Measure a similar item to the one you are thinking of buying,  that fits you well, using Toms instructions below.

Then compare with the measurements of the top you are considering.  The sizing charts are on every product page under the "sizing" tab, which is to the right of the "description" tab….**

HOW WE MEASURE TOPS

Shoulder:  fold the garment in half down the centre so that the back of the shoulder sleeve seams meet to find the centre of the back of the collar. Measure from the top of the arm/shoulder seam to the centre of the back of the collar and then (a) measure back down to the top of the other arm/shoulder seam, OR (b) double this measurement.

Length:  measure from centre of the bottom of the collar to the centre of bottom of garment.

Chest (P2P):  lay the garment flat and measure the full width from pit to pit (which may be slightly over the side seams if the back of the garment is slightly wider than the front).

Waist: lay the garment flat and measure across the narrowest part of the body (which may be slightly over the side seams if the back of the garment is slightly wider than the front).

Opening:  lay the garment flat and measure the full width between bottom of side seams (which may be slightly over the side seams if the back of the garment is slightly wider than the front).  If the shirt has gussets at the bottom of the side seams, the measurement is taken across the garment to the top of the gussets.

Arm: Measure from shoulder seam to bottom of arm on the outside of the arm.