CM to ring size converter
A finger that measures 5.45 cm around is a US size 7, a UK N½ and an EU 54.5. Enter the circumference in centimetres and get all three sizes plus the inside diameter in millimetres, ready to compare with a printed sizer. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Circumference
- 5.45
- Circumference mm
- 54.5
- Diameter mm
- 17.347888797
- Ring size
- 7
- Us
- 7
A finger circumference in centimetres is the measurement most people can take at home with a paper strip or a flexible tape, yet jewellery counters and online size charts speak in US numbers, UK letters, and European millimetre sizes. This centimetre-to-ring-size converter accepts that single positive length, converts it to an inside diameter with the circle identity diameter equals circumference over pi, and maps the result onto the three charts shoppers actually meet: the ASTM-style US numeric scale, the British letter scale, and the ISO European size that is the inner circumference in millimetres. You get the matching ring size plus the working diameter and circumference in millimetres, ready to compare with a printed sizer or a brand chart.
How centimetres become a matching standard ring size
A ring size is not an arbitrary label: it is a name for the inside circumference of a circular band. Start with the finger measurement in centimetres and multiply by ten to work in millimetres, the unit every jewellery chart uses. The inside diameter then follows the circle identity diameter equals circumference divided by pi, so a 5.45 centimetre finger is 54.5 millimetres around and about 17.35 millimetres across.
The US numeric scale used as the primary standard ring size here is the ASTM jewellery map: size zero has an inside diameter of 11.63 millimetres and each full size adds 0.812 millimetres of diameter. Rearranged, US size equals diameter minus 11.63, all divided by 0.812, then rounded to the nearest half size so the answer lands on the 6, 6.5, 7, 7.5 steps that retailers stock.
The European ISO 8653 number is simply that same inner circumference in millimetres, rounded to the nearest half, which is why a 54.5 millimetre finger reads as EU 54.5. The British BS 6820 letter chart starts at A with a 37.8 millimetre circumference and climbs 1.25 millimetres per full letter; the solver reports the nearest half letter (O, O1/2, P) and writes sizes past Z as Z+1. Because the maps are linear and the constants are fixed, the same centimetre value always produces the same US, UK, and EU triple.
That is the whole algorithm: no network lookup, no random sizer, and no clock.
How to measure, what to send, and how errors are reported
Measure the intended finger at the end of the day when hands are warm, not first thing in the morning. Wrap a narrow paper strip or a millimetre tape around the base of the finger and across the knuckle the ring must pass, mark the overlap, and lay the strip on a ruler. Read centimetres to one decimal place if you can; half a millimetre already moves some people across a half size.
If the knuckle is much larger than the base, record the knuckle and expect a slightly loose fit at the base, or record both and convert each. A formula string restates the three maps so a page can show the arithmetic next to the answer. A typical adult measurement of 5.45 centimetres returns US 7, a UK letter near O, and an EU size near 54.5.
Where a centimetre-to-ring-size converter earns its keep
Online jewellery shops need a centimetre box on the size guide, not a lecture about US versus UK charts. A shopper who has just wrapped a strip of paper around a finger can type 5.2 or 6.1 and see the matching standard size before adding a ring to the cart, which cuts the returns that come from guessing between a 6 and a 7. Gift buyers who cannot borrow the recipient’s ring still can ask for a finger circumference in centimetres by text and convert it privately.
Bench jewellers and resizing counters can confirm a client’s home measurement against the ASTM and ISO maps without opening a printed wheel. Marketplaces that list the same SKU in US, UK, and EU columns can fill those columns from one centimetre input, so a listing stays consistent when the catalogue is generated rather than typed.
What you can do with it
Gift buying without the ring
Convert a centimetre measurement sent by the recipient into a standard size so a surprise ring can be ordered without borrowing an existing band.
Bench and marketplace listings
Fill US, UK, and EU columns from one home measurement, or confirm a client strip-measure against the ASTM and ISO maps at the counter.
FAQ
How do I convert a finger circumference in centimetres to a ring size?
Send a positive finite number in the circumference field (centimetres).
How should I measure finger circumference in centimetres?
Wrap a paper strip or a flexible tape around the intended finger and across the knuckle the ring must pass, mark the overlap, and read centimetres on a ruler. Measure at the end of a warm day for a typical fit.
Which standard is the matching ring size?
The same response also includes uk (BS 6820 letter) and eu (ISO 8653, inner circumference in millimetres).
What if my measurement falls between two sizes?
US and EU values round to the nearest half size; UK letters round to the nearest half letter. If the knuckle is much larger than the base, convert the knuckle measurement so the ring can pass, and expect a slightly looser fit at the base.
Why is the EU ring size the same as my circumference in millimetres?
Because the European ISO 8653 size is simply the inside circumference in millimetres, rounded to the nearest half. A finger of 5.45 cm is 54.5 mm around, so it reads as EU 54.5.
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/life/cm-to-ring-size-converter \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"circumference":5.45}'const res = await fetch("https://api.kit.forhosting.com/life/cm-to-ring-size-converter", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"circumference": 5.45
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/life/cm-to-ring-size-converter",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"circumference": 5.45
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/life/cm-to-ring-size-converter", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"circumference":5.45}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"circumference":5.45}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/life/cm-to-ring-size-converter", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"circumference": 5.45
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "life.cm_to_ring_size_converter",
"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
Zero, negative, missing, or non-finite values fail with invalid_input and are not billed.
How centimetres become a matching standard ring size
The primary field ring_size is the US number, because that is the chart English-language shoppers mean when they type cm to ring size, and the UK letter and EU number travel beside it so a UK or continental chart can be read without a second conversion.
How to measure, what to send, and how errors are reported
POST a JSON body with one required field, circumference, a positive finite number of centimetres. The solver also accepts the aliases cm, value, finger, and circ, and it reports errors against the canonical name so a form can mark the right control. On success the body echoes the cleaned circumference, returns circumference_mm and diameter_mm as the working geometry, and names the matching size as ring_size (US, nearest half) together with us, uk, and eu. Missing fields, empty strings, non-numeric tokens, NaN, Infinity, zero, and negative values all fail with invalid_input and a message that says the circumference must be a positive finite number; those failures are not billed. Successful API calls use the published base of $0.002 per item; the free browser path runs the same solve function without consuming prepaid balance.
Where a centimetre-to-ring-size converter earns its keep
Keep the circumference positive and finite, pin ring_size and the UK and EU companions in your tests, and you get a reproducible size for a product page, a gift checklist, a jeweller’s intake form, or an edge worker without standing up a jewellery-CAD stack for a one-number conversion.
Integration use cases
Product-page size guides
Let a shopper type a finger circumference in centimetres and show the matching US, UK, and EU ring size before checkout, using the same numbers the catalogue API stores.
Technical questions
What does the centimetre-to-ring-size converter cost?
The browser tool is free. API requests use prepaid balance at $0.002 per successful call; invalid input is not billed.