Sleeve length is measured to a bone, not to the end of your hand

Find the bone first

Let your arm hang loose at your side and run a finger down the outside of your forearm to the wrist. There is a knob of bone that sticks out just before the hand begins — the head of the ulna, the smaller of the two forearm bones. Everything about sleeve length is measured against that lump, and it is the reason sleeve length is one of the few fit questions with a defensible answer.

A shirt sleeve should end at or just past that bone, so the cuff sits against the base of the hand and cannot ride up over it. That is not an aesthetic preference; it is a mechanical one. The cuff is a ring narrower than the hand, and if it stops short of the wrist bone the arm can pull it back up the forearm every time you move. If it stops just past it, the bone holds it in place.

This is also why an unbuttoned cuff behaves so differently. Open the cuff and you have removed the ring, at which point the sleeve is free to slide wherever your movement sends it. The length has not changed. The mechanism holding it has.

What the jacket sleeve is doing

A jacket sleeve is set against the shirt sleeve rather than against the arm. The usual convention is that a centimetre or so of shirt cuff shows below the jacket sleeve when the arms hang naturally, which puts the jacket sleeve a little short of the wrist bone.

That convention has a practical origin. The shirt is the cheaper, washable garment, and letting it take the friction at the wrist saves the jacket sleeve from wearing at the edge. It also creates a visual break between two garments, which is why the effect still reads as deliberate even to people who have never thought about it.

How much cuff to show is genuinely disputed, and any figure quoted as a rule is really a preference dressed up. Some tailors show almost nothing; others show a good deal more. What is not disputed is the failure mode: a jacket sleeve so long that it swallows the shirt entirely makes the arm look shorter than it is and, more importantly, means the jacket cuff is now taking the abrasion.

Where the alteration is done matters

Shortening a sleeve from the cuff is the ordinary job. The tailor unpicks the cuff or the vent, removes the excess, reassembles the end, and the garment is otherwise untouched. On a shirt it is straightforward, though a cuff with a placket and buttonholes takes real care to rebuild.

On a jacket there is a complication that surprises people who have just paid extra for it. Working buttonholes at the cuff — the ones that actually unbutton — cannot be moved, so a sleeve with functional holes can only be shortened by a centimetre or two before the buttons are sitting too close to the edge. Beyond that, the sleeve must be shortened from the top, which means taking it out of the armhole, recutting the sleeve head and setting it back in. That is a shoulder-adjacent job with a shoulder-adjacent price.

This is a fair reason to be sceptical about paying a premium for working cuffs on a jacket you have not had fitted. The feature is charming, it is a genuine mark of a certain kind of construction, and it converts a cheap alteration into an expensive one.

Pitch, not length, causes most sleeve complaints

A great many sleeve problems turn out not to be about length. Arms do not hang straight down at the side; most hang slightly forward, with the palm facing partly backwards, and the sleeve of a tailored jacket has to be set into the armhole at an angle that matches. That angle is called the pitch.

When the pitch is wrong, the sleeve fights the arm. Diagonal ripples appear on the front of the upper sleeve if it is set too far back, and on the rear if it is set too far forward, and no amount of shortening the cuff will remove them. It is a common fault in ready-made clothing for the simple reason that a factory has to pick one angle for every body that buys that size.

Rotating a sleeve is a moderate alteration: the sleeve comes out of the armhole, is turned a few degrees, and goes back in. It is not cheap, but it is a fraction of the cost of a new shoulder and it removes a fault most people notice without being able to describe.

Two arms, two numbers

Arms are frequently not the same length, and the difference is often large enough to see once you know to look. Tailors measure both as a matter of course and hem each sleeve separately, which is one of the small, invisible things that makes a fitted garment look settled.

Ready-made clothing is symmetrical by necessity, so it is worth deciding which arm you would rather have correct. Most people choose the dominant one, since it is the arm that moves and is looked at more. It is a compromise, and calling it anything else is dishonest.

One last piece of practical advice about measuring. Sleeve length changes with posture, so have it checked while you are standing normally rather than holding yourself straight for the tailor. A sleeve fitted to a posture you cannot maintain will be wrong within a minute of leaving the shop, and the fault will look like the tailor's rather than the fitting's.