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Hat Size Converter

Convert hat sizes from head circumference or labelled sizes. Measure above the ears and mid-forehead for the most reliable starting point.

Calculate

Measure head circumference above the ears 1 AROUND THE HEAD above ears · mid-forehead
How to measure for hats →

Updates as you type

Result

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The formula

US/UK fitted sizes are approximately head circumference in inches divided by π. EU labels often print circumference in centimetres.

US ≈ circ_in ÷ π · EU ≈ circ_cm · circ_cm = circ_in × 2.54

How to measure for hats

Measure above the ears and mid-forehead.

Measure head circumference above the ears 1 AROUND THE HEAD above ears · mid-forehead
  1. Wrap — Soft tape around the head at hat height.
  2. Level — Keep tape level; snug but not tight.
  3. Read — Note cm and inches.
  • Hair volume affects fit.
  • Between sizes, many people size up.

Worked example

  1. Head circumference 56 cm
  2. Inches ≈ 56 ÷ 2.54 ≈ 22.05 in
  3. US ≈ 22.05 ÷ π ≈ 7.02 → about 7

Result: ≈ US/UK 7 · EU 56

How hat sizes are converted

Hats fit a head circumference. Numeric US/UK sizes encode that circumference; many EU tags simply print centimetres.

Circumference hub

Inputs in cm or inches convert directly. US/UK numeric sizes convert as size × π × 2.54 → cm. EU is treated as centimetres (rounded in the output grid).

Fitted vs letter sizes

Baseball fitted caps often use fractional US sizes (7, 7⅛, 7¼…). Soft letter sizes (S/M/L) are broader bins — prefer centimetres when you can.

What we do not model

No crown height, brim styles, or stretch fabrics. Hair volume can push you up half a size.

Interesting facts

Circumference first

Head circumference in cm (or inches) is more reliable than S/M/L alone.

Fitted caps differ

Baseball fitted caps often use a separate fractional US scale (e.g. 7¼), not clothing letters.

US fractions vs cm

US hat sizes are roughly circumference in inches ÷ π — EU labels usually print centimetres.

Hair changes fit

Thick hair or styles that add volume can push you up half a size.

Between sizes, size up

If you are between sizes, many people choose the larger for comfort — especially for structured hats.

Frequently asked questions

Wrap a soft tape above the ears and across mid-forehead, level and snug. Enter centimetres (or inches) and read US/UK/EU equivalents.

Above the ears and across mid-forehead — where a typical hat brim sits.

Numbered sizes (or cm) are more precise. Letter sizes are rough bins that vary by brand.

Convert cm → inches, then divide by π. Example: 56 cm ≈ 22.05 in ÷ π ≈ size 7.

Many people size up for structured hats. Adjustable caps and stretch bands are more forgiving.

References

  1. NIST — length units — National Institute of Standards and Technology Exact centimetre/inch conversions for circumference.
  2. ISO clothing size framework — International Organization for Standardization ISO 8559 context for body measurement terminology.
  3. Head circumference — growth charts (clinical context) — U.S. Centers for Disease Control and Prevention (CDC) Clinical head-circumference charts (pediatric); illustrates why cm measurements are standard.