I tend to find yarn I like and buy it before I have a project in mind. Then when it comes time to knit a hat I wonder if I have enough. Usually the answer is yes, and especially so if I use two skeins of different colors.
In my experience in knitting hats, I’ve never used an entire skein of yarn, or run out of yarn. BUT… I often use more than one color, and it depends on the type of hat you make. Will it have a roll up cuff? Will you need to add a pom pom using yarn? Does it have earflaps? If it’s not a straight forward cap and you plan to use all of one type / color way, two skeins may be needed.
Simple Hat Calculator Page
The Earth Guild site has a page with information about choosing cast on amounts, length of hat, and how to decrease at the top. Click to view their Simple Hat Calculator.
Figuring the Cast On Stitch Amount
One thing to remember about a hat is that once it’s knit the circumference should be smaller than the head. This is called negative ease. You will find this with any item that needs to hug the body – socks are another example. So when you measure a head circumference you will want the hat measurement to be smaller. At Interweave they have a page about knitting hats and negative ease.
Once you have a number of knitted hats under your belt the finished hats can be a reference for starting a new project. Keep notes as you go and file them away for the future. Make note of the type, weight and leftovers of yarn used. If you have a scale, you can weigh yarn for exact amounts.
I’ve knit many hats from the “Hats On” book and have made notes about looseness, length and yarns right in the book.
For example, the hat below is called the Fana Cap and I cast on 108 stitches, size 6 circular needles, using Manos del Uruguay’s “Pescador” blue yarn (worsted). I made the hat for my daughter and she has a small head. The cast on amount was good, but the hat should have been a bit taller. I only used one skein of each color with some left over.
This tends to be the “live and learn” way of knitting, but practice makes perfect. Hats are fun to make because they come in many styles. Also, stranded knitting using two-colors (or more) looks wonderful on a hat. Circular knitting is easy and when it comes to decreasing at the top and using double-pointed needles (DPNs) it’s only for a short few rows.