Free tool
Gauge calculator
Enter the gauge from your swatch and the size you want, and this gives you the stitches to cast on and the rows to work. Gauge is how many stitches and rows fit in a measured square, usually four inches or ten centimetres. Always swatch first, because a small difference in gauge changes the finished size a lot.
Want this on every project, with your counts saved as you go? Get Worsted for iPhone.