How do you count gauge in knitted sweater?

Measure 4 inches on your piece of knitted work then count the number of stitches (including partials!).

Why? Because gauge determines the actual size of your piece. For example, a sweater knitted at a gauge of 5 stitches to the inch with 200 cast on stitches would be 40" around. Change the gauge to 4 1/2 stitches per inch and your sweater balloons to 44.4" around.