If I want a finished box cushion to be 22 x 22 x 3, what size should I cut the foam? Should I cut the bottom and top pieces the same size, or add seam allowances? I have done this before, but not recently and can't remember how I sized them. Thanks for your input.
Everyone does it a little different. For a 23 X 23 X 3 finished cushion I cut the foam 24 X 24 x 3-1/2. Then I wrap the the top, around the front and the bottom with Dacron. I don't wrap the sides. I always find this gives too much push and makes the cushion look strained.
I don't stock 1/2 inch sizes of foam. I always try to make my boxing finish at 2-1/2", 3-1/2", 4-1/2", etc.
ditto
Thanks so much. I'll cut the foam to 23 x 23 x 3 1/2 and the fabric the same size then.
I generally overcut foam by 1/2" on the perimeter. I do a lot of marine work and cushions frequently have to fit inside/behind coaming. So, if you overcut by 1/2" on all sides and you have multiple cushions to fill the area behind the coaming you will find yourself in a "world of hurt" in pretty short order (a lesson I relearned painfully not too long ago!).
So be careful with overcuts! only time and experience will give you the "key" to overcutting foam. And even then you can have your hindquarters singed by complacency (ask me how I know).
The foam I use is sold in even number heights. I'll add batting, but that's it. I cut boxing so that the finished boxing is 1/2-3/4" less than the finished height of the foam. I use a 1/2" seam allowance, and will occasionally use a 3/8" seam allowance.