This is a great tool to keep on you while crabbing. You can quickly measure and make sure the mud or sand (blue) crabs are within legal limits. The outer markings are for mud crabs and the inner markings are for sand crabs.
It has been designed to suit Queensland, Australia legal sizes (15cm mud and 11.5cm sand crabs). However I have included the solidworks source file which can easily be modified to suit the sizes in your own state.