Why running shoe sizes change from one brand to another

A size 42 can fit perfectly in one brand and feel short, wide or overly tight in another. This is not a manufacturing error or an unusual foot shape: shoe sizing is not fully standardized between manufacturers, and every model uses a different last, material selection and fit philosophy.

The most reliable reference is not the number on the box, but foot length in centimeters and the model’s internal measurements. Width, instep volume, toe-box shape and intended use also affect the final sensation. Choosing running shoes therefore requires more than repeating your usual size.

The number does not tell the whole story

European sizing is based on a scale expressed in points, but in practice each company may apply different tolerances. A European size does not describe the exact length of the internal space on its own, because the manufacturer must decide how much room to leave in front of the toes and how to distribute volume throughout the shoe.

The last, or internal mould used to build the shoe, also matters. Two shoes with the same size can have similar internal lengths and still feel completely different because one places more room in the toe box while the other concentrates volume around the midfoot. The last determines both fit and the perception of size.

The difference becomes more obvious when comparing racing shoes with daily trainers. Racing models often seek a closer fit and a faster response, while training shoes may offer a more forgiving platform and greater volume. The correct size may not change, but the most comfortable option for the same foot can.

The external shape does not reveal the real internal space. A bulky shoe may have a narrow toe box, while a streamlined model may hide a generous platform. Foam, lining, tongue and lacing system complete the fit. The appearance of the upper cannot replace measuring and trying the shoe on.

How to measure your foot reliably

Home measurement requires a sheet of paper, a pencil, a ruler or measuring tape and a few quiet minutes. Place the bare foot on a firm surface with the heel against a wall. Hold the pencil vertically to mark the furthest point of the longest toe without angling it forward.

Repeat the process with both feet, since it is normal for one to be slightly longer. Make the final decision using the measurement of the longer foot, not the average of both. A difference of only a few millimeters may be unnoticeable on a short walk but become obvious during a long run, when the foot expands.

It is best to measure at the end of the day or after an active period. Feet commonly swell with heat, load and time spent standing. Use the socks you normally run in. Measuring in the morning and buying for a completely rested foot can result in an internal space that is too tight.

Record the distance from heel to the longest toe in centimeters with reasonable precision. Not every foot has a longer second toe than big toe, so the measurement must reach the furthest point, regardless of which toe it is. Your actual foot takes priority over sizing conventions.

Length is not the whole measurement. If the foot leaves deep marks along the sides of the paper, if the toes spread widely or if the instep takes up considerable volume, width and depth are also part of the issue. A shoe can be long enough and still be unsuitable if it compresses the forefoot or presses against the top of the foot.

Centimeters, European sizing and internal room

Brand charts often show several columns: European size, foot length, product length or insole length. These values must be distinguished. Foot length describes the body; internal length indicates available room; and insole length may include areas that are not fully usable while walking or running.

The value labelled foot centimeters is usually the most useful starting point, but it does not always mean that the shoe measures exactly that much inside. Some brands show the recommended foot length while others publish internal length. Reading the column heading prevents comparing measurements that do not mean the same thing.

In a running shoe, the toes need room to avoid hitting the front during descents and to allow natural expansion. As a general guide, many runners are comfortable with around 8 to 12 millimeters of free space in front of the longest toe. This is not a universal law: the ideal amount depends on the activity, foot shape, sock thickness and shoe behavior.

Less room may work in a short-use model with a broad toe box, but it becomes risky in long sessions. Too much space is not a solution either: the foot may slide, wrinkles can form and friction can increase. The aim is freedom without losing security.

Trail shoes require a specific assessment. On technical terrain, some runners prefer a firmer fit to reduce movement inside the shoe, while long routes may require extra room because of swelling. The balance changes according to elevation gain, duration and terrain type.

The last changes the fit between brands

The last is the mould that defines the internal silhouette and affects length, width and volume. A narrow last can make the toes feel compressed even when the length is correct. A wide last can allow the foot to relax, but it may also create unwanted movement if the lacing system does not secure the midfoot.

The toe box is one of the areas that most changes perception. Some shoes end in a rounded shape and leave lateral room; others narrow progressively through the forefoot to create a precise sensation. On a wide foot, going up a size may create excessive length without solving side pressure. The right width is not always achieved by choosing a larger size.

Instep volume matters too. A high instep may feel pressure beneath the tongue or around the laces, even when the toes have enough room. A low instep may need a firmer fit to prevent heel lift. The solution may be a different last or lacing pattern rather than a larger size.

The heel should be secure without feeling strangled. While walking and running, the foot should not slide forward or lift noticeably at the back. A heel-lock lacing method can improve security, but it cannot fix a shoe that is too large. A stable heel and free toes are simultaneous goals.

Brands that offer different width versions make it possible to refine the fit without changing length so much. Availability depends on the market and model, so the technical product details should be checked before buying.

Why the same manufacturer can also change its measurements

Variation is not limited to comparisons between brands. A single company may use different lasts for road, trail, racing, stability or lifestyle lines. Even two successive versions of the same model can alter upper volume, toe-box shape or lacing tension.

