Blue luckybums size M helmet with ventilation holes for a comfortable fit. The helmet size is approximately 54cm in head circumference based on measurements. There is a dial on the back for size adjustment. It may be possible to adjust it to a smaller size, but this is uncertain. The inner diameter of the helmet is as shown in the photos, but there may be slight errors. - Brand: luckybums - Color: Blue - Size: M - Design Feature: With ventilation holes - Internal Structure: Padded Thank you for looking.