For baking, what do you use if you run out of Double-acting Baking Powder?

Can I use the same quantity of baking soda? Or double?

2 tablespoons of baking powder can be substituted by using 1/2 teaspoon baking soda and 1 teaspoon cream of tartar. Cream of tartar is the acid that is needed to create the same rising reaction that baking powder produces.

Baking powder is a chemical substitute for yeast. Yeast produces carbon dioxide naturally. Baking powder uses a mix of baking soda and another agent to produce a chemical reaction that produces the carbon dioxide. Since it's a chemical reaction, it's added last in recipes to allow the leavening to occur.