1 - With a measuring tape take a measure of the perimeter of your trunk, (also known as your thorax, or torso) at the height of your breasts. Should you be wearing an anorak , take it off. Please do not use a ruler to take the measures, as the results will not be trustworthy ;-)
2 - The result of these measures should be divided in two. This will give the width.
3 - Now take a measure of the length of your favorite T-shirt from the hem below up to the height of your shoulder or the end of the shoulder-strap. No !
That should be the neck height or the low neckline !
4 - Look on the table for the line with the width and height nearest to those measures you have taken. The size indicated on this line will be your size!
(*) Should you wish your belly-button to be seen and you do not possess a large chest, don't hesitate to ask for the M size for girls.