When measuring loft for sleeping bags, are manufacturers measuring both sides (top/bottom) of the bag, or only one side?

Why do I ask? Because 15-20 degree rated quilts from Jacks are Better have 2.5 inches of loft, and similarly rated sleeping bags have 5-6 inches of loft.

Jacks are Better seems to have a pretty good rep - but so does Feathered Friends and Western Mountaineering.

Still not convinced I'm ready to pay an extra $200-$250 to get half a pound off my back, but I'm thinking about it.

I'm not sure, but I will say quilts are obviously only measuring the top. If Feathered Friends and Western Mountaineering are measuring top and bottom together, it wouldn't make that much of a difference since most of the down (and thus most of the loft) is on top anyway. 2.5" of loft doesn't seem like enough for a 15°F rated quilt.

The sleeping bag is counting both sides, top and bottom.

My 0 degree underquilt has 4" of loft. Loft is loft, assuming good down stuffage and quality.

My Jacks R Better 2.5" of loft has kept me well warm, into the mid 20s or so.

yes bag manufacturers (unless stated) measure the total loft.
Usually it is 50-50 because, if you are the typical sleeper that turns taking the bag with you , at times the bottom is the top, so it needs to be the same,
On the plus side for the sb is that they can for most trap the heat better, for example as you move air still has to pass through the loft rather than escaping from the side.
On top of that, or bottom, the compressed down under you does have little R value but at the same time it does have a little R value.

Thank you everybody - that was very helpful!!!