Say you're in the middle of making pancakes, and you realize that you're out of baking powder. You may be tempted to simply substitute baking soda for baking powder in a panic—but don't do that. That's because baking soda is not a baking powder substitute. If you swap in an equal amount of baking soda for baking powder in your baked goods, they won't have any lift to them, and your pancakes will be flatter than, well, pancakes. You can, however, make a baking powder substitute by using baking soda.
All you need to make baking powder are two ingredients: baking soda and cream of tartar. According to our friends at MyRecipes, you should "combine ½ teaspoon cream of tartar plus ¼ teaspoon baking soda for every 1 teaspoon of baking powder." So if you need two teaspoons of baking powder, use a teaspoon of cream of tartar with a half-teaspoon of baking soda.
If you don't have cream of tartar on hand in your pantry, you can still use baking soda as a base for a baking powder substitute. You just need another acid to replace the cream of tartar, like lemon juice or white vinegar. Cooking Light recommends substituting one teaspoon of lemon juice or vinegar for every half-teaspoon of cream of tartar. So that means you'd use a teaspoon of lemon juice plus a ¼ teaspoon baking soda to make 1 teaspoon of baking powder.
The biggest issue with using white vinegar or lemon in place of cream of tartar is that the liquid will affect the taste, so you probably don't want to use vinegar in a sweet pancake batter. But otherwise, it's a good hack in a pinch. (Just be sure to add baking powder to your shopping list.)