Why Your Running Shoe Size Might Be Different to Your Normal Size

One of the most frequently asked questions from new runners is why their usual shoe size does not seem to fit properly in running shoes. You have worn a size 9 your entire adult life, but the size 9 running shoe pinches, causes blisters, or leaves your toenails bruised after a long run. This is more normal than you might think — and there are several clear reasons why it happens.

Your feet swell when you run

This is the main reason. During running, blood is pumped more forcefully to the feet, your muscles and tendons warm up and expand, and the repeated impact of landing causes the soft tissue in your foot to spread slightly. Over the course of a 60-minute run, your feet can swell by up to half a size.

Everyday shoes, and particularly dress shoes, are not designed with this in mind. A shoe that fits perfectly for standing or walking will feel tight at the end of a long run. Running shoes need to accommodate swelling, which is why the correct size is typically half to a full size larger than your everyday shoe.

Running shoes have more interior volume

Running shoes are built on a different last (the foot-shaped mold used to construct the shoe) than everyday footwear. They have a more voluminous toe box to allow toes to splay during the push-off phase, firmer heel counters that hold the foot securely, and typically a more generous overall interior than a casual trainer.

This means a size 9 running shoe is not the same shape as a size 9 dress shoe or casual trainer, even if the nominal size matches. The extra volume in the toe box can make the shoe feel too large at first for runners accustomed to slim-fitting everyday shoes.

Brand sizing is not consistent

There is no standardised sizing across running shoe brands. A UK 9 in one brand may be equivalent to a UK 9.5 in another. Hoka shoes in particular tend to run short and narrow compared to their stated size — many runners who are a 9 in Brooks find they need a 9.5 in Hoka.

Nike race shoes tend to run narrow by design. Adidas shoes vary by model. New Balance is generally consistent across their range. Always check the specific model's sizing reviews before buying, particularly for a brand you have not worn before.

The thumb's width test

The reliable rule for running shoes is to leave a thumb's width of space between your longest toe and the end of the shoe. With your running socks on, push your foot to the back of the shoe, then place your thumb on the end of the shoe at your longest toe. There should be a firm thumb's width of gap.

If your thumb is pressed hard against the front of the shoe, size up. If there is more than a thumb's width of space and your foot slides around, consider sizing down or adding an insole. This test works consistently across brands and is more reliable than simply going by the number.

When to try shoes in person

Buying running shoes from a physical retailer at least once is worth the extra effort, particularly if you have had sizing problems before. Running shop staff can measure your feet properly, check the fit, and have you run in the shoes before committing. Many runners discover their foot has changed shape or size since they last had it measured.

For future purchases, once you know the brand and model that fits you well, buying online is straightforward. Use RunSizer to compare prices across UK retailers and check who has your exact size in stock — prices on the same shoe vary considerably between retailers.

A practical tip

If you are between sizes, always size up rather than down. A slightly longer shoe is far preferable to one that is even slightly too short. The consequences of a shoe that is too short — black toenails, blisters under the toenails, and bruising that can persist for months — are much worse than the minor inconvenience of a shoe that has a little extra room at the front.