-For dark brown sugar from light brown sugar, use 1 cup firmly packed light brown sugar plus 1 tablespoon molasses; To make dark brown sugar from white sugar: 1 cup granulated sugar plus 1/4 cup molasses. read more
Light brown sugar has less molasses per total volume of sugar (about 3.5% according to Rose Levy Beranbaum) while dark brown sugar has more (6.5%). You can easily see the difference in their makeup using just your eyeballs: dark brown sugar is darker in color and looks more like molasses syrup. read more