Do Running Shoes Run Small?

Short answer: most running shoes are designed to run true to size — but there is significant variation between brands, and even between models within the same brand. A size 10 in Brooks is not the same as a size 10 in Nike, and the Nike Pegasus fits differently from the Nike Vaporfly.

This matters most when you are ordering online, switching brands, or buying a model you have never worn before. Getting it wrong means a return trip (if you are lucky) or black toenails on race day (if you are not).

This guide covers why running shoe sizing feels inconsistent, what the research says about sizing up, brand-by-brand notes based on general industry knowledge, and how to check before you buy.

Why Running Shoe Sizing Feels Inconsistent

The root cause is lasts. A last is the three-dimensional mould that a shoe is built around — it determines the shape, volume, and fit of the shoe before any cushioning or upper material gets added. Every brand designs their own lasts in-house, and there is no international standard requiring that a “size 10” last from Nike match a “size 10” last from Asics. They can differ by several millimetres in length and significant amounts in width.

Width compounds the problem. A shoe that is narrow in the forefoot will feel shorter than it is because your foot is splaying into the sides rather than forward. Two shoes can measure the same length but fit completely differently depending on their width profile and toebox shape.

Shoe purpose also affects perceived fit. A lightweight racing flat or carbon-plated shoe is intentionally built closer to the foot — snug upper, tighter toebox — to maximise energy return. A cushioned daily trainer like the Brooks Glycerin is built with more room. The same size in each will feel very different.

Finally, break-in period varies. Some uppers (particularly knit materials) stretch noticeably after a few runs. A shoe that feels snug on day one may fit well by week two. Others do not break in at all — what you feel in the store is what you get.

Do Running Shoes Run True to Size?

Most running shoes are designed to run true to size in length. The complication is that runners are commonly advised — and the running community broadly agrees — to size up half a size from their everyday shoe. This is not because running shoes run small; it is because running shoes are used differently from street shoes.

The core reason is the thumb width rule. Experienced runners and podiatrists consistently recommend leaving approximately one thumb's width of space between your longest toe and the end of the shoe. That gap accounts for two things: toe splay (your toes spread laterally on foot strike to aid balance and propulsion) and foot swelling. During a long run, your feet can swell by up to half a shoe size as blood volume increases in your extremities and repeated impact drives fluid into foot tissues. In hot conditions, swelling is greater. Over a marathon distance, a shoe that fits snugly at the start can feel like a vice at kilometre 35.

When to trust your usual size: if you are buying the same model you have run in before, your regular size is fine. If the brand is known to run roomy (Hoka Clifton, New Balance 1080), you may not need to size up. If you are doing short distances in cool conditions, the swelling argument is less compelling.

When to adjust: switching brands, buying a new model for the first time, or if past shoes in that brand have felt tight. When in doubt, go up — not down.

Brand-by-Brand Notes

These notes are based on general industry knowledge and runner consensus — not RunSized data. For model-specific community reports, use the links below each brand.

How to Check Before You Buy

Before committing to a size online, run through this checklist:

• Check the brand's size chart. Most brands publish a length-in-cm chart. Measure your foot (at end of day, weight-bearing) and match it directly.
• Read recent reviews that specifically mention sizing. Look for phrases like “runs small,” “sized up half,” or “true to size.” Prioritise reviews from the current model version — lasts sometimes change between versions.
• Use the RunSized tool. Find your size in any running shoe by entering your current shoe — the community reports show you what size runners with your reference shoe ordered and how it fit.
• Try in-store if possible. Running specialty stores will watch you jog in the shoe and give you a proper fit assessment. Worth doing for expensive purchases.
• Order from retailers with free returns. If you are genuinely unsure between two sizes, order both and return the one that does not fit.

When to Size Up in Running Shoes

Here are the situations where sizing up is clearly the right call:

• Long runs and races. Half marathon distance and beyond, foot swelling becomes significant. Many marathon runners go up a full size — not just half. Ultrarunners sometimes carry a second, larger pair for the back half of a race.
• If you are between sizes, go up. The risk of going too small is always higher than too large. Black toenails and blisters from a tight shoe are worse than mild heel slippage from one that is a touch long.
• Wide feet. If you have a wide forefoot and the shoe does not come in a wide variant, sizing up half a size can give you slightly more toebox room — though it is an imperfect workaround.
• Carbon plate shoes. Racing shoes are often built intentionally snug to maximise energy return from the plate. If your daily trainer is a 10, your carbon race shoe might need to be a 10.5 or 11.
• If your usual size felt tight in any shoe from that brand. Sizing patterns carry across models within a brand. If the Ghost felt tight in a 9, the Glycerin will likely feel tight in a 9 too.

Frequently Asked Questions