Question about a little math
 

Okay everyone I have a 16 patch that finishes at 10.5 inches want to combine with pinwheel of the same size, what size should I cut the squares to make the half square triangles to make the pinwheel. My thought is 5 7/8 but just not sure. Any and all help would be greatly appreciated.

Your block FINISHES at 10.5 inches. That is the size each block will be when they are sewn into the quilt? Just making sure here. That means that each of your four HST's for the pinwheel will FINISH at 5.25 inches. So add 7/8 inch to the finished size 5 1/4 plus 7/8 which would make 6 1/8 (or better, add 1 full inch and trim down to the exact size you need.

But if your block is now 10.5 inches, that means that the FINISHED size will be 10 inches and for the pinwheels, you need 4 squares whose FINISHED size would be 5 inches. In that case, you are correct, you would cut them at 5 7/8 inches. Again, I would round up to 6 inches and trim.

Cut the HST squares at 6.5" before making the HST.
Trim them to 5.5" for your pinwheel block.

Jan in VA

If you cut the squares at 5 7/8, they would FINISH at 5", so your pinwheel would FINISH at 10" not 10.5".
Finished means completely sewn into the quilt.

thanks ladies just couldn't wrap my mind around it.

I always round up the 7/8" to an even round number so I would cut 6" squares, starch, press and then trim.