If you accidentally added baking soda instead of baking powder to your cookie recipe you will need to add something acidic to allow them to rise. This could be lemon juice, yogurt, cocoa powder or cream of tartar.
Most cookie recipes only need a small amount of raising agent to give them softness. When I used baking soda instead of baking powder in my sugar cookies they turned out the same because they contained some self-raising flour.

Baking soda is a pure form of sodium bicarbonate which releases carbon dioxide when mixed with an acid. Baking powder is sodium bicarbonate with an added acid like cream of tartar. This means that the cookie recipe does not need to contain the acid for the reaction to occur. Baking powder often has added cornstarch to bind and stabilize the product on the shelf.

If your recipe includes an acidic element like cocoa powder, lemon juice, yogurt or cream of tartar you can use baking soda instead of baking powder.
If the recipe contains some self-raising flour you will get some lift from the raising agent in the flour.

I baked the same cookie recipe, one with baking soda and one with baking powder. The cookies turned out exactly the same with the same amount of lift.
The other ingredients included plain flour, self-raising flour, butter, sugar, golden syrup and cinnamon.

What to do if you accidentally using baking soda instead of baking powder in cookies
If you accidentally use baking soda instead of baking powder it is important to add an acid to the recipe to make sure the cookies still rise. Here are some easy ways to add acid to your recipe so the cookies rise properly.
1. Add ½ a teaspoon of cream of tartar
If you accidentally add baking soda instead of baking powder, you can fix it by adding a small amount of cream of tartar. Add ½ a teaspoon of cream of tartar to the recipe and mix it in. This will add a small amount of acid which will allow the cookies to rise when they bake.
2. Add cocoa powder
Adding cocoa powder to the mixture is a great way to add acid and to allow the baking soda to react and release carbon dioxide. Add 2-3 tablespoons of cocoa powder to your mix to make the cookies chocolatey and rise.

3. Add buttermilk
Adding a small amount of buttermilk is a great way to add acid to the recipe. You only need to add a tablespoon to help the cookies to rise.
If you don’t have buttermilk at home you can make some yourself. Get a cup of milk and add tablespoon of vinegar or lemon juice. After around 5 minutes you will have soured milk. You can replace the soured milk with the water or milk in your recipe.
Add 2 tablespoons of buttermilk or soured milk to the recipe. Add some extra self-raising flour if the mixture has become too wet to work with.
4. Add lemon juice
Add a tablespoon of lemon juice to your recipe to give it a lemony hint and to help the baking soda to create carbon dioxide. This is perfect for vanilla or sugar cookies that are not chocolate based.
The lemon juice or even zest will give your cookies a yummy tang and help the baking soda to react.
5. Add yogurt
Adding yogurt is another great way to add acid to the cookie recipe. You only need to add a tablespoon of plain, natural or Greek yogurt to the cookie batter to add enough acid to help them to rise.
What happens if you use baking soda instead of baking powder in cookies?
Most of the time there will not be any difference in the cookies that you bake with baking soda or baking powder. Most recipes only include ½-1 teaspoon of baking powder which adds a small amount of lift.
To be sure that they still rise it is best to add some acid to help the baking soda to react. If you are already making cookies with cocoa powder then they will be fine as they already contain acid.

Is it OK to use baking soda instead of baking powder in cookies?
It is fine to use baking soda instead of baking powder in cookies if they contain an acidic ingredient. The easiest way to do this is to add ½ a teaspoon of cream of tartar. This naturally acidic ingredient will help the cookies to rise rather than leaving them flat and hard.
Accidentally Used Baking Soda Instead of Baking Powder in Cookies | Summary
Most cookie recipes will be fine if you accidentally use baking soda instead of baking powder. To be sure that they still rise add a small amount of acid like lemon juice for sugar cookies or cocoa powder for chocolate chip cookies. The cookies will still taste delicious and will rise nicely.
I am an accredited practicing dietitian, experienced gardener and a dedicated cook. I love writing and sharing my experience so you can learn from my successes and mistakes.