It's confusing, we know, but it works
There's no feeling of panic quite like the one when you realize that a recipe calls for baking soda, and you only have baking powder available in your pantry. Even though they look pretty similar, baking soda and baking powder are two different ingredients, and if you just make a one-for-one substitution—like using one teaspoon of baking powder in place of a teaspoon of baking soda—your baked good will turn out all sorts of wonky. So how exactly do you substitute baking soda?
It turns out that if a recipe calls for baking soda, you can swap in baking powder without adding any additional ingredients, but if a recipe calls for baking powder, it's more difficult to swap in baking soda, because you'd also need to add cream of tartar or another acid. That's because baking powder is made from baking soda; it's technically a combination of baking soda and a dry acid. So if you're replacing baking soda with baking powder, you're playing with proportions rather than adding more ingredients.
To substitute baking powder for baking soda, simply use three times the amount of baking powder as you would baking soda. This counteracts the addition of the dry acid and creates the right chemical reaction in your batter or dough. So if a recipe calls for a teaspoon of baking soda, use three teaspoons of baking powder instead.
It's not that easy, though—because nothing these days is. As Bri McKoy explains on her blog Our Savory Life, there is usually a bit of salt in baking powder, so she recommends that you, "omit any salt the recipe calls for" when you're making this substitution. But if you forget to forget the salt, it's not that big of a deal; your baked goods might be a tad on the salty side, but they probably won't be inedible. If you really want to be sure you're doing it right, though, just get a box of baking soda the next time you go to the store so you can avoid making any substitutions.