Measure around your fullest part, with the measuring tape under your armpits, making sure the tape is flat across your back. 

2XS
Chest: 88-91cm / 34.6-35.8"
XS Chest: 92-95cm / 36.2-37.4"
S Chest: 96-99cm / 37.7-38.9"
M Chest: 100-103cm / 39.3-40.5"
L Chest: 104-107cm / 40.9-42.1"
XL Chest: 108-111cm / 42.5-43.7"
2XL Chest: 112-115cm / 44-45.2"
3XL Chest: 116-119cm / 45.6-46.8"
4XL Chest: 120-123cm / 47.2-48.4"
5XL Chest: 124-129cm / 48.8-50.7"
6XL Chest: 130-135cm / 51.2-53.1"
7XL Chest: 136-141cm / 53.5-55.5"