Free ring size calculator
Find your ring size in two seconds. Measure your finger with a strip of paper, or measure a ring you already own, and enter the millimetres — the ring size calculator returns your US, UK, and EU size at once, with the full conversion chart and the inside diameter and circumference, updated live, as you type.
On this page15 sections
| US | UK | EU | Diameter (mm) |
|---|---|---|---|
| 3 | F | 44 | 14.1 |
| 4 | H | 47 | 14.9 |
| 5 | J | 49 | 15.7 |
| 6 | L | 52 | 16.5 |
| 7 | N | 54 | 17.3 |
| 8 | P | 57 | 18.1 |
| 9 | R | 59 | 18.9 |
| 10 | T | 62 | 19.7 |
| 11 | V | 65 | 20.5 |
| 12 | X | 67 | 21.4 |
| 13 | Z | 70 | 22.2 |
Conversions follow standard US / ISO 8653 EU sizing; UK letters can vary by up to half a size. See wide-band sizing
Results are estimates. Consult a professional.
What is your ring size?
Your ring size is simply a label for the inside size of a ring — the inside diameter, or equivalently the inside circumference, of the band that has to slide over your knuckle and sit comfortably at the base of your finger. Every sizing system in the world describes that one physical measurement; they just label it differently. This ring size calculator takes a measurement in millimetres and returns your size in all the common systems at once.
The four systems you will meet most often are US (a number, used in the US and Canada), UK / Australia (a letter), EU (the inside circumference in whole millimetres, per ISO 8653), and Japan (a number on its own scale). A US 7 is a UK N and an EU 54 — three labels for the same ring.
How to calculate ring size from a measurement in mm
If you already have a finger measurement in millimetres — from a string, a paper strip, or a ring you own — the conversion is straightforward arithmetic. The US size comes from the inside circumference, and the EU size simply is the circumference rounded to a whole number.
So a finger that measures 54.4 mm around the inside works out to (54.4 − 36.5) ÷ 2.55 ≈ 7.0, a US size 7. If you measured the diameter instead — say 17.3 mm — first turn it into a circumference with π × 17.3 ≈ 54.4 mm, then run the same step. The calculator above does both conversions for you and snaps the result to the nearest standard size.
How to measure your ring size at home (string or paper method)
No ring sizer to hand? You can get a good measurement with a strip of paper or a piece of string and a millimetre ruler. Paper is better than string because string stretches.
- Cut a thin strip of paper (or a length of non-stretchy string) about 10 cm long and a few millimetres wide.
- Wrap it snugly around the base of the finger you'll wear the ring on — snug, not tight, and make sure it can still slide over your knuckle.
- Mark where the end overlaps with a pen, exactly where the strip meets itself.
- Lay the strip flat and measure to the mark in millimetres. That length is your inside circumference.
- Enter that millimetre figure into the calculator (circumference mode) to read your US, UK, and EU size.
A worked example using the ring size calculator
Sofia wraps a paper strip around the base of her ring finger, marks the overlap, and measures it flat: 54.4 mm. Here is exactly how the calculator turns that into a ring size.
Step 1 — Enter the circumference
Sofia leaves the calculator in circumference mode and enters 54.4 mm — the length she measured. (If she had measured across a ring she owns instead, she would switch to diameter mode and enter that width.)
Step 2 — The calculator finds the US size
It applies the standard relation: (54.4 − 36.5) ÷ 2.55 ≈ 7.0, which snaps to a US size 7. The inside diameter is 54.4 ÷ π ≈ 17.3 mm.
Step 3 — Read every system at once
From the conversion chart, a US 7 is a UK N and an EU 54. Sofia now knows her size in whichever system the jeweller uses.
