Hat Size Calculator
Estimate numeric and letter hat size references from head circumference.
Last updated · View algorithm, constants and default inputs (JSON)
About this calculator
This model divides head circumference by pi to estimate a numeric hat size, then applies the selected fit allowance. The letter ranges are broad references based on metric circumference. Hat sizing and preferred ease vary by maker, so check the product chart.
Worked example
calculated with the default values below
Head circumference (selected unit) 22 · Measurement unit Inches · Preferred fit Regular US hat size 7.00 · UK hat size 6.87 · Metric size (cm) 55.9 cm · Letter size S
General background references
Disclaimer
These calculators give general estimates. Sizing, pricing, availability and standards vary by brand, retailer and country, so treat the output as a starting point and confirm it against the seller's or authority's own figures.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
US = (c+a)/π; UK ≈ US−0.13; cm = 2.54(c+a)
- c
- Head circumference, in
- a
- Fit allowance, in
- US
- US hat size
- cm
- Metric size, cm
US numeric size ≈ head circumference in inches ÷ π plus the selected ease adjustment; UK and letter-size outputs are broad approximations. Check each maker’s sizing chart.
Frequently asked questions
How does this calculator estimate a numeric hat size?
It divides head circumference in inches by pi and applies the selected snug, regular or loose allowance. Letter sizes use broad circumference bands. This is an approximation; check the hat maker’s size chart and fit guidance.