Good Fit Guide
Blazers & Suit Jackets
About 2cm of shirt collar should be revealed by the jacket collar.
The shoulder seam should lie on the edge of your shoulder. The aim is to reduce the amount of buckling, as the shoulders should have no apparent wrinkles or divots while the arms are down at the sides.
Similarly to shirts, armholes must be sufficiently high, and alternatively should not be cutting into your armpit. The arms should move somewhat independently of the jacket during normal motions.
While buttoned, the jacket should not pull across the chest (fabric making an "X" shape across your abdomen). Similarly, it should not pull across the shoulders when arms are folded.
Holding your hand flat, you should easily be able to fit it inside the jacket under the lapels.
The jacket's second button from the bottom should lie just above your belly-button, never below.
With your arms at your sides, the sleeves should cover the wrist bone.
If a jacket doesn't fit your shape properly, sometimes the bottoms will flare out, a product of the jacket being too slim in the waist, so your hips push out the fabric.
A suit jacket's length should always cover your backside