Sprocket Nomenclature

Outside Diameter: 
The diameter of a sprocket at it's external circumference

Pitch Diameter:
The diameter through which the chain pin centers move around the sprocket. This is the area of the tooth that bears most of the load.

Root Diameter:
Denotes the diameter of the sprocket at the root of the tooth. Tooth height is calculated by comparing the outside diameter with the root diameter. Used only with even tooth count.

Caliper Diameter:
Denotes the diameter of the sprocket at the valley between two teeth. This measurement is used when the sprocket has an odd number of teeth. 

Bore Diameter:
This is the diameter of the mounting shaft. The sprocket will be bored to suit the shaft diameter.

Sprocket Width:
Measures the thickness of the sprocket. When measuring sprockets with multiple tooth sets (double, triple, double-single etc.), measure the total width across all teeth. Face width is restricted by load requirements: too thin and the sprocket won't be able to sustain sufficient working load.

Hub Outside Diameter:
Measures the diameter across the outside of the hub. Minimum hub thickness is determined by required bore size and load requirements. 

Length Through Bore
The total length of the bore, through both the hub and face. The minimum LTB is determined by the required chain load: a sprocket with a bigger LTB will be more stable and have a higher load capacity.