How to Find Your Running Shoe Size — The Complete UK Guide

One of the most common mistakes new runners make is buying running shoes in the same size as their everyday shoes. Running shoes require more space than casual footwear — and getting the sizing wrong is one of the most reliable ways to end up with blisters, black toenails, and discomfort that puts you off running entirely. This guide walks you through exactly how to find the right running shoe size.

Why running shoe sizing is different

When you run, your feet swell. Blood flow increases, your tendons and muscles expand slightly, and your foot spreads as it absorbs impact with the ground. This swelling is completely normal and can add up to a full shoe size over the course of a long run. A shoe that feels perfect when you're standing still can feel uncomfortably tight after 10 or 15 kilometres.

Running shoes are also designed with more interior volume than dress shoes or casual trainers. The toe box is broader, the heel counter is firmer, and there is typically extra length built in to account for foot movement during the running gait. All of this means most runners wear running shoes that are half to a full size larger than their everyday shoes.

How to measure your feet at home

The most accurate way to find your running shoe size is to measure your feet in the afternoon or evening, when they are at their largest after a day of activity. Place a piece of paper on a hard floor, stand on it with your full weight, and trace around your foot. Measure the distance from the heel to the tip of your longest toe. Repeat for both feet — most people have one foot slightly larger than the other — and always size for the larger foot.

Match your measurement against the brand's size chart. UK shoe sizing varies slightly between brands, which is why it is always worth checking the specific chart rather than assuming a consistent conversion. A UK 9 in Brooks may fit differently to a UK 9 in Adidas or Hoka.

The thumb's width rule

The traditional fitting rule for running shoes is to leave approximately a thumb's width of space between your longest toe and the end of the shoe. With your running socks on and your foot pressed firmly to the heel of the shoe, press your thumb against the front of the shoe behind your big toe. If there's a firm thumb's width of space, the length is approximately right.

Runners who tackle downhills regularly often prefer slightly more space than this to prevent toenail bruising on descents. If you are planning trail running or hilly routes, err on the slightly larger side of your fitting range.

Width matters as much as length

Shoe size is not just about length. Width matters equally for comfort on the run. Most running shoes are made in a standard width, but several brands offer wide and extra-wide options. New Balance is the standout here, with many popular models available in standard (D), wide (2E), and extra-wide (4E) widths.

If you regularly experience blisters on the sides of your feet, numbness across the top, or find your toes curl downward in shoes, the shoe is likely too narrow. Conversely, if your heel lifts or your foot slides from side to side inside the shoe, the width may be too generous.

Brand-to-brand sizing differences

Running shoe sizing is not standardised across brands. Hoka shoes tend to run slightly short and narrow compared to their stated size — many runners size up half a size, particularly in the Clifton and Bondi. Brooks and Asics generally run true to size for most foot shapes. Nike race shoes like the Vaporfly run narrow and snug by design, which can be uncomfortable for wider feet.

When buying from a new brand for the first time, always check the sizing notes on the product page. Customer reviews on specialist running retailers frequently flag when a model runs long, short, narrow, or wide, which is more useful than the brand's own guidance.

Find your size in stock with RunSizer

Once you know your size, finding that shoe in stock across UK retailers is often the next challenge. Popular sizes sell out quickly on in-demand models. RunSizer lets you search any running shoe, select your UK size, and instantly see which retailers have it in stock and at what price — without clicking through five different websites. It's the fastest way to find your shoe without paying more than you need to.