In some recipes, you can substitute 3/4 cup milk plus 1/3 cup butter or margarine for 1 cup of heavy cream. If your recipe calls for whipping that cream, it won't work. You need the fat that's in heavy cream in order to get whipped volume.
When a recipe calls for half-and-half, you can substitute 7/8 cup milk plus 1/2 tablespoon of butter or margarine.
Source: Marge Perry