janjj

Not sure where I found this but here goes:
Ya start with whatcha want, and ya divide it by whatcha got.
So
begin with your goal-- it's the reason you have to deal with quilt math in the first place. What you want is a 10" block, so punch "10" into your calculator first. Hit the division key, then enter the number 'ya got," which is 12. Press the "=" key The number 0.83333333333 pops up.
The copy machine wants a percentage, so move that pesky decimal point to the right by two spots, and then you're done. Because this is a quilt, not a suspension bridge,you don't need all of the decimal points, so ignore them. You need to reduce the 12" patters to 83.3% to make a 10" block.
Now lets work it the other way and make it a little more complex. You have an applique pattern for a 6" x 7 l/2" heart, and you decide you'd like to make it at least 8" wide, but you're clueless how tall that will be. Ask yourself, what is it you want" An 8" wide block. what do you have"? A 6" wide block 8 divided by 6 = 1.3333. This is what I call the 'proportion number." Move the decimal point two places to the right and you've determined you need to enlage the heart pattern 133.3%. How tall will it be" in this case you multiply the original height (7 1/2") by the proportion number, which is 1.3333. so 7.5 x 1.3333 = 9.99975. the heart will be about 10" tall.

The proportion number will always be greater than 1.0000 if you are enlarging something and will always be less than 0.999 if you want to make something smaller.

When changing the sizes of blocks, always do the proportion calculations with the numbers for FINISHED sizes, not cut sizes. This is because you use different numbers to add seam allowances to a triangle (7/8" for half-square triangles or 1.25" for quarter-square triangles) than you add for the seam allowances of a square or rectangle (1/2"). Do all the proportion calculations for the finished pieces first, and then add the seam allowances.

Hope this is clear
Janice