calculating yardage is basic arithmetic, but if you’re new to sewing and/or have never had to estimate how much material to buy, it can seem really difficult. (like barbie says, math is hard!) seriously, this is a very easy task once you understand how fabric “works”.
let’s start by pretending you’re making a 20 inch pillow and it requires three pattern pieces: a 20 inch square front, i.e. 20 inches long x 20 inches wide, and two overlapping backs that are 20 inches long x 15 inches wide.
(note: pillow covers are made to the same size of the form. the finished cover will be an inch smaller in width and height to give the pillow a “full” look. feel free to swap out 20 inches for whatever size form you want to work with. the backs are always the same length as the front but the width(s) are equal to 75% of the front. if you want to make a 16 inch pillow for instance you’d multiply 16 x .75 which is 12 inches. just to be clear, you would need a 16 inch square front and two 16 inch long x 12 inch wide backs.)
I find it works best for students to draw out the pattern pieces just like any pattern designer would; this helps you visualize the math stuff. btw, there’s also no shame in using a calculator or consulting a yardstick; both of these tools are super helpful!
let’s create our cutting diagram(s). grab a pencil and paper and draw a square and a rectangle. label the square “front” and the rectangle “back(s)”. write “cut 1” on the front and “cut 2” on the back(s). draw a long rectangle to represent your fabric and label the length as 36 inches and the height as 54-60 inches. next, add the width(s) of all three pattern pieces together, i.e. 20 + 15 + 15 = 50 inches. jot this measurement down. your fabric needs to be at least 50 inches wide to accommodate all 3 pieces.
we already know the length we need; all three pieces are 20 inches long, so write this down as well. a yard is equal to 36 inches in length and is most often sold in increments of a 1/8th yard. what’s 1/8th of a yard? if you divide 36 by 8 it equals 4.5 inches. therefore 1/8th of a yard is 4.5 inches, 2/8ths (1/4) is 9 inches, 3/8ths is 13.5 inches, 4/8ths (1/2) is 18 inches, 5/8ths is 22.5 inches, 6/8ths (3/4) is 27 inches, 7/8ths is 31.5 inches and 8/8ths (a.k.a. 1 yard) is 36 inches. come to think of it, this would make an excellent tattoo! so the answer is, you need at least 5/8ths of a yard, since we need 20 inches and 5/8ths of a yard (22.5 inches) is the closest to this measurement.
whew! it’s a good idea, however, to buy more than this bare minimum, as it will make cutting much easier and guarantee that you have enough to work with. to be safe, I’d buy at least 3/4 of a yard if you are working with a 54-60 inch wide material (and if you’re in nyc like me, there’s a pretty good chance that you have a one-yard minimum anyway, which is even better! you can always use the leftovers for other things.)
now, you may be asking yourself, what if I want to use a 45 inch wide fabric instead? well, you’ll still need 50 inches in width, but because the fabric isn’t wide enough (45-50 = -5 inches), you’ll have to lay the pattern pieces side by side instead. now you’ll need at least 40 inches in length (20 inches in length times 2), and the closest you can get to that amount is 1 1/8 yards (36 + 4.5 = 40.5.) again, this is cutting it a bit close (pun intended) so get at least 1 ½ yards.
want to make a pillow like I’ve described here but don’t have a pattern? follow the sewing instructions for “the envelope, please” in sew everything workshop or take a 2-hour pillow making workshop at make. (either way you’ll need to just swap out the size of the pillow!)