Ring size converter
Enter the ring size you know and see it in every system: a US 7 is a UK O, an EU 54 and an inside diameter of about 17.35 mm, with the circumference worked out too. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Size
- 7
- Input scale of 'size'
- us
- Input size
- 7
- Input scale
- us
- Us
- 7
- Uk
- O
Ring size converter tools exist because engagement, wedding, and fashion jewellery is sold under at least four incompatible label systems. A shopper in New York sees a US seven, a London jeweller marks the same band with a British letter O, a continental catalogue prints a European fifty-four, and a mandrel or caliper reports an inside diameter near seventeen point three five millimetres. Ordering the wrong scale wastes shipping, resizing, and trust. This capability accepts one known size plus its scale, looks it up in a fixed deterministic conversion table covering common US half sizes from three through thirteen with matching UK letters, EU numeric labels, and inside diameters, and returns every equivalent plus the derived inside circumference.
How US, UK, EU and millimetre ring sizes relate
Physical fit is governed by the inside diameter of the band, or equivalently by the inside circumference around the finger. The US scale is a numeric sequence that rises in half-size steps for most adult catalogue ranges from three to thirteen, with each step corresponding to a fixed increment of diameter on standard jewellery charts. The UK scale uses letters, often with half sizes written as letter one half, following British practice closely related to historical BS tables and everyday high-street markings.
The EU or ISO-style numeric scale that shoppers meet on continental packaging is typically a rounded expression of inside circumference in millimetres, so a European fifty-four is in the same family as a band whose circumference is near fifty-four millimetres. Measuring with a ring sizer, a strip of paper, or digital callipers ultimately produces millimetres of diameter or circumference; converting that measurement into a labelled size is a table lookup, not a marketing claim.
This tool stores a fixed industry-aligned table of known combinations and derives circumference as pi times diameter so geometry stays consistent. It does not interpolate unofficial intermediate sizes, claim medical fit for swollen fingers, convert shoe or clothing sizes, or call external product databases. It answers the conversion intent people mean when they search for a ring size converter and expect US, UK, EU, and millimetre columns side by side for one known input.
Where ring size conversion shows up in real work
Online jewellery retailers and marketplace sellers list inventory with mixed US and EU tags and need one ground-truth row before shipping internationally so returns from wrong-scale orders fall. Wedding planners and proposal gift shops help clients translate a partner’s UK letter from a high-street sizer into a US web checkout without walking them through a spreadsheet. Manufacturing and CAD teams store mandrel diameter in millimetres while marketing copy must print local labels, and a single conversion call keeps both systems honest.
Customer support bots that answer “what is a US eight in EU?” can invoke this capability instead of hard-coding brittle charts that drift between documents. If you later need shoe size conversion, clothing size charts, general length unit conversion, or product search against a live catalogue, treat those as separate search intents and landings rather than overloading this ring size converter page.
What you can do with it
Cross-border jewellery checkout
Map a customer’s US or UK size to EU and millimetre labels before international shipping so the band that leaves the warehouse matches the finger that ordered it.
Mandrel and CAD to catalogue labels
Convert an inside diameter from a mandrel or design file into US, UK, and EU labels for product pages and packaging.
FAQ
Which ring size systems does this converter support?
US numeric (3–13 half steps), UK letter sizes (e.g. F, J1/2, O, Z+1), EU/ISO-style numeric sizes, and inside diameter in millimetres. Circumference is derived as π × diameter.
What happens if I enter an unknown size?
The table does not invent intermediate sizes; only known chart rows are returned so catalogue labels stay honest.
How precise is diameter matching?
Diameter inputs match a table row within about ±0.06 mm so common rounded values still resolve. US, UK, and EU inputs require an exact table value.
Does this measure my finger?
No. You must already know a size or diameter. Measuring a finger is a physical step; this capability only converts known labels among the supported scales.
What US size is a UK J½?
A UK J½ is a US 5. Enter the letter with its half and choose the UK system, and the converter returns the matching US, EU and millimetre values from the same row of its table.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/conv/ring-size \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"size":"7","scale":"us"}'const res = await fetch("https://api.kit.forhosting.com/conv/ring-size", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"size": "7",
"scale": "us"
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/conv/ring-size",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"size": "7",
"scale": "us"
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/conv/ring-size", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"size":"7","scale":"us"}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"size":"7","scale":"us"}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/conv/ring-size", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"size": "7",
"scale": "us"
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "conv.ring_size",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
Unknown sizes fail with a clear invalid_input error instead of inventing a near match.
What you send and what you get back
Send two fields: size and scale. Size is the value you already know: a number string such as seven or seventeen point three five for numeric scales, or a UK letter string such as O, J one half, or Z plus one. Optional alternate keys value or ring for the size and system, from, or unit for the scale are accepted for convenience. Classic checks include US seven mapping to UK O, EU fifty-four, and diameter near seventeen point three five millimetres; UK J one half mapping to US five; and diameter seventeen point three five millimetres resolving to the same row. Scale names the system of that value and accepts us, uk, eu, and mm, with aliases including usa, british, iso, diameter, and inside diameter. Missing fields, empty strings, non-finite numbers on numeric scales, and completely unknown combinations fail with invalid_input and are not charged; the error message points at supported ranges so clients can correct the label rather than guess. On success the response echoes input_size and input_scale, returns us, uk, eu, diameter_mm, and circumference_mm with stable twelve-significant-digit cleaning, nests the same four values under scales, and includes method table_lookup plus a short formula string. Pricing for successful API calls is the published base of $0.002 per item; validation failures do not consume prepaid balance.
Where ring size conversion shows up in real work
Data pipelines cleaning product feeds with fields like size_us, uk_size, and ring_mm can normalise every SKU through the same table and reject unknown codes early. Because this capability is a pure parse engine, API pricing is the published base of $0.002 per successful item and failed validation does not consume balance.
Integration use cases
Support macros and chatbots
Answer “what is a size 7 in UK and EU?” with one deterministic JSON row instead of copy-pasting from a PDF chart.
Technical questions
Is the browser tool free?
Yes. The tier-A browser widget runs locally at no charge. API requests use prepaid KIT balance at $0.002 per successful item; invalid input is not charged.