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?

This is exactly what I do!!!

Take a quilt square ruler and measure from 8" to 0 diagonally across the ruler.

No, the formula is used on FINISHED sizes/measurements.

The seam allowances are either added to or subtracted from the sizes, after or before the calculation, depending on what you're trying to figure out.

If I've got an 8" square block and I want to know how wide it will be on point, it's 8" x 1.414 = 11.3" (then round up).
The unfinished size will be ~12", but that has nothing to do with the calculation.

Let's say I've got a design where I want six on-point blocks/squares that are ~10" across (because I want a 60" wide top).
I take 10" divided by 1.414 = 7.07" (and round up or down).
I would THEN add the seam allowance to cut my squares.
For blocks, I'd want them to be the 7" (or 7.125" or 7.25", depending on your round) FINISHED.

The seam allowance is always a constant .5".
You wouldn't want to perform the calculation by including it.
Actually, you'd pretty much exclude it for any calculation (not just this example).

correct.
The theorem can be written as an equation relating the lengths of the sides a, b and c, called the Pythagorean equation: where c represents the length of the hypotenuse (the diagonal), and a and b represent the lengths of the other two sides.

Sorry, but that does not take the seam allowance into account. We just had that discussion on the German forum. It appears that Mrs. Pythagoras had it figured out, but her husband didn't listen. So the seam allowance is not part of the Pythagorean Theorem.

:? math major???? Totally impressed. :thumbup:

Show off! LOL huh??

Middle School Math teacher, lol. You caught me!

This works. Just have a calculator handy as it is much safer.

ali

Oh Boy, Do I have to understand all this. I'll wait until I need to know and ask or I'll bookmark this one!!!! EEEIKKEES

Half square triangles - If cut from a square - the theoretical amount to the add to the finished size is 7/8 inch. I actually add an inch and trim the HST units down after sewing them.

(Example: Four inch finished size (two pieces make a square unit) - cut 4-7/8 inch squares

Quarter square triangles - if cut from a square - the theoretical amount to add to the finished size is 1-1/4 inch

(Example: Four inch finished size (four pieces in the square unit) - cut 5-1/4 inch squares.

I am so happy I have no plans to do this! LOL

You have to go back to Pythagoras' Theorem:

http://www.mathguide.com/lessons/Pythagoras.html

I am totally confused, MATH isnt a strong point for me !!!

Thanks everyone, this was really helful for those of us who like to do our own thing!

Salute all these clever girls!!!!!!!! But after all thank you.

WOW!!, you are really smart. You must have done well in your math classes

I needed to know that formula the other day and now I can't remember why. I hate getting old. I saved the formula for when I remember why I needed it.

That was my first thought, too!

Bonnie Hunter has computed them all. go to her website . www.quiltville.com

Quilt University offers a class called Math for Quilters that is one of their most popular classes. It's taught by Dena Crain who is an excellent teacher - I've had her for other classes.

The Math class is next offered starting April 27th and covers all sorts of things that are frequently asked here on the board...planning a cutting layout, on point quilts, drafting blocks in different sizes, figuring yardage requirements, calculating the cost of a quilt, setting a price for it.

Info on this and other QU classes at http://www.quiltuniversity.com/catalog.htm

GOODNESS this is SO MUCH EASIER doing it this way!

I draft out the darn thing on graph paper and measure with a ruler.

I can't wait to go home and try this and use it!

Thank you all for the lesson(s).

The 1.414 works with a SQUARE which is also a rectangle.
The A squared + B squared theory works with any right triangle (or diameter of a rectangle) meaning that the angle of intersection of the sides has to be 90 degrees. Ann in TN

This is exactly the reason I'm not a math major!!!!LOL

Quick math is size of block multiplied by 1.5 (actual measurement is 1.414 or something, but 1.5 will get you almost the exact number.

I recently learned this trick from a longarmer. I'm going to demo it at our next guild meeting. It is way too simple & easy so at first I wondered if it could be right. But. . .it is! Have used on several different size blocks. Much easier than math.

Sorry - the trick I learned was using the diagonal line on the cutting mat & trimming down after triangle was sewn to the diagonal of the square.

I have a book called "All in one quilters reference tool" written by Harriet Hargrave, Sharyn Craig, Ales Anderson, and Liz Aneloski. i use this reference all of the time for everything..it has easy to follow charts, tables, and illustrations for yardage requirements, cutting instructions, setting secrets,choosing supplies, piecing techniques, number conversions and the list goes on. I highly recommend it as a quick reference for just about everyone. It is put out by C&T publishing for $16.95..that is the price on mine which admittedly is a few years old. It does have a table for diagonal measurements for squares amd lists them for sq. from 1.5 inches -12.5. If you want to pm me and let me know the sq. size you are interested in i would be glad to look it up for you.

Try this

http://www.mathopenref.com/squarediagonals.html

I did a search on Bing and found a complete list of sizes for putting it on-point.

http:/www.quiltville.com