2 1/2"

Depends on what kind of batting but the standard I mostly use is 2 1/2". :)

I agree that the width should be determined by the size finished binding you want. Like Holice, I start with my finished width but then I multiply by 7 for a handstitched seam. Since I don't do handstitching very much but do almost all bindings totally by machine, I multiply my finished binding width by 7 then add 3/8th to give me enough extra to do that machine stitching.

As Holice and others have said, practice different ways and decide what works best for you.

It's really personal preference! I belonged to a smallish guild that had speakers two months in a row. The first one told us she cuts her binding 1 3/4ths and gave her reasons. The next month the speaker said she cut hers at 3 inches and gave her reasons. Do what looks good to you!

Was that for French double-fold binding? Because that seems really, really tight.
Do you remember what her reasons were? Just curious.

Edited to add "French" - I was thinking it but apparently didn't type it. :roll:

It was for French fold binding which is the name of what you're calling double fold binding. I don't remember what her reasons were.

Double fold binding is still only one layer thick. It just means both raw edges are folded to the center. Single fold binding means one raw edge is folded to the center.

Anywhere from 2 1/2 - 3 1/2 depending on how wide I want the finished binding to be. I personally tend to like them wider.

I cut 2 1/4 if I'm going to stitch it onto the front and hand-sew it to the back. I cut 2 1/2 if I'm going to wrap to the front and do a decorative stitch.

2 1/4

I cut it 2.5 and fold in half. My binding is always wider on the back, but that's how I want it. I sew with 1/4" seam on the front.
Sue