It's 3" foam.
It's pretty firm, but it works well, and wont overpower the frame.
If you're planning on one large cushion I suggest a heavy fabric. The fabric on mine is by Adonis, its smooth on the backside and I think this helps it from creeping around on the cushion, if you know what I mean.
kcbrains
Your fabric has a backing of some sort, that's all.
Hawaiian, the creeping can be minimized by covering the foam with polyester upholstery batting. (Some places just call this "dacron".) That way you can use any fabric you want.
The batting has to be adhered to the foam with a good quality spray adhesive---3M Heavy Duty is good and you can get it at Home Depot and Lowe's and places like that.
Be sure to use upholstery batting and not quilt batting. They look the same and are made of the same stuff but quilt batting is softer and will not hold up well at all. (I tried it once. Big mistake.)
Look at Maharam/Kvadrat wools--they are what was used originally on Danish furniture and the quality is excellent. Hallingdal, Tonus, Tonica are all great. I like Steelcut for a smoother look. There are some boucles that are nice too--Outback, Tundra and one or two others, I think. They will send you samples for free if you register on their site but they require a tax ID number to buy directly from them.
(Some of the Kvadrat fabrics are very contemporary synthetics---i would just avoid those even if the colors are tempting.)
http://www.maharam.com/search?only=products&query=kvadrat
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