Sleeping bag to quilt conversion

Well, you could strip all the down out: that would save a bit of weight…
Me, I took the heavy zip out and left it at that. Kept ALL the down!

Cheers

It really depends on how much wasted space there is. Lay the bag out flat and measure the top from corner to corner, then compare that with the width of the quilt you want it to turn into. 50" is pretty narrow unless you're hammocking, 55" is a common standard, and 60" would be luxuriously wide for most people. Depending on the sleeping bag, you might end up cutting a way a lot of material or a little.

If I were to venture a guess, I'd say you'll get 2-3 oz for the zipper and minimal weight savings after that. Probably no more than 1 oz for each 5" of width you cut away, a little more if you take the down with it.

If you're cutting away a lot of fabric and want to moderately increase the temperature range of the bag, you could move the down toward the center and leave the same amount of down distributed over a smaller area.

I converted an old cheap 20F synthetic bag into a winter overquilt by cutting out the zipper and the hood. The difference in weight was 5 oz, but I'd expect less for a nicer bag with lighter fabric/insulation.