Calibrated, not assumed
Three calibration methods, each with its own stated accuracy. The tool refuses to draw a millimetre it cannot justify — auto-detect declines on unknown hardware rather than guessing.
Ring sizer · US · UK · EU · India · Japan
Find your ring size on a screen that has been calibrated first.
Every other on-screen sizer draws a circle and calls it a millimetre. A pixel is not a millimetre. Calibrate your display against your device, its size, or any bank card — then lay your ring on the glass and read the answer.
- ±0.15 mm
- Card calibrated
- 3
- Calibration methods
- 5
- Sizing standards
- 0
- Sign-ups or uploads
On-screen sizer
Match your ring
Not calibrated
The circle below is not yet a real millimetre
Hold any bank or ID card flat against the screen and drag until the outline disappears behind it. Every card is 85.60 mm wide by international standard, so this works on any device.
The outline is as wide as this screen allows. If your card is still larger than it, this screen is too narrow to calibrate against a card — use instead.
Checking your device…
· Not listed? Use .
Enter your display's diagonal and we compute its density from the native resolution.
Check it — is this bar exactly one card wide?
85.60 mm — ISO ID-1 card width
Drag the lines, or use − and +
This circle is not a real millimetre yet
Calibrate your screen once and it becomes exact — about twenty seconds with a bank card.
13.0 mm24.0 mm
Your ring size
US—
- UK / AU
- —
- EU / ISO
- —
- India
- —
- Japan
- —
- Diameter
- —
- Circumf.
- —
Band width
Guessing the width costs you up to a full size..
Why this one is different
Most on-screen ring sizers cannot be right, and quietly admit it in the small print.
Proof you can hold
After calibrating, the page draws an 85.60 mm bar. Hold your card against it. If they match, every measurement that follows is sound. No other sizer lets you check its own ruler.
Presets, not fiddling
Preset mode draws every size at once at its true diameter, the way a printed sizer works. Lay your ring across them and tap the circle that vanishes underneath it — no dragging anything until it looks right.
The band is measured too
A wide band needs up to a full size more. Everywhere else you pick that from a dropdown; here you measure the band on the same calibrated stage and the allowance follows from it.
Honest error bars
Every result carries the tolerance of the method that produced it, in millimetres and in US sizes, and the error budget behind each one is published. A number without an error bar is a number you cannot act on.
Calibrate once
Your screen calibration is remembered, so the second visit starts at the measurement rather than the setup.
The three calibration methods
Pick the one that suits your device. Each states what it can and cannot do.
- 01±0.3 mm
Auto-detect device
Your screen resolution and pixel ratio are matched against a table of known phones and tablets with documented pixel densities. Instant, and nothing to type.
Declines on desktops and unrecognised hardware instead of guessing.
- 02±0.5 mm
Screen size
Enter your display diagonal in inches. Combined with the native resolution the browser reports, that gives the pixel density directly.
Only as accurate as the diagonal you enter — check the spec sheet.
- 03±0.15 mm
Card calibration
Resize an on-screen rectangle until it matches a real bank or ID card. Every such card is 85.60 mm wide by international standard, anywhere in the world.
Works on any device, assumes nothing. The recommended method.
Each tolerance above is an error budget, not a slogan.See how ±0.15 mm, ±0.3 mm and ±0.5 mm are derived.
