This post is a part of my "How to Knit" series that is aimed at teaching beginner knitters the basics of knitting. Click here to view the other posts in this series.
Watch out, wordy post ahead (and math!).
Someone asked me what method I use or how I calculate decreases when it came time to shape the crown for a hat. She had to adjust the pattern for size and found that the decrease section in the original pattern wouldn't work for the decreases she needed to do in hers. While there are many ways to shape the crown of a hat (all providing different effects for the shape of the crown), there is a quick and easy way to calculate how many decreases you need to do each row evenly and keep that same rhythm in each row following it.
Get a piece of paper out and draw up two columns. Label column one "# of Stitches" and column two "Decreases". Let's say you have 60 stitches in your hat. Write that at the top... then write all the numbers that 60 can be divided by. 60 can be divided by 2, 3, 4, 5, 6, 10, 12, 15, 20, 30. Pick any of those numbers... I'm going to choose 10 and write that under the "# of Stitches" column. 10 goes into 60 six times; so I'll write six in the "Decreases" column. So in my first decrease row, there will be six sections of ten stitches. In order to get those six decreases evenly across my row, I will need to do a k2tog in each section of 10 stitches. Subtract two from 10. 10 - 2 = 8. I end up with, k8, k2tog (started with 10 stitches, decreased 1 stitch down to 9 stitches). Working *k8, k2tog* the first row will decrease your hat 6 stitches. 60 - 6 = 54.
What next? It's very easy. Once you have your first row figured out, you just work down the line of numbers. So after the *k8, k2tog* row, the next decrease row would be *k7, k2tog* (9 stitches in each section, this still works because 9 goes into 54 six times!), then *k6, k2tog* (8 stitches), etc., all the way down to *k2tog*. You can insert a row of k sts in between each decrease row to add a bit of length to the crown (I usually stop those types of rows when I get down to smaller numbers such as *k3, k2tog*).
Let's try a different number. I want to finish my hat quickly, so I want lots of decreases. I'm going to pick the number 5. 5 goes into 60 12 times. 12 decreases. So I begin with *k3, k2tog*. Then maybe a k row, then *k2, k2tog*, *k1, k2tog* then *k2tog*. Voila!
As you can see, this method works best for simple hats suck as stockinette hats. If there is a pattern that is worked all through out the hat, including the crowning, you will have to play with numbers for a bit in order to get what you want. While lots of math is involved, it is rather simple math and taking the time to calculate and put your brain cells to work will pay off!