I recently started working on a bedspread for my king size bed. I'm using a variety of 12-inch square block patterns I've found on Ravelry to make a sort of sampler afghan. I want this afghan to be generous in its size so my husband and I both have plenty of blanket at night. Since a king size mattress is 76x80 inches, my plan was to make this bedspread 8-by-8 squares (so, 64 total) plus a narrow edging.
Then I made my first square.
It came out 13 inches square, not 12.
The thing is, that I like the fabric I made at that larger gauge. I'm using the yarn I want to use, in the colors I want, and I don't want to use a smaller hook than the one I chose. But eight 13-inch squares are just shy of nine feet, not the eight feet (plus border and seams) for which I was hoping.
In order to get an appropriately sized blanket using my new gauge, I'm going to make 56 squares (7x8 squares), not 64. I don't mind the blanket being a bit on the wide side, but I don't want it to be too long.
Relatively small differences in gauge can make a really big difference, even with things that don't have to fit anything. And even if the difference in finished size didn't matter all that much to me, making eight fewer squares amounts to a major difference in both the yardage (over 1000) and work needed to make something that will work for my needs. Thankfully, since the finished product isn't something that is fitted, adjusting my numbers to account for the gauge I'm using is pretty easy.
Always check your gauge, and always check your math.