Materials also evolve. Stretchy fabric can adapt to foot width, while a more structured mesh keeps a rigid shape. Heat-bonded reinforcements, protective pieces and side panels change usable space. The fit of one model cannot automatically be transferred to the entire collection.

The midsole has an indirect influence. A strongly curved platform can move the foot into a different position, while a broad base can make the runner feel more stable even when the upper is close-fitting. The sensation when trying on a shoe does not depend only on internal length measured with a ruler.

Women’s and men’s versions should not be understood simply as a color or numbering difference. They often vary in width, heel volume and last proportions, although many feet do not fit neatly into those broad categories. Individual anatomy should take priority over the collection label.

Manufacturing changes, different production facilities and industrial tolerances can create small differences. They are usually minor, but they matter for runners who are between sizes. Keeping the exact model reference and checking the return policy is especially useful when buying remotely.

How to read a size chart without making a mistake

The official chart for the specific model should be checked before looking at general conversions. Universal charts are useful as an initial guide, but they can hide important differences between manufacturers. The safest process is to compare your foot measurement with the centimeter recommendation for that exact model.

If the brand provides its own guide, check whether it refers to foot length or internal length. Also verify whether the measurements refer to the removable insole, usable space or a recommended fit. An accurate chart still requires accurate reading, especially when you are between two options.

Conversions between European, United Kingdom and United States sizes do not always maintain a perfect mathematical equivalence. A US size can vary according to the collection’s gender category, and a UK size may not exactly match an automatic conversion shown by a retailer. The converted number helps locate the chart, but it should not make the final decision.

When the measurement falls between two sizes, intended use and model shape matter. For long runs, heat, mountain routes or thick socks, the larger option may provide more room. For a close-fitting racing shoe and short sessions, some runners prefer the smaller option, provided the toes are not compressed. Training duration and conditions change the choice.

Reviews from other users can offer clues about whether a model runs short, long, narrow or wide, but they require caution. One person may describe a shoe as narrow while it feels normal on another foot. Objective measurement and personal experience carry more weight than one isolated opinion.

Signs that the size is wrong when running

The first test should take place at home on a clean surface with your usual socks. Once standing, the toes should be able to move and the longest toe should not touch the front. While walking, the heel should remain reasonably secure and the instep should not feel localized pressure.

A shoe that is too short often leaves clear signs: sensitive toenails, repeated impact against the toe box, numb toes or pain on downhill sections. Pressure may appear only after several miles, not during the first few minutes. Initial comfort does not guarantee that the fit will work during a long session.

When the shoe is too large, the foot may slide forward and create rubbing around the toes. Blisters can also appear at the heel, along with folds beneath the forefoot and a feeling of poor control during changes of direction. Tightening the laces does not always solve the issue and can create pressure across the instep.

Incorrect width appears through marks on the sides, tingling, overlapping toes or a constant need to loosen the laces. In other cases, runners increase the size to avoid side pressure and end up with excessive length. Lateral pain often points to a last problem, not only a sizing problem.

Fit changes with activity. During a run, the body produces heat, the foot absorbs thousands of impacts and tissue volume may increase slightly. A shoe that feels exact while sitting can become restrictive in motion. Testing should therefore include walking, stairs and foot flexion, not simply standing in front of a mirror.

Buying online without relying on your usual size

Online shopping replaces physical fitting with a more careful process. The product page should show the model-specific chart, last information and exchange conditions. If it only presents a generic conversion, the information is not enough for a confident decision.

A previous shoe can be a useful reference only when it belongs to the same model and a similar version. Two brands may use the same size while offering different lengths and widths. Your usual size is a clue, not a guarantee.

When the shoes arrive, try them indoors at the end of the day with your running socks. Move the foot backward inside the shoe before checking the front space. After tying the laces, walk around and observe heel movement, instep pressure and toe freedom.

The removable insole can help explain the fit, although its length does not always equal functional space. If the foot clearly extends beyond the front or sides when placed on the floor, the model probably does not provide enough room. The insole is a useful reference, not an absolute measurement.

Read the return conditions before removing tags or using the shoes outdoors. An easy exchange makes it possible to correct a poor choice; a limited policy can turn a difference of a few millimeters into a difficult expense. This precaution matters even more when buying an unfamiliar brand or a newly updated model.

A choice based on the foot, not the label

Different sizing systems reflect the complexity of athletic footwear. No universal number can describe every foot, and no conversion can fully predict how a shoe will feel. Measuring in centimeters, reading the official chart and considering the last is a much more reliable combination than remembering one familiar number.

The best fit leaves room for the toes to work, keeps the heel controlled and does not force the foot to adapt through pressure. In a store, this can be assessed during a short fitting; online, it requires checking the data, trying the shoes at home and remaining critical of automatic conversions.

The correct size is not necessarily the one you have worn for years. It is the one that keeps the foot stable and comfortable when temperatures rise, the route gets longer and the body is no longer still. Measuring before buying turns an uncertain choice into a far more reasoned decision.

You may also like: Salomon runs large or small: a clear guide to get it right.

Este artículo contiene enlaces de afiliado: si compras a través de ellos, se genera una pequeña comisión sin coste adicional para ti. Más información.