Fit
Measure head circumference and check bicycle helmet fit
A circumference measurement is a useful first filter, but it is only one part of fit. The goal is a helmet that follows the manufacturer's instructions, sits in the intended position and remains stable when fastened—not simply one whose printed range contains your measurement.
Measure in a repeatable way
Use a flexible measuring tape and keep it level as it passes around your head. Giro's US bike helmet size guide describes this method and labels its chart as a guide rather than a substitute for trying the helmet on. Repeat the measurement if it sits at an angle or changes substantially between attempts.
Use the exact model's chart
Size letters are not universal. Compare your measurement with the exact model and market page, and look for the head-circumference range attached to each size. The Aries Spherical, for example, is listed in S, M and L, while Giro's general chart assigns those sizes to 51–55, 55–59 and 59–63 cm respectively. Those edges overlap in the published table, so a boundary value still deserves a try-on rather than a forced choice.
Check position and retention
Follow the instructions supplied with the helmet. Giro's guide says the helmet should sit low enough to protect the forehead without obstructing vision, and recommends trying it on before riding. Make sure the retention system and straps can be adjusted as described and that the helmet stays in place. Do not improvise with extra pads, replace parts or alter the strap path unless the exact manufacturer instructions allow it.
Do not overlook head shape
A chart generally describes circumference, not every dimension or contour of a head. If the shell pinches at one area while leaving space at another, or if the fit system cannot stabilize it without discomfort, the size label alone is not enough. Try another size or model and ask a qualified retailer or the manufacturer when the instructions do not settle the question.
Fit is a personal physical check. The finder can compare stated size ranges, but it cannot evaluate pressure points, vision, strap comfort or retention on your head.