Why Sizing Discrepancies Exist

Why a person can have different shoes sizes, and why sizing is an unsolved challenge in the footwear industry

One of the biggest problems in the footwear industry is size discrepancies. It’s common for people to have multiple sizes of shoes because sizing is inconsistent across brands and hard to standardize.

How a shoe fits and feels depends on multiple factors:

• The shoe last that was used to design the shoe
• The materials that the shoes are made of
• The specific factory or machines used

Length isn't the only indicator of fit

The heel to toe length of the foot is normally the main (and sometimes only) measurement used to determine shoe size. However, for best fit, you need to look at the shape and form of the entire foot because there are a large variety of foot shapes and dimensions. This is why specific foot measurements and features can indicate your customer may need to go up or down in shoe size.

Footwear Sizing Chaos

A big contributor to the chaos surrounding sizing is because there are several sizing systems around the world with unique systems:

The UK System - based on size grading interval of 8.47mm, starting at 101.6mm (4 inches). Originally developed with regard to shoe or last length but redefined in terms of foot length.

The US System -  also an 8.47mm interval, but starting at 99.5mm (3 11/12 inches). Corresponds with the UK system, but with an arbitrary off-set of one size for men's and usually two sizes for women's footwear.

The EU System - defined in ISO 9407 and is based on foot length.

The Mondo point System - based on size grading interval of 6.67mm, starting at zero. May be interpreted as being based on last length or foot length depending on the country of origin.

Source: ISO/TS 19407:2015

Size discrepancies can exist even within the same shoe style

Sizing can vary not only across brands but also from model to model within the same brand. As seen below, the best fit for a size 9 of the Nike Vomero differs drastically for each edition.