I need some math help...
 

I want to make a throw quilt that is 50'' x 65''.
I just want to use 9 patch blocks, using 2.5'' squares to make the blocks.
How many blocks do I need to make to get to that throw size?

2.5" squares will finish to a 6" block, so you have to adjust your finished size accordingly. You'd have to make your throw either 48x66 ( 8x11 blocks =88) or 54x66 (9x11 blocks =99). Hope this helps you.

I'm not great at math but I do know that nine 2 1/2" x 2 1/2" squares will make a 6 1/2" unfinished block (6" finished block).
If you make 88 9-patch block this size, you could made a quilt that is 48" x 66" by using 8 blocks across & 11 blocks down. You could enlarge the quilt width slightly if it needs to be 50" x 65" by putting on a border around it. Or you could make 99 blocks & put 9 across the top & 11 blocks down & that would make your quilt 54" x 66".
Now please wait before you start making all those blocks & see if someone else comes along & lets me know if I did the math right!!

Are you using sashing between blocks? If so you could make 48 blocks set 6 X 8. With sashing between them

I drew it up in EQ for a visual.
As osewme said, with the parameters you want, you will end up with a 6" block. This layout is 8 by 10 blocks with a 1" border all around, which results in a quilt that is 50x62.

Wonderful, Peckish!