→ 1 - 1 1/4 teaspoon baking powder per cup of all-purpose flour
→ 1/4 teaspoon baking soda per cup of all-purpose flour
When both baking powder and soda are called for in a recipe, it's usually because there is an acidic ingredient in the the recipe. Some of the baking soda or powder goes toward neutralizing the acidity, so you need a little extra to make sure your cake or muffin still rises. If your recipe contains an acidic ingredient like yogurt or buttermilk, include an extra 1/4 - 1/2 teaspoon of baking soda per cup of an acidic ingredient.
• Breads and Rolls: Yeast is a much more forgiving ingredient than baking soda and baking powder! Yeast-risen breads and rolls can be doubled without needing to adjust any of the quantities. If you're tripling or quadrupling, however, I'd recommend weighing your ingredients rather than measuring them by volume to make sure the ratios stay consistent.
If you're going the other way and looking to cut these recipes in half, the same general principles apply. Check the ratios of baking powder and soda, but halving the other ingredients should give you great results.
What tips do you have for doubling recipes for baked goods?
(Image: Chewy Molasses Cookies/Faith Durand)