{"openapi":"3.1.0","info":{"title":"Size Conversion API","version":"1.0.0","description":"International size conversion as an API, for the everyday wearables that ship in different scales around the world. The shoe endpoint converts a shoe size between EU (Paris point), UK, US men, US women, Mondopoint and foot length in centimetres — derived from the standard barleycorn and Paris-point relationships — and reports both the exact figure and the nearest standard half-size. The ring endpoint converts a ring size between US numeric, ISO 8653 (inside circumference in millimetres, used across most of Europe), inside diameter, inside circumference and the UK alphabetical scale. The hat endpoint converts a hat size between head circumference (centimetres or inches), the EU metric size, US and UK, with a simple S/M/L band. Everything is computed locally and deterministically, so it is instant and private. Ideal for fashion and footwear e-commerce, jewellery shops, size-guide widgets, returns-reduction tooling and international checkout. Pure local computation — no key, no third-party service, instant. Live, nothing stored. Guidance only — manufacturers' sizing varies, so always check the brand's own chart for a critical fit. 3 endpoints. This is wearable-size conversion; for plain length/weight unit conversion use a unit-conversion API.","contact":{"name":"PremiumApi","url":"https://www.oanor.com/by/premiumapi"}},"servers":[{"url":"https://api.oanor.com/sizes-api","description":"oanor gateway"}],"tags":[{"name":"Sizes"},{"name":"Meta"}],"components":{"securitySchemes":{"oanorKey":{"type":"apiKey","in":"header","name":"x-oanor-key","description":"Get your key at https://www.oanor.com/developer/keys"}}},"security":[{"oanorKey":[]}],"paths":{"/v1/hat":{"get":{"operationId":"get_v1_hat","tags":["Sizes"],"summary":"Convert a hat size","description":"","parameters":[{"name":"value","in":"query","required":true,"description":"Head circumference or size","schema":{"type":"string"},"example":"57"},{"name":"from","in":"query","required":true,"description":"cm|in|us|uk","schema":{"type":"string"},"example":"cm"}],"security":[{"oanorKey":[]}],"responses":{"200":{"description":"OK","content":{"application/json":{"example":{"data":{"note":"US hat size = head circumference (inches) ÷ π in 1/8 steps; EU = head circumference in cm.","input":{"from":"cm","value":57},"sizes":{"eu":57,"uk":7,"us":7.125,"band":"M","head_circumference_cm":57,"head_circumference_in":22.44}},"meta":{"timestamp":"2026-06-03T17:42:01.234Z","request_id":"4b32f7bf-a80e-4461-87e8-e23286e7a288"},"status":"ok","message":"Convert a hat size","success":true}}}},"401":{"description":"Missing or invalid x-oanor-key header"},"402":{"description":"Active subscription required"},"429":{"description":"Rate-limit or monthly quota reached"},"502":{"description":"Upstream did not respond"}}}},"/v1/ring":{"get":{"operationId":"get_v1_ring","tags":["Sizes"],"summary":"Convert a ring size","description":"","parameters":[{"name":"value","in":"query","required":true,"description":"Size, circumference, diameter or UK letter","schema":{"type":"string"},"example":"7"},{"name":"from","in":"query","required":true,"description":"us|iso|eu|circumference|diameter|uk","schema":{"type":"string"},"example":"us"}],"security":[{"oanorKey":[]}],"responses":{"200":{"description":"OK","content":{"application/json":{"example":{"data":{"note":"US numeric uses ID = 11.63 + 0.8128·size mm; ISO/EU = inside circumference mm (ISO 8653). 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