Shoe Size Calculator

A shoe size calculator estimates a starting size from foot length and width, converts international size systems and checks whether a shoe or removable insole provides enough internal space.

Choosing footwear online becomes difficult because one number does not describe the entire fit. A useful calculator starts with measurements, shows several systems and makes clear that the brand, model and last still determine the final fit.

What the calculator can do

• measure left and right foot length separately;
• record the width of both feet;
• use the longer and wider foot for the recommendation;
• work in millimetres, centimetres or inches;
• estimate EU, UK, US, JP, KR, Mondopoint, CN and BR sizes;
• handle adult men, adult women, unisex and children profiles;
• adjust toe allowance for casual, running, hiking, boots, dress shoes, heels and sandals;
• account for sock thickness and preferred fit;
• apply a cautious correction when a known model runs small or large;
• classify width as narrow, standard, wide or extra wide;
• check shoe or insole internal length and width;
• export results to CSV.

Why foot length is the best starting point

ISO 19407:2023 bases international footwear conversion tables on foot length. The standard covers major systems including Mondopoint, European, United Kingdom, China, Japan, Korea and the United States. This approach avoids assuming that the same printed size means the same internal shape across every market.

Foot length gives the calculator a measurable reference. It adds a configurable allowance to estimate the target internal or last length before calculating EU, UK and US values.

Measure both feet while standing

Measure in the socks that you expect to wear with the shoes. Stand with normal body weight on the foot, keep the heel against a straight reference and mark the longest toe. Measure from the rear of the heel to that mark.

Repeat the process for the other foot. Nike and New Balance advise using the larger measurement when the feet differ, so the calculator does not average the two lengths.

Measure foot width separately

Length and width describe different parts of fit. Measure width across the widest part of the forefoot, usually around the ball of the foot. Do not pull the measuring tool tightly enough to compress the foot.

The calculator compares width with foot length and returns an approximate category with familiar width codes. Manufacturers still use different lasts and grading rules.

New Balance publishes separate length and width guidance and offers multiple width options. That illustrates why a length conversion cannot replace a brand-specific width chart.

How the fit allowance works

A shoe needs more internal length than the bare foot. The extra space depends on footwear type, socks, activity and personal preference. Running and hiking footwear often need more toe room than close-fitting dress shoes or sandals.

The calculator combines three inputs:

Total allowance = toe-room setting + sock adjustment + fit preference adjustment.

The user can edit the toe-room value because no single allowance suits every product. Running shoes, heels and sandals use different constructions.

EU shoe size calculation

The traditional European system uses Paris points. One Paris point equals two-thirds of a centimetre. A practical formula based on estimated last or internal length is:

EU size ≈ target internal length in centimetres × 1.5.

The formula produces a technical estimate. Retail charts may map the same foot length to a different EU number because brands choose their own last allowance and rounding.

UK and US size estimates

Traditional UK and US systems use barleycorn grading, where one full size represents one-third of an inch. Adult UK sizing can be estimated from target internal length in inches:

UK adult size ≈ 3 × target length in inches − 25.

Common approximate offsets then produce:

• US men ≈ UK + 1;
• US women ≈ UK + 2.5.

Official tables do not align perfectly across every brand, so the calculator labels the output as estimated and shows neighbouring options.

JP, KR, China and Mondopoint

Japan commonly marks footwear in centimetres, while Korean labels often use millimetres. Mondopoint also builds its designation from defined foot measurements. ISO 9407:2019 specifies Mondopoint as a sizing and marking method based on the measurements of the foot that the footwear should fit.

The calculator reports:

• JP as estimated foot length in centimetres;
• KR and Mondopoint as foot length in millimetres;
• CN as a length-based millimetre estimate;
• BR as an approximate conversion from EU.

A “CM” box label may not equal measured foot length. Nike warns that its printed CM value can differ from the foot-length column.

Why international conversions disagree

Conversion tables combine different concepts. Some systems mark foot length, others refer more closely to a last or internal length, and manufacturers introduce model-specific allowances. A narrow racing shoe and a wide casual sneaker may share the same printed size while fitting very differently.

Other causes include:

• different rounding to whole, half or third sizes;
• separate men’s and women’s US scales;
• child and youth numbering resets;
• brand-specific last length and toe shape;
• regional labels applied to the same physical shoe;
• manufacturing tolerances and upper materials.

Converting a known size

Conversion mode estimates a target length, removes the selected allowance and calculates the other systems from one common reference. A known EU 42 can still represent different foot lengths across brands, so use the output for comparison rather than as proof of fit.

Whole, half and EU third sizes

Retailers offer different grading steps. UK and US footwear often uses half sizes. Some European brands use thirds such as 42⅓ and 42⅔, while others sell only whole EU sizes.

The calculator can round upward to:

• the next half size;
• the next whole size;
• an EU third-size step.

Rounding upward is a cautious default when a measurement falls between sizes, but excessive extra space can also reduce stability. Compare both neighbouring options when possible.

Checking internal shoe or insole dimensions

The fit-check mode compares the larger foot with measured internal dimensions:

Length clearance = shoe internal length − longer foot length.

Width clearance = shoe internal width − wider foot width.

A removable insole gives a convenient reference but cannot describe toe-box height, upper stretch, heel hold, instep volume or last curvature.

Category-specific fit considerations

Running shoes

Feet can move forward and expand during activity. Nike’s running-fit guidance describes roughly a thumb’s width ahead of the longest toe as a common check, although preferences and models vary.

Hiking footwear and boots

Thicker socks and downhill movement can require more room. At the same time, the heel should remain controlled to reduce sliding.

Dress shoes and heels

These designs often use less toe allowance, but the toe box must not compress the toes. A pointed shape can require a different size or width even when the measured length appears correct.

Sandals

The foot should remain supported by the sole without hanging over the front, side or heel. Open construction changes how internal length relates to fit.

Children’s shoe sizes

Children’s systems use child and youth suffixes such as C and Y. The calculator estimates these markings from length, but the manufacturer’s child chart remains essential because growth allowance and model construction vary.

Measure both feet regularly. Nike’s children’s guide recommends measuring to the longest toe, using the longer foot and sizing up when the measurement falls between available sizes.

Common shoe-sizing mistakes

• measuring only one foot;
• measuring while sitting instead of standing;
• using an old shoe size instead of current measurements;
• ignoring width and toe-box shape;
• confusing box “CM” labels with actual foot length;
• assuming EU, UK and US conversions are universal;
• forgetting the socks used with the shoe;
• using the outside sole length as internal length;
• buying only by gender label instead of the brand chart;
• adding too much extra space for children.

How to verify the recommendation

Open the exact brand’s current chart, compare the longer foot with its foot-length column and read model-specific width notes. During a fitting, stand and walk in both shoes, checking toe room, forefoot pressure and heel hold.

Persistent pain, numbness or unusual pressure needs professional assessment. The calculator supports shopping and comparison; it does not diagnose foot conditions or prescribe footwear.

Frequently asked questions

Official references and size guides

• ISO 19407:2023 — Footwear sizing conversion systems
• ISO 9407:2019 — Mondopoint sizing and marking
• Nike — How to measure foot size
• Nike — Men’s footwear size chart
• Nike — Children’s footwear size chart
• New Balance — Size and width guide

Sources reviewed: 21 June 2026.