XS - 20" PTP
Small - 21" PTP
Medium - 22" PTP
Large - 23" PTP
XL - 24" PTP
XXL - 25" PTP
These are a relaxed fit and if you like clothing slimmer fitting I would go a size down!
*PTP is the measurement from one arm pit to the other.
If unsure on the fit, compare measurements to another item you have that fits.