Brown sugar is one of the easiest foods to substitute, in that it is a combination of white sugar and molasses.

If you need a cup of light brown sugar, add 1 to 2 tablespoons of molasses to a cup of granulated sugar. If you need a cup of dark brown sugar, add 2 to 3 tablespoons of molasses to a cup of white sugar. Similarly, if you have light brown and need dark brown sugar, add a tablespoon of molasses per cup.

To go in the other direction, if you have dark brown sugar and need light brown, mix dark brown sugar with granulated sugar on a one-to-one basis (1/2 cup dark brown sugar plus 1/2 cup granulated sugar to equal 1 cup light brown sugar).