O'neill Swimwear Sizing
 
Mens' Waist (inches) Waist (cm)
S 29 - 31 74 - 79
M 30.5 - 32.5 77.5 - 82.5
L 32.5 - 34.5 82.5 - 88
XL 34.5 - 36.5 87.5 - 93
 
Boys' Waist (inches) Waist (cm)
S 23 - 24 58.5 - 61
M 25 - 26 63.5 - 66
L 27 - 28 68.5 - 71
XL 29 - 30 74 - 76
 
See our complete fitting guide here