I don't have enough yarn

(OK, in this case, I really don't!)

Previously posted on Ravelry, reposted here for posterity:

I just figured out I don’t have enough yarn for a project and thought I’d share my methods. Feedback welcome. (I’m sad that I can’t make it, but happy I figured it out now rather than partway up the front and having to quit _then_…!)

I had several skeins of Worsted Border Leicester/Corriedale in “Black Orchid” from Rabbitworks Fibre Studio. She had also thrown in a swatch/scarf she’d started in the wool, so my first order of business was to frog that, skein it, wash it, and hang it to get rid of the wigglies. Once I did that, I counted the strands at one end of each skein to get the yardage (my niddy-noddy measures 1-yd skeins).

So now, I had 84 + 76 (the two frogged skeins) + 60 + 130 + 130 (the labeled skeins) = 490 yards.

I grabbed the smallest skein, cast on about 5” wide in stitches, and began knitting for a swatch. A little more than halfway through the ball, I realized that it would be more useful to knit up the entire thing and measure it for my intended project, Boogie Vest. It knit up quickly on 8s, so I kept going, and the end product measured 8x12”.

Since we know 60 yards made 8x12” of fabric, we can extrapolate from that to estimate yardage needed for a vest.

I used the measurements from Ann Budd’s Knitter’s Handy Book of Patterns to estimate fabric needed. I drew a rectangle for the lower half of the vest, and then modified rectangles for the upper part (narrower width on the edges to show the armholes, and a v-neck in the middle). For my size, I would have

Front:
                   lower rectangle 22 x 14 x 1 piece = 308 square inches (sq in)
                   upper rectangle, the square outer part 4.75 x 10 x 2 of these = 95 sq in
                   V-neck part consists of two triangles, so we can do width x height once
                   to get the area for both: 7 x 10 = 70 sq in
                   Total: 473 sq in for front
                  
                   Back:
                   Roughly, one big rectangle
                   22 x 24 x 1 piece = 528 sq in
                  
                   Grand Total: 1001 sq in needed.

My knitted piece is 96 sq in from a 60-yd skein, yielding 1.6 sq in per yard (96/60 = 1.6 sq in).

So my 490 yards at 1.6 sq in per yard would yield 784 sq in, more than 200 sq in less than I need.

Of course, this is all estimated, but a back-of-the-envelope calculation can certainly give you the answer you need (but don’t want): don’t even start it.