The fact that leather shoes stretch is something that most people learn when they are children. In what way and how much they stretch, however, is another matter, and not something for which there is a fixed formula. There are lots of different aspects that affect how much or how little a shoe stretches, here I go through some of them.
One aspect that affects is how much stitching there is on the shoe. It’s partly about the seams holding things together a bit, but also about overlapping of leather here which generally means less stretch than when it’s one piece of leather, so to speak. So a wholecut usually stretches a bit more than a full brogue, but seams also stretch, and contrary to what some people think, it’s no problem to stretch also a full brogue.
It’s also important to remember that shoes stretch differently in different places. At the heel and toe they stretch almost nothing, while at the ball of the shoe, its widest part, they can stretch quite a bit.
An important variable for how much the leather expands is how much stretch there is in the leather, and this is determined by a variety of factors. Of course, different types of leathers from animals of different ages, tanned differently etc. highly affect the stretch of a hide. Then, there can be some difference between the stretch in different hides even though they come from the same batch, and it can even vary between different parts of a hide. A soft part closer to the belly tends to stretch more than a more solid grain structured part close to the spine. In addition, stretch is affected by the type of lining leather used, as well as how much reinforcement is used and where it’s placed between the upper leather and the lining leather. As many manufacturers vary between different suppliers of both lining and uppers, even for the same colour, this makes it even more difficult to make calculated predictions.
It then also affects how tight the leather was pulled during the lasting process. On factory-made shoes this difference is smaller, but with hand-lasted shoes it can vary a lot depending on the person who lasted the shoe, although of course you try to have a standardised level so to speak. When you hand-last, you can also pull the leather harder than with a machine, as you have more control over the process. Machine lasting should work for all shoes in a batch without changing the settings and then they are set to last a bit looser by default.
The material also matters here, a classic example is Hungarian Vass whose smooth leather shoes are very hard lasted, and therefore stretch relatively little as a rule, while suede and cordovan which have less tensile strength are not lasted as hard, and therefore often both perceived as larger than a smooth leather shoe even though they are made on the same last in the same size, and then also tend to stretch more (not cordovan though, since it has very little give). Also embossed grain leather often are stretched a bit softer, to not remove all the embossing. This above also do apply for factory-made shoes, but can be a bit smaller differences than in some hand lasted cases, but still highly noticeable.
The construction method used can also play some role. A Goodyear welted shoe with a very soft cork filling will be compressed a bit over time as they are worn, becoming more roomy especially at the widest part of the shoe where the cavity is the largest. A shoe that is hand welted, Blake/Rapid-stitched or Blake-stitched does not change as much here, as there is a smaller cavity on these that does not sink in the same way, although the filling and the leather insole also mould well and do compress to some extent.
In other words, to simply assume that a shoe “stretches half a size” or similar, as you may sometimes hear, is not very realistic. There are so many different parameters to take into account, and in many cases it’s very difficult to determine in advance what the stretch will be. When even a shoe of the same size and model from the same manufacturer can vary depending on the material, the person who lasted it, and so on, it sure makes it difficult.
Read more on the topic in this follow up article, where I go through other aspects of what affects how shoes stretch, above it was mainly technical aspects of materials and making, in that post it’s more on how they fit and are worn plays a role.