One K Helmet Size Chart
| Size | Centimeter Measurement | Hat Size |
| X-Small | 51-52 cm | 6 3/8 - 6 1/2 |
| Small | 53-54 cm | 6 5/8 - 6 3/4 |
| Medium | 55-56 cm | 6 7/8 - 7 |
| Large | 57-58 cm | 7 1/8 - 7 1/4 |
| X-Large | 59-60 cm | 7 3/8 - 7 1/2 |
| XX-Large | 61-62 cm | 7 5/8 - 7 3/4 |
| Size | Centimeter Measurement | Hat Size |
| X-Small | 51-52 cm | 6 3/8 - 6 1/2 |
| Small | 53-54 cm | 6 5/8 - 6 3/4 |
| Medium | 55-56 cm | 6 7/8 - 7 |
| Large | 57-58 cm | 7 1/8 - 7 1/4 |
| X-Large | 59-60 cm | 7 3/8 - 7 1/2 |
| XX-Large | 61-62 cm | 7 5/8 - 7 3/4 |
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