How much buttermilk should you use instead of 1 cup of milk?

To substitute buttermilk for 1 cup of milk, use 1 cup of buttermilk. If you don't have buttermilk on hand, you can also make a quick substitute by adding 1 tablespoon of lemon juice or white vinegar to a measuring cup and filling it with milk to the 1-cup line. Let it sit for about 5-10 minutes before using.