{"success":true,"version":"v1","request":{"tool":"get_calculator_schema","calculator_id":"ring-size"},"result":{"entity_type":"calculator","id":"ring-size","calculator_id":"ring-size","canonical_url":"https://tttkmbb.com/everyday/ring-size","name":"Ring Size Converter","title":"Ring Size Converter – US, UK, EU/ISO 8653 and Japanese Sizes, Diameter and Circumference","category":"everyday","category_name":"Everyday Life","tool_name":"convert_ring_size","featured_mcp_tool":false,"description":"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.","use_when":"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.","inputs":[{"name":"value","label":"Size","type":"string","required":true,"description":"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).","example":"7"},{"name":"system","label":"Sizing system","type":"enum","required":true,"values":[{"value":"us","label":"US / Canada (numbers 0–16, quarter sizes)","aliases":["usa","canada","us_canada","north_america"]},{"value":"uk","label":"UK / Ireland / Australia (letters A–Z)","aliases":["british","ireland","australia","au","nz"]},{"value":"eu_iso","label":"EU / ISO 8653 (inner circumference in mm)","aliases":["eu","iso","iso_8653","europe","european","german","french"]},{"value":"japan","label":"Japan (numbers 1–27)","aliases":["jp","japanese"]},{"value":"diameter_mm","label":"Inner diameter in mm","aliases":["diameter","inside_diameter"]},{"value":"circumference_mm","label":"Inner circumference in mm","aliases":["circumference","finger_circumference"]}],"description":"System of the entered value.","example":"us"}],"outputs":[{"name":"diameter_mm","label":"Inner diameter","type":"number","unit":"mm","decimals":2,"description":"Inside diameter of the ring."},{"name":"circumference_mm","label":"Inner circumference","type":"number","unit":"mm","decimals":1,"description":"π × diameter; the ISO 8653 / EU size before rounding."},{"name":"us_size","label":"US / Canada size","type":"number","decimals":2,"description":"(diameter_mm − 11.63) / 0.8128, unrounded."},{"name":"us_size_nearest","label":"US size (nearest ¼)","type":"number","decimals":2,"description":"US size rounded to the nearest quarter size, the finest step rings are made in."},{"name":"uk_size","label":"UK / Ireland size","type":"string","decimals":4,"description":"Nearest letter or half letter (A–Z) of the comparison table."},{"name":"eu_iso_size","label":"EU / ISO 8653 size","type":"integer","decimals":4,"description":"Inner circumference in mm rounded to a whole number."},{"name":"japan_size","label":"Japan size","type":"integer","decimals":4,"description":"1 + 3 × (diameter_mm − 13) rounded: size 1 = 13.0 mm, one third of a millimetre per size (omitted below 13 mm)."},{"name":"diameter_in","label":"Inner diameter","type":"number","unit":"in","decimals":3,"description":"Inside diameter in inches."},{"name":"note","label":"Note","type":"string","decimals":4,"description":"Rounding conventions and fit caveat."}],"input_schema":{"type":"object","properties":{"value":{"description":"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).","type":"string","examples":["7"]},"system":{"description":"System of the entered value.","type":"string","enum":["us","uk","eu_iso","japan","diameter_mm","circumference_mm"],"examples":["us"]}},"additionalProperties":false,"required":["value","system"]},"output_schema":{"type":"object","properties":{"diameter_mm":{"description":"Inside diameter of the ring. Unit: mm.","type":"number","x-unit":"mm"},"circumference_mm":{"description":"π × diameter; the ISO 8653 / EU size before rounding. Unit: mm.","type":"number","x-unit":"mm"},"us_size":{"description":"(diameter_mm − 11.63) / 0.8128, unrounded.","type":"number"},"us_size_nearest":{"description":"US size rounded to the nearest quarter size, the finest step rings are made in.","type":"number"},"uk_size":{"description":"Nearest letter or half letter (A–Z) of the comparison table.","type":"string"},"eu_iso_size":{"description":"Inner circumference in mm rounded to a whole number.","type":"integer"},"japan_size":{"description":"1 + 3 × (diameter_mm − 13) rounded: size 1 = 13.0 mm, one third of a millimetre per size (omitted below 13 mm).","type":"integer"},"diameter_in":{"description":"Inside diameter in inches. Unit: in.","type":"number","x-unit":"in"},"note":{"description":"Rounding conventions and fit caveat.","type":"string"}}},"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)","method":"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).","sources":[{"name":"Ring size – US, UK, ISO 8653 and Japanese scales with comparison table (Wikipedia)","url":"https://en.wikipedia.org/wiki/Ring_size","type":"reference","retrieved_at":"2026-09-24"}],"freshness":{"type":"static","max_age_seconds":null,"note":"Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output."},"examples":[{"name":"US 7","inputs":{"value":"7","system":"us"},"expected":{"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},"url":"https://tttkmbb.com/api/v1/calculate/ring-size?value=7&system=us"},{"name":"18 mm inner diameter","inputs":{"value":"18","system":"diameter_mm"},"expected":{"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},"url":"https://tttkmbb.com/api/v1/calculate/ring-size?value=18&system=diameter_mm"}],"faq":[{"q":"How do I measure my finger?","a":"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."},{"q":"Does band width change the size?","a":"Yes. Bands wider than about 6 mm feel tighter; jewellers usually recommend going up a quarter to a half US size for wide bands."},{"q":"Why does the UK letter not match some charts exactly?","a":"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."}],"tags":["ring size converter","ring size chart","us to uk ring size","ring size in mm","iso 8653 ring size"],"related":[{"calculator_id":"shoe-size","reason":"Shoe sizes between US, UK, EU and cm."},{"calculator_id":"circle","reason":"Circumference and diameter of any circle."}],"links":{"html":"https://tttkmbb.com/everyday/ring-size","markdown":"https://tttkmbb.com/everyday/ring-size.md","json":"https://tttkmbb.com/everyday/ring-size.json","api":"https://tttkmbb.com/api/v1/calculate/ring-size","schema":"https://tttkmbb.com/api/v1/calculators/ring-size","openapi":"https://tttkmbb.com/openapi.json","mcp":"https://tttkmbb.com/mcp"},"version":"v1","updated_at":"2026-09-24"},"timestamp":"2026-09-24T03:45:01Z","next_actions":[{"tool":"run_calculator","calculator_id":"ring-size","reason":"Run Ring Size Converter with the inputs above."}],"links":{"markdown":"https://tttkmbb.com/everyday/ring-size.md"}}