By Robin Strobel (an expert quilter who is also an experienced teacher, technical editor, illustrator, and author)
"I’ve always had trouble with numbers. I stand in awe of those of you who can manipulate them with skill and finesse. I break into a sweat if I need to figure out what numbers to punch into the copy machine in order to enlarge a 12″ appliqué pattern into 15″. Are you with me here? Do you have a math phobia too? If you don’t and number among the math adept, please feel free to smirk and feel superior. I envy you.
Quilting has a way of sneaking past phobias. Thanks to a friend, I was actually able to learn how to change the size of quilt-block patterns, and before I knew it, I was competently working with proportions and percentages of all types! The hardest part was getting past my belief that I couldn’t do quilt math because it involved numbers. Here’s how it works. All you need is an inexpensive calculator, a little bit of courage, and the phrase:
“Ya start with whatcha want, and ya divide it by whatcha got.”
Imagine a darling little appliqué that’s just perfect for your wall, but the 12″ block is too large. You decide you’d like to make it 10″ square. You take the 12″ pattern to a photocopier with the intent to make it smaller, but what percentage should you make it? In the words of my friend, “Ya start with whatcha want, and ya divide it by whatcha got.”
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″ pattern to 83.3% to make a 10″ block. Yes, it’s that easy to figure out.
Let’s work it the other way and make it a little more complex. You have an appliqué pattern for a 6″ x 7 1/2″ 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 ÷ 6 = 1.3333. This is what I call the “proportion number.” Move the decimal point two places to the right, and you’ve successfully determined you need to enlarge 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 1.333 = 9.99975. The heart will be about 10″ tall.
One way to check that you did the math correctly is to remember the following. The proportion number will always be greater than 1.000 if you’re enlarging something and will always be less than 0.999 if you want to make something smaller. Always. If you want to reduce a pattern but you have a proportion number larger than 1.000, you likely entered the wrong number into the calculator first. I do it all the time.
Remember, “Ya start with whatcha want.”
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 do 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.
I hope you were able to face all this number stuff without getting a headache, though I’m certain those of you who find math easy are rolling your eyes by now. Remember, to keep quilts in proportion, start with what you want, and divide it by what you’ve got."