This is a must have if you are crabbing or lobstering. The Promar Lobster and Crab Gauge makes sure you stay legal but make sure the measurements are are accurate to your local. It&';s important to have the correct Promar Lobster and Crab Gauge and some are designed by your state or your county. Model: AC-320 measures 3 1/4" and 4" California regulations set the minimum carapace length requirement for harvesting spiny lobster at 3.25 inches. Rock crab carapace width must be a minimum of 4 inches across for harvest.