My T-shirts are classically cut and unisex. 

I've estimated UK women's sizes here as best I can following feedback, however, sizing is tricky isn't it!? A 12 in Zara is a 16 in H&M and oversized to some will feel true to size to others.

To be 100% sure please lay one of your favourite T-shirts flat and measure in cm as follows:

A - chest (armpit to armpit)
B - length (neck to hem)
C - sleeve (shoulder to cuff)

 

Size Approx A B C
XS 6-8 46 66 19
S 8-10 49 69 20
M 12-14 52 72 21
L 16 55 74 22
XL 18 58 76 22
XXL 20 61 78 23
3XL 22-24 64 80 24