Every blue crab state measures the same way: point to point across the shell, from the tip of one lateral spine to the tip of the other. Not the body. Not the claws. The widest span of the carapace including the spikes.
The measurement
Lay the crab flat and gauge across the back at the widest point. If a spine is broken off, the crab measures what's there; Natural Resources Police don't give credit for a missing spike. A crab that reads 4-15/16" on a 5" day is a short, and the tolerance for shorts in a bushel is not generous anywhere.
What the legal number is depends on where you are and, in Maryland, what day it is. Every state's minimum is here. Short version: New York and New Jersey are 4-1/2", Maryland is 5" until July 15 and 5-1/4" after, everyone else with a limit is 5".
Cull stick or gauge
A cull stick is a bar with a notch cut to the legal width. The crab either drops through the notch or it doesn't. It's faster than a caliper and it works one-handed, which is the point when you're culling a full pot with the next one coming over the rail.
The stick is only as good as the notch. Plastic sticks are sold about a sixteenth under nominal because the makers know crab shell wears the notch open; the stick starts short and wears toward true. That means it passes shorts early and throws back keepers late. More on that here. Aluminum doesn't wear. Ours is machined from 5/8" 6061 bar and held to .005".
Grading at the dock
Legal is the floor. The money is in the grade. Buyers pay by size, and the breaks vary by buyer and season, but the common ones are:
- Number twos: legal but small, roughly 5" to the low 5-1/2" range
- Number ones: the standard market crab, usually starting somewhere between 5-1/2" and 5-3/4"
- Jumbos: 6" and up
Ask your buyer where his breaks are. Then grade to a notch, not by eye. A 6" jimmy sorted into the ones basket at 4 AM is dock price you caught and didn't get paid for. That's why our stick carries grading notches at 5-3/4" and 6" on the same bar as the legal notch. One reach, three answers.
Females
Mature females are exempt from the hard crab minimum in most states, and recreational possession of any female is prohibited in Maryland. Immature (V-apron) females are protected in North Carolina and commercially in Louisiana. Sponge crabs are protected nearly everywhere, with dated exceptions in Virginia and Alabama. Know your water; the state pages have it.