Ring Size Calculator with Circumference and Diameter

Convert a millimetre measurement into a US, UK or EU ring size, then check it against the factors that change how a ring actually fits — knuckle clearance, band width, and the time of day you measured.

This ring size estimator works from one measurement in millimetres. A ring size is not a property of a person: it is a label attached to one of two physical measurements, the inside diameter of the band or the inside circumference. Every system in use — the numeric US and Canadian scale, the British letter scale shared with Australia and Ireland, and the ISO 8653 circumference sizing used across continental Europe — describes one of those two figures under a different name. Once you have a measurement in millimetres, you have every size at once.

That is worth knowing because the arithmetic is the easy part. A half size is about 1.25 mm of circumference, which is a small enough distance that ordinary measuring errors move the answer. A finger measured on waking and the same finger measured in the evening commonly differ by around half a size, and heat, salt and exercise widen the gap further. The calculator converts whatever you give it faithfully; the guides below cover how to give it something worth converting.

How to use this calculator

You need one measurement in millimetres, and it can be either of two things. Circumference is the distance around your finger — what you get from wrapping a strip of paper around it. Inside diameter is the distance straight across the inside of a ring you already own. Enter whichever you have; the calculator handles the rest.

If you enter both, the tool uses whichever field you edited last, and tells you if the two disagree by more than about 1.6 mm — which almost always means one of them was measured wrong rather than that your finger is unusual.

The two numbers are the same measurement

Circumference is diameter multiplied by pi. A 17.3 mm inside diameter is a 54.4 mm circumference — the same ring described two ways. Every sizing system in the world is a label stuck on one of those two figures, which is why the millimetre columns in the chart below never change even though the size names do.

The full ring size chart

US and Canadian sizes are numbers. The UK, Ireland, Australia and New Zealand use letters. Most of continental Europe uses the ISO size, which is simply the inside circumference in millimetres. Find any one value and read across.

Ring size conversion chart. Inside diameter and circumference are the physical measurements; every regional size is a label for one of them.
US / Canada	UK / AU / IE	EU / ISO	Inside diameter	Inside circumference
3	F	44	14.1 mm	44.2 mm
3.5	G	45–46	14.5 mm	45.5 mm
4	H	47	14.9 mm	46.8 mm
4.5	I	48	15.3 mm	48.0 mm
5	J½	49	15.7 mm	49.3 mm
5.5	K½	50–51	16.1 mm	50.6 mm
6	L½	52	16.5 mm	51.9 mm
6.5	M½	53	16.9 mm	53.1 mm
7	N½	54	17.3 mm	54.4 mm
7.5	O½	56	17.7 mm	55.7 mm
8	P½	57	18.1 mm	57.0 mm
8.5	Q½	58	18.5 mm	58.2 mm
9	R½	59–60	18.9 mm	59.5 mm
9.5	S½	61	19.4 mm	60.8 mm
10	T½	62	19.8 mm	62.1 mm
10.5	U½	63	20.2 mm	63.4 mm
11	V½	64–65	20.6 mm	64.6 mm
11.5	W½	66	21.0 mm	65.9 mm
12	X½	67	21.4 mm	67.2 mm
12.5	Y½	68–69	21.8 mm	68.5 mm
13	Z½	70	22.2 mm	69.7 mm

Where the EU column shows a range, the circumference falls on a half-millimetre and retailers genuinely stock either neighbouring size. Go by the millimetre figure rather than the label when you have the choice.

Three things that make the answer wrong

A ring size calculator is only as good as the measurement you feed it. In practice three factors account for most bad fits, and all three are avoidable.

You measured at the wrong time of day Fingers are smallest in the early morning and largest in the late afternoon and evening. Across a normal day the difference is commonly around half a size, and heat, salt, exercise and long-haul flights widen it further. Measure at the end of the day, at normal room temperature, and measure more than once. When to measure your finger
You measured the base of the finger but not the knuckle A ring has to pass over the knuckle to get on and off. If your knuckle is noticeably wider than the base of your finger, the base measurement will give you a ring you cannot get past it. Measure both and use the larger, then check the ring will still sit reasonably at the base. Other measuring mistakes
You ignored how wide the band is A wide band covers more of your finger and grips more of it, so it wears tighter than a slim band of the same nominal size. From roughly 6 mm of width upward, most jewellers recommend going up half a size. Sizing wide bands

What you can use this calculator for

People arrive here with four different jobs. The tool handles all of them from the same two input fields.

Buying a ring online when you have never been sized

Measure your finger with a paper strip, enter the circumference, and order against the result. This is the most common case, and the one where the measurement matters more than the arithmetic. How to take the measurement

Working out the size of a ring you already own

Measure straight across the inside of the band and enter the diameter. Useful for insuring, resizing, replacing a lost ring, or checking what size a hand-me-down actually is. How to read inside diameter

Converting a size quoted in another country's system

A UK jeweller quotes letters, a German retailer quotes a number in the fifties, a US site quotes a number under 15. Find the millimetre figure for the size you know and read across to the one you need. How each system works

Sizing a ring for someone else

Borrow a ring they already wear on the right finger, measure it, and enter the diameter. This is the most reliable way to size a surprise, and it takes a few minutes. Discreet sizing methods, ranked

How this compares to other ways of getting sized

A calculator is not the only option, and it is not the most reliable one. Here is where each approach sits.

