How a jacket should fit

Start with the shoulder

The shoulder is the one part of a jacket that cannot really be altered, so it decides everything. The seam should end where your shoulder bone ends. Too far in and the sleeve pulls; too far out and a dent appears at the top of the arm.

Then the collar

The jacket collar should rest against your shirt collar all the way round, with about a centimetre of shirt showing above it at the back. A gap here usually means the jacket is too big across the upper back.

The sleeve

Let your arms hang naturally. The jacket sleeve should stop at the wrist bone so that a little shirt cuff — around a centimetre — shows. Sleeve length is the easiest alteration there is.

The length

A classic jacket covers the seat of your trousers, or comes to around the knuckles of your cupped hand. Shorter reads more modern; longer, more traditional. Either way, buttoned, it should close without the fabric pulling into an X around the button.

Measure one you already love

The simplest guide of all is a jacket you already wear often. Lay it flat and measure across the chest, armpit to armpit, and down the back from the collar seam to the hem. Compare those numbers with the measurements on each product page.