Insert Ring/Loop Sizing
The first measurement in a Tip-Top is the Inside Diameter (I.D.) of the ring (eyelet) in Millimeters. Generally, the I.D. of the ring will match the smallest guide on the rod. Standard sizes are 6, 8 and 10mm for conventional rods, while heavier saltwater rods may go as high as a 16mm ring.
The second measurement in a Tip-Top is the Inside Diameter (I.D.) of the tube, measured in 1/64ths of an inch. Tube sizes generally range from 3.5 to 12, but they go all the way up to 32 (.500") in Big Game roller tops. The tube size should correspond to the tip size given by the manufacturer of your blank.