T-shirt sizes are unisex.
For Mens; they'll run small; so if trying to decide between sizes, order the size up.
For Ladies; order down from your usual. (e.g. we estimate a Unisex Medium = Women's UK 12)
Measure if you feel unsure - As we don't do exchanges for wrong size -
There are two measurements in the table below; Width (across the chest armpit to armpit) and Length (shoulder to hem).