Getting the perfect fit is easy. Just grab your favorite tee shirt, lay it flat on a table, take a few measurements and pick the closest matching shirt. If you are in between 2 sizes always go on the larger, chances are your favorite tee has shrunk a little since you got it. Shirt sizes can vary by +/- 3cm, and will shrink by 1-3 cm over time. If in doubt, go on the bigger size!
Lay your shirt flat on a table and measure the width under the arms (Measurement W on diagram)
Measure the length of your t-shirt from the seam at the neck to the bottom of the garment. (Measurement H on diagram)