Decision guide
How a blazer should fit.
The shoulder seam decides almost everything else. If it sits where your own shoulder ends, the rest of the jacket can be tailored to work; if it does not, no amount of alteration elsewhere will fix it. Start there, then check chest, waist, sleeve, and length in that order.
The short answer
Fix the shoulder first; a tailor can fix the rest.
A jacket's shoulder seam is the one measurement a tailor cannot meaningfully move without remaking the garment. Chest, waist, sleeve length, and hem can all be taken in or let out by a competent alteration. If the shoulder is wrong, walk away from the jacket regardless of how good the rest looks in the mirror.
Once the shoulder is confirmed, everything else is a matter of degree rather than pass or fail. A slightly loose waist or a sleeve half an inch long are easy, inexpensive fixes.
Five checks, in order
Where to look, and what correct feels like.
Seam ends where your shoulder ends
Look in a mirror or have someone check from behind. The shoulder seam should sit exactly at the edge of your shoulder bone, not out over the arm and not short of the edge. Divots or pulling fabric at the seam signal the wrong size.
Buttons close without an X-shaped pull
Fasten the top button and look for horizontal stress lines radiating from it. A small amount of room to slide a flat hand inside the closed jacket is correct; visible strain across the chest means the size is too small.
Follows your shape without gaping
The jacket should taper in slightly at the waist rather than hang as a straight box, but should not pull or crease when buttoned. A tailor can take in a waist that gaps; they cannot easily let out one that already pulls.
A half-inch of shirt cuff shows
With your arms relaxed at your sides, the jacket sleeve should end at the wrist bone and let roughly half an inch of shirt cuff show beyond it. A sleeve that fully covers the shirt cuff reads as too long.
Roughly covers the curve of the seat
A common guide is that the jacket hem should reach the point where your fingers curl when your arm hangs naturally at your side, covering the seat without extending well past it. Proportions vary by height, so use this as a starting point, not an absolute rule.
Structured or unstructured
The checks change slightly by construction.
A structured jacket, built with internal canvassing and shoulder padding, holds its shape and makes shoulder fit especially important since there is little natural give. An unstructured or soft-shoulder jacket moves more with the body and forgives a slightly imperfect shoulder line, but still should not pull or bunch at the seam.
For business casual and travel wear, an unstructured jacket usually tolerates more day-to-day variation in fit. For interviews, weddings, and formal occasions, a structured jacket with a correct shoulder is worth the extra attention.
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