New to Piecing: Need Help with math
 

I am creating a quilt for my father's 90th birthday. I have a 10.5" center square. I want to surround it with a 10.5" square cut from 3 strips of different fabrics. Can someone please help me with the measurements of each strip width?? Is there some kind of calculator or website or?? that would help me with this?? I hope this makes sense!!! Thanks sew much for your assistance.
If I need to change the size of my center square that is also totally do-able.

(3) 4" strips when sewn together will = 10 1/2" (4" unfinished = 3 1/2" finished---3 1/2" X 3= 10 1/2"
to figure out sizes first subtract the seam allowance- do the math then add the seam allowance back in.

You're going to need some strange sizes to get a 10.5 inch square. The closest I could come up with are strips cut at
3 7/8 inches which totals 11 5/8 total but you lose one inch when you sew the strips together giving you 10 5/8 as close to 10.5 as I can figure. Maybe just make your seams a thread wider than 1/4 inch. Or just sew some sample strips together and see how they measure. Hope that helps. This should work if your center square is 10.5 unfinished.

OK.... DH came to my rescue and between us we figured it out... we think. I will need my center strip to measure 4" and 2 outside strips to measure 3.75. When seam allowances are taken all strips will be equal @ 3.5.

ckcowl is correct if you want the finished size to be 10.5". Apparently you want the finished size to be 10" instead? In your example with DH's help, the outside strips will be still be smaller than the inside strip when all are sewn into the quilt.

If you are just trying to frame the inside block with three fabrics, why not just do that instead of making blocks?

ckcowl is correct. I took 10.5 and divided it by 3 which = 3.5. Now you have to add seam allowances and each of the three strips will need a 1/4" on each side of it so that is 1/2" per block x 3 = 1.5. So... 3.5 + .5 = 4.0

hmm, when you join this block to the next one, the outside strips will loose another 1/4", so they will wind up being 3.25, is this what you want?
Is the 10.5" the finished or unfinished measurement?

In the end I changed the square I was working on so all my mathematical deductions went out the window!Anyway, everything did work out with the strips and the piecing. I appreciate very much everyone's suggestions. Thank you

That is how I did my calculation too. Therefore each strip should measure 4 inches.