# Hat Size Calculator

> Hat size calculator: convert head circumference in inches or centimetres into US, UK, and metric hat sizes instantly. Free, private, runs locally, no signup.

- **Canonical URL:** https://boringtoolskit.com/hat-size-calculator/
- **Category:** Converters
- **Evidence level:** authored-methodology
- **Reviewed:** 2026-09-20
- **Last verified:** 2026-10-12
- **Privacy:** Inputs and results stay in this browser unless the page clearly labels an external request.

## How it works

US hat size = head circumference in inches / pi, so the labels run ~6.5 to 8 while the tape reads 20.5 to 25 inches. Small-medium-large mappings compress roughly two hat sizes into each letter.

## Assumptions

- Manufacturer charts vary; conversion outputs are the standard-mapping midpoint.
- Measurements are body measurements unless the tool labels them as garment measurements.
- Between-size results round per the rule stated in the FAQ.

## Common questions

### Is S/M/L reliable?

It compresses a ~4 inch circumference range into three letters, so two people in the same letter can differ by 3/4 inch. Where it matters (fitted caps, felt), use the numeric size.

### What if I fall between sizes?

Round the direction that matches use: clothing and shoes generally round up.

## Sources

- [Aggregated manufacturer sizing conventions](https://boringtoolskit.com/methodology/)

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