Here is what I remember about the sub... You need baking soda and an acid like vinegar. For 1 tsp of baking powder, try mixing a bit of soda with some vinegar??? I think it is 1/4 teaspoon of soda, but I am not entirely sure about the vinegar part. You might need more or less depending on your wet ingredients. Just be careful because it fizzes up right away and you have to mix it into the batter fast before the reaction dies down... Hopefully this helps even though I am not 100% certain on the exact measurements for bran muffins specifically. Check another source just to be safe