Before I cut wrong
 

I watched a you tube video and sewed 4 strips of a jelly roll together.[2 1/2 inches]. the video said to cut 8 inches to make a square. I think it should 8 1/2 inches to make a square. Please help me with math. thanks

I think whatever the width of your strip is now which would be 8.5 if your 1/4" seam is accurate, would make you right...it should be 8.5 or it won't be square????

seems right to me. the outsides of your strips still have 1/4 inch seam allowance on each side.

I agree with the 8.5". Four strips sewn together takes 3 seams @ half an inch each, or 1.5" total. Four strips measure 10 inches unsewn, subtract the seams and you get 8.5".
However, as Tartan suggested, I'd measure your sewn strip set and use that width as the measurement in case your seams are bit off.

Maybe the wording was wrong. Probably should have said cut 8.5 inches to make an 8 inch finished square.

As long as you are consistent with cutting and seams and cut the fabric to a unfinished square, you will be fine.

Looks like the instructions forgot seam allowance of 1/2"