Teacher4lyfe,

Welcome!

I'm guessing you're crocheting. The key to calculating your finished afghan size will be finding your gauge. Your gauge will tell you how many stitches you're getting per inch.

A gauge swatch is making a square with your chosen yarn and hook. Your square should be at least 4 inches x 4 inches (or larger - some say at least 5x5 is optimal). When you measure out how many stitches in your square and divide by the number of inches (avoiding edges in this calculation), that will tell you how many stitches per inch.

For example, let's say you find you're crocheting 5 stitches per inch and your pattern is 100 stitches across. So 100 divided by 5 is 20 inches.

There's no one number that can be given for all projects, since gauge is different for everyone. You and I could crochet the same number of stitches with the same fiber and the same hook and still have completely different gauges. And almost nobody likes making swatches. Especially when you really want to get started on your project! But swatches are well worth the effort; the alternative is to make something, get almost or all the way through the project, only to find out the size is wrong, particularly if it's a garment. So unless the size of a finished project really, really doesn't matter (a scarf, for instance), swatching is always the best way to ensure your project isn't too large or too small.