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Ring Size Converter

Converts a ring size between US/Canada numbers, UK/Ireland letters, EU/ISO 8653 (inner circumference in mm), Japanese numbers and the inner diameter or circumference in millimetres, using the US relation diameter = 11.63 mm + 0.8128 mm × size and the Wikipedia ring-size comparison table.

When to use

You know a ring's size in one system, or have measured a ring's inside diameter or a finger's circumference, and need the equivalent sizes in the other systems.

Do not use when: The finger has not been measured (measure its circumference at the knuckle with a strip of paper first), or you need a jeweller's fitting for a very wide band; sizes are geometric conversions, not fit advice.

Formula

diameter_mm = 11.63 + 0.8128 × US = circumference_mm / π = 13 + (Japan − 1) / 3; circumference_mm = π × diameter_mm = EU/ISO 8653 size; UK: nearest entry of the letter table (A = 37.8, F = 44.2, N = 54.4, P = 57.0, Z = 69.7 mm circumference, ≈ 1.28 mm per letter, half sizes at the midpoints); Japan = 1 + 3 × (diameter_mm − 13)

US whole sizes differ by 0.032 in (0.8128 mm) of inner diameter with US 3 = 14.07 mm; ISO 8653:2016 designates rings by inner circumference in mm; Japanese sizes step by 1/3 mm of diameter from 13 mm at size 1; UK letters follow the Wikipedia comparison table. Conversions are purely geometric, so unrounded values show where a ring falls between made sizes (¼ US, ½ UK letter, whole EU and Japanese sizes).

Inputs

Parameter	Type	Unit	Required	Description
`value`	string		yes	Size in the chosen system: a number (7, 7.5, 7¼, 54, 17.3) or, for UK, a letter with an optional half (N, N½, N1/2).
`system`	enum: us \| uk \| eu_iso \| japan \| diameter_mm \| circumference_mm		yes	System of the entered value.

Outputs

Output	Type	Unit	Description
`diameter_mm`	number	mm	Inside diameter of the ring.
`circumference_mm`	number	mm	π × diameter; the ISO 8653 / EU size before rounding.
`us_size`	number		(diameter_mm − 11.63) / 0.8128, unrounded.
`us_size_nearest`	number		US size rounded to the nearest quarter size, the finest step rings are made in.
`uk_size`	string		Nearest letter or half letter (A–Z) of the comparison table.
`eu_iso_size`	integer		Inner circumference in mm rounded to a whole number.
`japan_size`	integer		1 + 3 × (diameter_mm − 13) rounded: size 1 = 13.0 mm, one third of a millimetre per size (omitted below 13 mm).
`diameter_in`	number	in	Inside diameter in inches.
`note`	string		Rounding conventions and fit caveat.

Example

US 7: {"value":"7","system":"us"} → {"diameter_mm":17.32,"circumference_mm":54.4,"us_size":7,"us_size_nearest":7,"uk_size":"N","eu_iso_size":54,"japan_size":14}

18 mm inner diameter: {"value":"18","system":"diameter_mm"} → {"diameter_mm":18,"circumference_mm":56.5,"us_size":7.84,"us_size_nearest":7.75,"uk_size":"O½","eu_iso_size":57,"japan_size":16}

GET https://tttkmbb.com/api/v1/calculate/ring-size?value=7&system=us

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/ring-size (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/ring-size · Markdown: https://tttkmbb.com/everyday/ring-size.md · JSON definition: https://tttkmbb.com/everyday/ring-size.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="ring-size"
OpenAPI operationId: convert_ring_size
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

Ring size – US, UK, ISO 8653 and Japanese scales with comparison table (Wikipedia) (reference)

FAQ

How do I measure my finger?

Wrap a strip of paper snugly around the base of the finger, mark where it overlaps and measure the length in mm; enter it with system circumference_mm. Measure at the end of the day when fingers are largest, and make sure the ring passes the knuckle.

Does band width change the size?

Yes. Bands wider than about 6 mm feel tighter; jewellers usually recommend going up a quarter to a half US size for wide bands.

Why does the UK letter not match some charts exactly?

Charts differ by up to half a letter because British sizes were historically defined by diameter steps that do not align with US quarter sizes; this tool uses the Wikipedia comparison table.

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