How to measure a shirt you already own

Measuring your body is awkward. The tape slips, you breathe in without meaning to, and the number changes depending on who is holding the other end. A shirt lies still. It also already contains the answer to the question you care about, which is not how big you are but how big a shirt you like to wear.

So pick the one you reach for. Not the one that fitted two years ago, and not the one you wish fitted. The one you put on when it matters.

Before you start

• A soft tape is easiest. A metal one works if you keep it flat against the cloth. A ruler works too, it is just slower.
• Use a table or a hard floor. A bed is the classic mistake: the shirt sinks into the duvet and every width comes out short.
• Do up the buttons, all of them. An open placket lets the front spread and adds half an inch you did not earn.
• Smooth the shirt flat with your palms, from the middle outwards. Do not pull it. Knits in particular will give you an extra inch if you ask them to.
• If it has just come out of a hot wash, wait until you have worn it once. Cotton relaxes again with wear, and that is the size you live in.

Pit to pit

This is the one that decides whether a shirt fits, so take it twice. Find the point under each arm where the sleeve seam meets the side seam. Measure straight across the front between those two points.

Write down the number you read and leave it alone. Sellers and brands quote this measurement flat, as a half, and so do we. Doubling it gives you the distance around the shirt, which is a different number with a different job. Pit to pit vs chest goes through the difference.

Shoulders

Turn the shirt over. Measure across the back from the top of one sleeve seam to the top of the other, in a straight line. Ignore the curve of the yoke and the dip of the collar.

Shoulders are the part a tailor cannot fix, which is why it is worth knowing yours to the half inch. If the shirt has raglan sleeves, the kind where the seam runs diagonally up to the collar, there is no shoulder point to measure. Skip it and lean on pit to pit instead.

Sleeve

Lay one sleeve out flat and straight. Start where the sleeve joins the shoulder and follow the top edge down to the very end of the cuff.

One warning. American dress shirts are sold by a sleeve number like 34/35, and that number starts at the centre of the back, not the shoulder. It runs about eight inches longer than the measurement above. If a size chart gives a sleeve of 34 or 35 for a medium, that is the one it means, and the two cannot be compared.

Length

On the front, start where the shoulder seam meets the collar and measure straight down to the hem, keeping the tape parallel to the buttons. On a shirt with a curved hem, measure to the lowest point of the curve.

Some brands measure length down the back from the base of the collar instead, and the two can differ by an inch. When a chart says which, believe it. When it does not, assume the front.

What the numbers are for

You now have a size chart with one row, and it is the only row you know fits. Put it next to the chart of anything you are thinking of buying and the letter on the label stops mattering.

Secondhand listings are where this pays off fastest. A seller on eBay or Grailed who gives pit to pit and length has told you everything the tag would, and more honestly.