How to measure a blazer that fits

What you need

• A flexible measuring tape
• A flat surface (a made bed works)
• The blazer you already reach for, as the reference
• Somewhere to write five numbers you will reuse for years

Measure the garment, never your body against a listing. A tailored chest carries ease over the body it is cut for, and how much depends on the cut, so the only honest comparison is jacket against jacket. The blazer you already wear is the ruler.

The five measurements

1. Pit to pit

Button the jacket, smooth the front, and measure straight across from armhole seam to armhole seam, just below where the sleeves join. Double that number for the chest circumference. Take it without pulling; a stretched tape flatters the listing and punishes the first wearing.

2. Shoulder, seam to seam

Turn the jacket over and measure across the back, from the point where one sleeve seam meets the shoulder seam to the same point on the other side. Cuts differ, and an extended shoulder measures wider than a natural one on the same chest, which is why this number needs a reference jacket more than any other.

3. Sleeve from the shoulder

Run the tape from the shoulder seam down the outside of the arm to the end of the cuff. Convention puts the cuff near the wrist bone with a little shirt showing, but the convention matters less than the comparison. Your reference jacket already knows your number.

4. Back length

Measure from the base of the collar, where it meets the body at centre back, straight down to the hem. This is the number that decides whether the jacket reads cropped or long on you, and a camera notices it before you do.

5. Waist at the button

Fasten the closing button and measure straight across at its height, then double it. This number decides whether the jacket closes without the cloth pulling an X across the front.

What each number forgives

A chest or waist can come in about an inch at the side seams, sometimes two if the pockets allow, and letting out is limited to whatever cloth the maker left inside, often an inch or less. A jacket slightly big through the body is a candidate. A jacket slightly small is a bet on the seam allowance of a stranger.

Shoulders are surgery. Narrowing them means detaching the sleeves and rebuilding their heads, sometimes the collar too, and the bill lands near the price of another jacket. Buy the shoulders.

Sleeve length is the cheap fix, a seam and a press at the cuff, unless the buttonholes are functional. Working cuffs mean shortening from the shoulder, slower and costlier, so ask the seller to photograph the buttonholes before you decide. What each of these jobs is worth paying for is its own subject, and the journal keeps a note on alterations in Toronto for it. Back length barely moves: a hem can rise a touch before the pockets sit wrong, and it almost never drops.

Questions to ask before you pay

• Was the jacket buttoned and laid flat for these measurements?
• Are the sleeve buttonholes functional or decorative?
• Has the body or sleeve been altered already?
• Is the shoulder natural or padded?
• Centimetres or inches?

The same method, the rest of the wardrobe

Five numbers make a tailored listing legible the way five different ones do for denim, and the habit transfers: measure the garment you love, keep the numbers in your phone, and read every listing against them. A blazer that fits at the shoulders and nowhere else is a project. A blazer that fits everywhere but the waist is a fitting.