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A fitted cap is sized in eighths of an inch and has no adjustment at all.
The increments are small because the tolerance is small. With no closure, the cap either sits correctly or it does not, and a quarter-inch is the difference between those two states.
Head circumference is one number describing a three-dimensional shape. Crown depth, how far down the cap sits, and head shape both affect how a fitted wears, and neither is captured by the measurement.
A structured fitted holds its shape by design. Some give develops in the sweatband over time, but the crown is built not to relax, which is the same property that keeps the cap looking right after a year.
Measure properly, and go up when between. How to measure → · Fitted caps →
Every shop, address and detail is in the full guide, and the store map covers 21 states.