Actually, there are two convetnions to be aware of. Bags are usually described in terms of total loft - the fluffed-up bag laid on a flat surface and the total height measured. On the other hand, Charts that show how much insulation is required for a given temperature usually refer to a single layer - so if the chart recommends 2" of loft for the temperature you want to be comfortable in, you need 2" of loft above you, or roughly 4" of total loft. f course, all the reccomendations for amounts of insulation have to be interpreted by you for your own needs - you may sleep warm or cold.