I've got some figures put together, to figure out how much fabric I need for sashings. Anybody better at math want to review my #'s?

I have 12 blocks, will be 4 across and 3 down
Each block is 12.5"
I want sashing to be 2" (cut @ 2.5")

My figures:
I need 2 horizontal strips, each 57.5"
I need 9 vertical strips, each 12.5"

If my fabric is 40" wide, I would need 3 for the horizontal strips. How many would I need for the vertical sashes? Just 1? Would I need about a 1/2 yard of fabric?

Thanks!

I'm assuming the 12.5" is the measurement before you assemble into rows?

So you'll be sewing 4 blocks into a row using 3 sashing strips, right?

Then you'll take the 3 rows and use 2 LONG sashing strips to make the top?

Okay, based on my assumptions above, you do need the nine (9) 12.5" vertical sashing strips and two (2) 54.5" strips (not 57.5" - think in terms of "finished" sizes and then add the last seam allowance, so 4 blocks * 12"/block + 3 sashings * 2"/sashing + .5" seam allowance = 48 + 6 + .5 = 54.5")

Altogether you need 6 WOF sashing strips. You can get 3 verticals from each WOF (totals 37.5") and then, as you said, will need 3 for the horizontal strips (piecing them either diagonally or straight). So 6 WOF strips * 2.5"/strip = 15". IF your fabric is cut perfectly straight and you NEVER make any cutting mistakes, then 15" is all you need. If you think you might need a 7th WOF strip to fix a cutting mistake or you think the fabric will need to be straightened, then get 18" or better yet 21".