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What does this converter do?
Hat size calculator: convert head circumference in inches or centimetres into US, UK, and metric hat sizes instantly. Free, private, runs locally, no signup.
- Hat size 7-1/8 = 22.5 in circumference = medium.
- Measure across the mid-forehead, just above the ears, the way the hat will sit.
- Most quality hats include a sizing band worth 1-2 sizes of adjustment.
How this calculator works
Formula
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.
Sources
Sources support the method, terminology, or planning context. Editable assumptions remain your responsibility.
Assumptions to verify
- 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.
Frequently asked 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.
Cite this tool
BoringToolsKit. “Hat Size Calculator.” boringtoolskit.com/hat-size-calculator/ (reviewed 2026-09-20). Free to reference in articles, syllabi, and answer posts with a link.
Privacy: Inputs and results stay in this browser. Conversion runs locally in your browser; review the output before relying on it.
A worked example: 22.8 inches lands on 7 1/4 (L)
The calculator holds eight anchor points: 21.5, 22, 22.5, 23, 23.5, 24, 24.5, and 25 inches, mapped to the labels 6 7/8 (S) through 7 3/4 (XXL). It computes the absolute distance from your input to each anchor and keeps the smallest. Say you measure 22.8 inches around your head. The distances are: 21.5 gives 1.30, 22 gives 0.80, 22.5 gives 0.30, 23 gives 0.20, 23.5 gives 0.70, 24 gives 1.20, 24.5 gives 1.70, and 25 gives 2.20. The minimum is 0.20, so 22.8 maps to 7 1/4 (L).
The tie-breaking rule matters at exact midpoints. At 21.75 inches, the distance to 21.5 is 0.25 and the distance to 22 is also 0.25. The code only replaces the current best when the new distance is strictly smaller, so the first anchor in the list wins and 21.75 returns 6 7/8 (S), the smaller of the two valid sizes. That is the conservative choice for apparel: a slightly loose hat is more salvageable than one that compresses. The same tie at 24.75 would return 7 1/2 (XL) rather than 7 5/8 (XL).
No interpolation ever happens. Every input inside 18 to 26 inches collapses to one of exactly eight outputs, so two heads measuring 23.4 and 23.49 both get 7 3/8 (L/XL). That mirrors how hats are actually sold, since manufactured sizes are discrete.
Boundary behavior: what the input clamps to and where it refuses
The input window is hard at both ends. Below 18.0 inches or above 26.0 inches, the calculator returns an error message instead of a size, with no nearest-match fallback. A 17.9-inch measurement, plausible for a toddler, is rejected even though 18.0 would have returned 6 7/8 (S). At the top, 26.0 is valid and returns 7 3/4 (XXL), but 26.1 is rejected, so heads past that point need a size system this table does not cover, such as Alpha sizing (L/XL buckets) or custom fitting.
Because the largest anchor is 25 inches, any input from 25.0 to 26.0 returns 7 3/4 (XXL), meaning the final inch of the valid range all maps to one size. The same compression happens at the bottom: anything from 18.0 to 21.74 returns 6 7/8 (S), the smallest size on the chart. In practice those bottom-band inputs often describe head shapes better served by youth or women's-fit lines, which use different crown depths.
Real-world fitting adds friction the formula cannot see. Hair volume, hat material stretch, and measurement placement above versus across the ears can shift a tape reading by a quarter inch, which is enough to move you from 7 (M) to 7 1/4 (L) at 22.3 inches. Leather and straw hats do not stretch, while knit caps can absorb a full size of error. If you land exactly between two anchors, the tie rule gives you the smaller one, which is the safer default for stiff hats but may read tight on elastic ones.