Any time you knit something, you should first make a gauge swatch - a small square of knitting from the yarn and needles you think you'll be using. It gives you a chance to see if the resulting fabric has the qualities you want in your finished piece, and, if you're working from a pattern, to make sure that you're matching the numbers the designer used. If you're not getting the same number of stitches and rows per inch, your finished object will not be right size.
I hate making swatches. I want to knit a mitten, not an aimless tube. However, without an existing pattern, I don't know how many stitches to cast on for a mitten, so a gauge swatch I must make.
I cast on some 30 odd stitches, and since I was going to be working in the round, I made my swatch in the round. I knit for a good three inches, cast off, and whipped out the measuring tape.
Here's how it breaks down:
My stitch gauge, measured over several spots on the swatch, averaged at 8.5 stitches per inch.
The row gauge averaged at 10.5 rounds per inch.
I'm knitting these mittens for my four year old. His measurements are:
Wrist circumference: 4.75"
Hand circumference at base of thumb (the widest point of the mitten): 6.25"
Hand circumference above base of thumb: 5.25"
Wrist to tip of pinky finger (traditional spot for decreases to begin): 3.1"
Wrist to tip of middle finger: 4.1"
Thumb length: 1.75"
If I multiply each circumference measurement by 8.5, I'll have the minimum number of stitches I'll need to fit around each part of the hand. I like to have a little wiggle room in mittens (and I'll need a little space for the lining, too), so I will add 1/2" to each circumference before doing the multiplication. I also like to work with a set of five needles, so I'm going to round the numbers down or up whenever possible to have a multiple of 4 stitches to evenly divide across the needles.
Stitches to go around the wrist: 44
Stitches to around widest part of hand: 56
Stitches to go around the hand after dividing for the thumb: 48
The math for the length will only matter for the increases to the thumb and the decreases to the top of the hand - otherwise the number of rows doesn't need to be notated - you just knit to the desired length. I'll deal with those numbers when I reach those sections.
We're almost ready to cast on!