Shoe size is a length, and most fitting problems are width

One number for a three-dimensional problem

A shoe size is a length and nothing else, and the systems used to express it are more arbitrary than most people assume. British and American sizing descends from the barleycorn, an old unit of a third of an inch; continental European sizing counts Paris points of two-thirds of a centimetre. Neither was chosen for a reason that still applies.

Worse, the number does not describe your foot. It describes the last the shoe was built on, and a last is always longer than the foot it is meant for, because toes slide forward as you walk and need somewhere to go. How much longer is a decision the maker takes rather than a published standard.

So the length is a rough sort. What people actually complain about — burning across the joints, a shallow feeling over the instep, a heel that will not stay put — is rarely length. It is girth, and the size number says nothing about girth at all.

The last is the shape underneath everything

A last is a solid form, historically wood and now usually plastic, around which the upper is pulled and to which the sole is attached. It carries far more information than a size: width across the ball joints, volume through the instep, the depth of the heel cup, the height of the toe box.

Two lasts can share a length and disagree on every one of those. That is the real reason a size which has worked for years suddenly does not in a different style, and it is why the advice to simply know your size is close to useless once you are buying anything other than trainers.

The measurement that matters most, and that almost nobody takes, is heel-to-ball rather than heel-to-toe. It fixes where the shoe is designed to flex. If your foot is long in the toes and short from heel to joint, a shoe long enough for the toes will bend in the wrong place, and it will crease across the wrong part of the upper for the rest of its life.

Where the foot is supposed to sit

The widest part of your foot should sit at the widest part of the shoe. Everything else follows from that. If the joints are sitting forward of the widest point the shoe is short, regardless of what the toes are doing; if they are behind it the shoe is long, and the foot will slide.

At the front you want a little room ahead of the longest toe, which is not always the big one. A thumb's width is the usual guide, and it exists because the foot lengthens under load. A shoe fitted while sitting down will be tight on a pavement.

A small amount of heel movement in a new leather shoe is normal and usually disappears as the sole flexes and the upper settles. Movement that comes with the whole foot sliding forward is a different problem, and lacing will not fix it. Lacing controls volume over the instep. It cannot make a heel cup narrower than it was made.

Feet are not stable objects

Feet swell through the day, in heat, on long flights and after hours of standing. The difference is enough to change how a shoe feels, which is why fitting in the late afternoon gives a more honest answer than fitting first thing.

They also change over years. Arches settle, ligaments relax, and feet tend to get longer and broader with age, so a size that fitted at twenty is not a permanent fact about you. Pregnancy commonly changes foot size, and the change is often not fully reversed.

And they are not a matched pair. Most people have one foot noticeably longer or wider than the other. Fit the larger one and adjust the smaller with lacing or a thin insole, because there is no way to add room to a shoe that is short, while taking up slack is easy.

Leather moves, and most other things do not

Breaking in a leather shoe is real, but it works in one direction. The upper stretches across the width and moulds over the joints, and the sole becomes flexible where the foot bends. Length does not meaningfully increase. A shoe that is short when you buy it will be short when you throw it away.

Unlined leather moulds most, since there is nothing behind it resisting; a lined shoe moves less, and patent barely gives at all. Synthetic uppers and moulded rubber are largely fixed geometry, and knitted uppers stretch immediately and then keep stretching, which is comfortable at first and is why a knitted trainer often feels loose long before the sole is worn out. None of them reward buying tight and hoping.

Buying against the constraint

Try both shoes, stand up, and walk on a hard floor rather than a carpeted one. Wear the sock thickness you actually intend to use.

If a width fitting is available, use it before you use length. Going up a half size to solve a width problem gives you a shoe that is wide enough in one place and too long everywhere else, and the foot then slides forward into the space and defeats the fix.

Insoles and tongue pads reduce volume, which is a genuine adjustment for a shoe that is roomy over the instep. A cobbler can stretch a leather shoe across the joints or over a single pressure point. What nobody can do is add length, narrow a heel cup, or move the position of the joint. Those were decided on the last.