I have a block made that measure 10 x 10 . I want to make it 12.5 x 12.5 . Do I add a 2.5 inch border ?

Add 2 3/4 inch
this is because a 1/4 inch will be used up from the ten inch block and a 1/4 inch will be used up from the strip when sewing them together.

Don't you want a three inch border? You'll lose 1/4" from each the block and the strip, so that 1/2". 10+3=13-1/2=12 1/2.

I will add a 3 inch border. Then the person recieving can square it up to whatever size they need. This block is being sent to someone on this site to make a quilt for a child with cancer. Thanks everyone!

10. + .25 + 2.50 + .25 = 3" strip to end up with a 12.50" block.

If you add a three inch border won't you get something that is 15 X 15. I assume you are adding the border on all four sides.

QuiltSage is right. The borders need to be cut at 1 3/4 inches. You can cut them at 2" and square to 12 1/2" if you wish.