Does anyone know if there is a formula for figuring the diagonal measurement of a quilt block? Say, if I make an 8" square 9-patch block, is there a formula that I can use to figure the diagonal measurement from one corner to the opposite corner? Oh, I know I could just make a block and then measure it's diagonal, but was hoping there is some easy method to figure it out on paper. I'm wanting to make a quilt with sashed blocks on point and want it to end up a certain size, but don't exactly know how big to make my blocks and sashing. Any help would be appreciated.

The diagonal of a square is 1.414 times the length of one side. Best to round up and trim down.

Pythagorean Theorem? A squared + B squared = C squared is the formula for a right triangle. 8 squared + 8 squared = C squared. 64 + 64 = C squared. 128 = C squared. The square root of 128 = 11.3

Your way is much easier.

.... or instead of "making" it, you could cut draw it on paper.
Don't forget the seam allowances!

the 1.414 number is used in both directions- if you have a square you need the diagonal of- times the size of the square by 1.414- if you want to figure out the size of setting triangles for said square- to make it on point you divide by 1.414 make a square that size- then cut in half diagonally for your setting triangles.
also- if you have a square- which you multiply by 1.414- that gives you the size the square will be when put on point.

It's the same as yours, just more universal. If the sides are 1, the diagonal equals the square root of 2 which is 1.414. ;)

You need to add in the seam allowance before doing the math. So the 8" finished block becomes 8.5". Plugged into the formula, you get 12.02. Just to demonstrate that you can use the formula OR the "Multiplying by 1.414" trick, multiplying gives you 12.019. Round UP to the nearest 1/8 for ease in cutting, so sashing strips would need to be cut at 12 1/8.

For quick no-math measurements I measure diagonals on my cutting mat. Quick and easy, no brain strain. Don't forget to add seam allowances.

Sashing would be the length of the side of the block, just like straight set quilts. You sash the sides, not the diagonals, using the unfinished length.

Diagonals are needed for setting and corner triangles and to compute that measurement, you use the finished block size plus the finished sashing width....not adding the seam allowances. Here's a chart for you to use.
http://quiltville.com/onpointmath.shtml

How big do you want your quilt to be?