Ring size chart: US, UK & EU conversion
The chart below converts between US, UK, and EU ring sizes and the inside diameter and circumference in millimetres. Because every system measures the same band, you can read across any row to convert. The EU size is the circumference in millimetres (ISO 8653); the UK column is sourced from the standard international conversion chart.
| US | UK | EU | Inside diameter (mm) | Inside circumference (mm) |
|---|---|---|---|---|
| 4 | H | 47 | 14.9 | 46.7 |
| 5 | J | 49 | 15.7 | 49.3 |
| 6 | L | 52 | 16.5 | 51.8 |
| 7 | N | 54 | 17.3 | 54.4 |
| 8 | P | 57 | 18.1 | 56.9 |
| 9 | R | 59 | 18.9 | 59.5 |
| 10 | T | 62 | 19.7 | 62.0 |
| 11 | V | 65 | 20.5 | 64.6 |
| 12 | X | 67 | 21.4 | 67.1 |
| 13 | Z | 70 | 22.2 | 69.7 |
US whole sizes with the nearest UK letter and EU (ISO 8653) size. The calculator above also handles every half size in between. UK conversions vary by up to half a size between jewellers.
Inside diameter vs. circumference — which should you measure?
Both work — they are two ways of describing the same circle, linked by circumference = π × diameter. The right one to measure depends on what you have in hand.
- Measure the circumference when you are sizing the finger itself — wrapping a paper strip or string around it gives a circumference directly.
- Measure the diameter when you have a ring that already fits — laying it over a ruler and reading straight across the inside is fast and accurate.
Tips for an accurate ring size measurement
Fingers change size through the day and with the weather, so a few habits make the measurement far more reliable:
- Measure when your hands are warm and at the end of the day, when fingers are at their largest — a ring sized to a cold morning finger will feel tight by evening.
- Measure two or three times and take the consistent reading; fingers swell and shrink, so one measurement can mislead.
- Don't measure when hands are cold, swollen, or just exercised — wait for a neutral moment.
- Account for the knuckle. If your knuckle is much wider than the base of your finger, size to slide over the knuckle, then split the difference so the ring doesn't spin.
- Use paper, not string, in a pinch — string stretches and reads large.
Sizing for wide bands
Band width changes the fit. A wide band covers more of your finger, so it sits tighter than a thin band of the same nominal size. The wider the ring, the more it grips.
- For a wide or chunky band (over ~4 mm), go up about half a size from your standard measurement for the same comfortable fit.
- For a very thin band (under ~1.3 mm), you can size down slightly, as it slips on more easily.
- For comfort-fit (domed inner) bands, many people find they can stay at — or drop a touch below — their standard size, since the rounded interior glides on.
Common ring size questions
What is the average ring size?
For women, US sizes 6 to 7 are the most common (around a 16.5–17.3 mm inside diameter); for men, US 9 to 10½ are typical (around 18.9–20.1 mm). These are just averages — your own measurement is what matters.
Can my ring size change over time?
Yes. Weight changes, pregnancy, temperature, time of day, and even what you ate can shift finger size by half a size or more. Heat swells fingers and cold shrinks them, which is why warm, end-of-day measurements are recommended — and why a well-fitting ring can sometimes be resized by a jeweller.
How do I measure a ring I already own?
Lay the ring flat and measure its inside diameter straight across the middle in millimetres, then enter that figure in diameter mode. This is usually the most accurate way to find a size without a professional sizer.
How this calculator works and sources
This ring size calculator converts a millimetre measurement into US, UK, and EU sizes using the standard relationships: the US linear scale (each size = 0.032 in of inside diameter), the ISO 8653 EU convention (size = inside circumference in mm), and circumference = π × diameter. UK letters are taken from the widely published international conversion chart; because charts disagree slightly, UK conversions can vary by up to half a size between jewellers. The maths runs in your browser — nothing you enter is sent anywhere.
ISO 8653:2016 — Jewellery — Ring sizes — Definition, measurement and designation.Frequently asked questions about the free ring size calculator
About this ring size calculator
This ring size calculator runs entirely in your browser. The measurement you enter never leaves your device — nothing is sent to a server, logged, or shared. It converts your millimetre measurement to US, UK, and EU sizes using the standard US scale, the ISO 8653 EU convention, and circumference = π × diameter, updating instantly as you type.
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