Awesome chart from Lisa R. Myers’ The Joy of Knitting Companion:

Gauge Sq.In/Yd Yd/Sq.In
3.0 2.13 0.47
3.5 2.04 0.49
4.0 1.72 0.58
4.5 1.60 0.63
5.0 1.46 0.68
5.5 1.29 0.78
6.0 0.98 1.02