2 tsp cream of tartar + 1/4 tsp baking soda = 1 tsp baking powder; 1 tsp double-acting baking powder = 1/4 tsp baking soda + 1/2 cup buttermilk, sour milk or yogurt (to replace 1/2 cup liquid in recipe); 1 tsp double-acting baking powder = 1/4 tsp baking soda + 1/4 cup molasses (reduce liquid in recipe by 1/4 cup; adjust sweeteners); 1 tsp double-acting baking powder = 2 tsp quick-acting baking powder; 1 tsp single-acting baking powder = 3/4 tsp double-acting baking powder

Baking powder substitute

For 1 teaspoon, use: 1/4 teaspoon baking soda plus 1/2 teaspoon cream of tartar; or 1/4 teaspoon baking soda plus 1/2 cup buttermilk, sour cream or yogurt (to replace 1/2 cup liquid called for in recipe).