Ways to establish a ring size, and what each one requires.
Approach	Relative reliability	What you need	Tests knuckle clearance?
Jeweller's metal sizing rings	Highest	A visit to a shop	Yes
This calculator, from a measured ring	High	A ring that fits, and a ruler	No
Plastic strip sizer	High	A sizer, often free from a jeweller	Yes
This calculator, from a paper strip	Moderate	Paper and a ruler	No, unless you check separately
Printable paper sizer	Moderate, and only if print scale is verified	A printer, and a bank card to calibrate	No
Uncalibrated on-screen sizer	Not meaningful	Nothing, which is the problem	No

The reliability column reflects how much error each approach typically introduces, not a measured score. The pattern behind it is straightforward: approaches that put a physical object on your finger beat approaches that measure it indirectly, and anything that has not been calibrated against a known physical size is not measuring anything at all. Why uncalibrated screen sizers fail

If you would rather browse everything at once, the full index of ring sizing guides groups every page on the site by what you are trying to do.

Which measuring method should you use?

Not all the popular home methods are equally good. Ranked by how much error they typically introduce:

Measuring a ring that already fits — most reliable at home: You are measuring a rigid object with a fixed size, so there is no tension error and nothing to squash. How to measure a ring's diameter
A plastic strip sizer — reliable, and cheap: A reusable sizer behaves like a real ring, including passing over the knuckle. Many jewellers will post one free.
A paper strip around the finger — good enough for most purchases: Paper does not stretch, which is what makes it acceptable. How to measure circumference
String or thread — the least reliable, use only as a last resort: String stretches under tension and compresses the skin, and both errors push in the same direction. Why string is the least reliable option

How accurate is this?

Ring sizing is not governed by a single enforced standard. Published charts differ between manufacturers by up to roughly a quarter of a size, and a home measurement adds error of its own. Treat every figure here as a close estimate. For a ring that cannot easily be resized, have the finger measured on a metal ring sizer by a jeweller before you order.

Ring sizing questions

What is the most common ring size?

For women, US 6 and 7 are the sizes ordered most often, and jewellers commonly keep US 7 as a sample size. For men, US 9 and 10 are the equivalent, with US 10 the usual sample. These are the middle of a wide distribution, not a safe guess for a specific person — plenty of people sit two or three sizes either side.

Are men's and women's ring sizes different scales?

No. A US 7 is a US 7 on anybody. The scale measures the finger, not the wearer. The only real difference is where each group tends to fall on it, and the fact that rings marketed to men are often wider — which does change the size you need.

Should I size up or down if I am between two sizes?

Usually up. A slightly loose ring can be worn today and sized down later; a ring that will not pass your knuckle cannot be worn at all. The exception is a ring so loose it could spin off. Working through the between-sizes decision.

Can I measure my ring size with a printed paper chart?

Only if you verify the print scale first. Browsers and printers routinely rescale a page by a few percent. Because a full US size is only about 2.5 mm on a circumference of roughly 54 mm, a scaling error of about 4% moves you a whole size. Any printable chart worth using includes a calibration bar — measure it with a ruler, or check a bank card against the card outline, before you trust anything else on the page.

How much does resizing a ring cost?

It depends far more on the ring than on the number of sizes. A plain metal band is straightforward for a jeweller to alter. A ring with stones set all the way round, or one made in tungsten, titanium or ceramic, may not be resizable at all. Check before you buy, not after. What can and cannot be resized.

Does this calculator store my measurement?

When you press the button, the site records the measurement, the resulting size and whether the two input fields agreed. It does not record your name, your email or anything that identifies you, and there are no advertising trackers on the site. What is recorded, in full.

How to check this site is worth trusting

Ring sizing figures get copied between websites without anyone verifying them, so an error introduced once spreads quietly. Rather than ask you to take these numbers on faith, here is what stands behind them — each of which you can check.

The figures are generated, not transcribed: Every size on every page comes from one reference table, and circumference is calculated from diameter rather than copied. That is why the two millimetre columns always reconcile — multiply any diameter here by 3.1416 and check.
278 automated checks run against the data: They confirm diameter and circumference reconcile through pi, that each UK letter matches its own published circumference to within 0.05 mm, that EU/ISO values round correctly, and that the data sent to your browser is identical to the data used to print the charts.
The limits are stated, not buried: Ring sizing has no single enforced global standard and published charts differ between manufacturers. Every page carrying a number says so, and the site tells you when a jeweller will beat it. What this site cannot tell you
Nobody pays to appear here: No advertising, no affiliate links, no sponsored placements. There is no account to create and nothing to buy. How the site is funded
A named person is accountable: The guides are maintained by Eleanor Whitfield, and corrections from jewellers and manufacturers are acted on. About the editor
Guidance last reviewed 22 August 2026: The review date appears in the page footer and in the structured data, so it cannot quietly drift out of step with the content.