A typical shoe is quite a complex product, it’s assembled in a number of layers from many parts which are glued, stitched and/or nailed together. Obviously, it’s hard to give a floppy piece of leather structure, so a form called a last is used. The last is the only fixed measure when making a shoe and it is from the length of the last that the shoe size is based. However only the length is used in calculating a shoe size, therefore factors such as shoe width, the shape of the vamp, toe box, etc are not accounted for when listing shoe sizes, and you’ll find that two different designs from the same factory will have fit differently.
Traditional lasts are made of a material into which nails can be hammered to hold the leather in position. Used with permission © Jeremy Engleman
Starting out I measured the widths as this seemed to cause the most problems, but it’s become apparent that there are other things that seem to affect things such as the cut and materials, since the widest shoe measured happened to be the tightest fit. The next stage will be to look at shoe designs and materials and comparing them to foot shapes.
Slightly OCD but I was curious how much variation there was. Between different manufacturers there was more than 0.5mm despite being the same size
However the one recommendation I will make and will always make is that you should never buy a pair of shoes you can’t return. You will also save a lot of time if you went to an actual shop where you can try the shoe out.