Measure yourself:
With a tape measure, measure from under the arm around the fullest part of the chest with your arms down, keeping the tape horizontal. Use the number to find your size below accordingly; (Please note the following numbers are in Inches)
XS: 32-34
S: 35-37
M: 38-40
L: 41-43
XL: 44-46
2XL: 47-49
3XL: 50-53
4XL: 54-57