Quick reference
Common ring sizes
| Ø mm | Circ. mm | US / CA | UK / AU | EU / ISO | India | Japan |
|---|---|---|---|---|---|---|
| 14.9 | 46.8 | 4 | H | 47 | 7 | #6 |
| 15.7 | 49.3 | 5 | J | 49 | 9 | #9 |
| 16.5 | 51.9 | 6 | L | 52 | 12 | #11 |
| 17.3 | 54.4 | 7 | N | 54 | 14 | #14 |
| 18.1 | 57.0 | 8 | P | 57 | 17 | #16 |
| 18.9 | 59.5 | 9 | R | 60 | 19 | #18 |
| 19.8 | 62.1 | 10 | T | 62 | 22 | #21 |
| 20.6 | 64.6 | 11 | V | 65 | 25 | #23 |
About the tool
A ring size calculator you can check against something real
What this ring size calculator does
This ring size calculator measures your ring size on screen, in about a minute, without a tape measure or a trip to a jeweller. It works in five standards at once — US and Canada, UK and Australia, EU/ISO, India and Japan — so a single measurement answers the question wherever you are ordering from. Everything runs in your browser: nothing is uploaded, no account is needed, and the tool is free.
How to find your ring size on this page
On-screen sizing only works if the screen is calibrated first, because a CSS pixel has no fixed physical width. So start with calibration: hold a bank card against the on-screen rectangle and drag until the two match, pick your device from auto-detect, or type in your display diagonal. Card calibration lands within roughly ±0.15 mm — here is where that figure comes from.
Then lay a ring that already fits flat on the glass and match its inside edge to the circle — or use preset mode, which draws every size at once at true scale, the way a printed sizer does. The answer appears as a US number alongside its UK letter, EU circumference, and Indian and Japanese equivalents.
How to measure ring size at home without a screen
If you would rather not measure on glass, three other methods are reliable, and the how to measure ring size at home guide walks through each. Measuring the inside diameter of an existing ring with a caliper is the most accurate of all. A printable ring size chart is next: print the sheet at exactly 100% scale, check the printed calibration bar against a bank card, then lay your ring over the circles. The paper strip method — wrap a 6 mm strip around the base of the finger, mark the overlap, measure to the mark — gives a circumference you can look up directly, and is the usual answer to how to determine ring size with nothing else to hand.
How to read a ring size chart
A ring size chart converts one inside measurement into every national system. US sizes follow a straight line: circumference in millimetres equals 36.537 plus 2.5535 × the US size, so each whole size adds 2.55 mm of circumference, or 0.81 mm of diameter. The US ring size chart runs from about size 2 to size 15 in quarter steps. UK letters run A to Z and beyond in half-letter steps, one letter being roughly half a US size, and EU/ISO sizes are simply the circumference rounded to whole millimetres.
The men’s ring size chart is the same table read higher up — most men order between US 8 and 12, with 10 the single most common size. The average ring size for women is about US 6 to 6.5, a useful starting point for a surprise but no substitute for a real measurement.
Band width, timing and which way to round
Two things change the size you should actually order. Band width comes first: a wide band presses against more of the finger and wears tighter, so add half a size from about 5 mm of width and a full size from about 9 mm. The calculator measures the band on the same calibrated stage and applies that allowance for you.
Timing comes second. Fingers are smallest when cold and first thing in the morning, and swell in heat and after exercise, so measure in the late afternoon at room temperature — a ring sized at your largest fits all day rather than only part of it. If you fall between two sizes, go up: a loose ring can be resized or fitted with a sizing bead, while one that will not pass the knuckle cannot be worn at all. Every result here carries the tolerance of the method behind it, in millimetres and in US sizes, so you know how much weight to put on it before you order.
Questions
Ring sizing, answered
Why does the screen need calibrating first?
A CSS pixel has no fixed physical size. The same circle is 40% smaller on a dense phone than on an old monitor, so any tool that draws a "17.3 mm" circle without calibrating is drawing a guess. Calibration establishes how many pixels cover one millimetre on your screen, which is what makes the rest of the measurement meaningful.
Which calibration method is most accurate?
Card calibration, at roughly ±0.15 mm. It compares the screen against a physical object of known size, so it assumes nothing about your hardware. Device auto-detect is about ±0.3 mm, and screen-size entry about ±0.5 mm because it depends on the diagonal you type in. The accuracy page sets out the error budget behind each figure.
Why is a credit card a good ruler?
Bank cards, debit cards, most national ID cards and many driving licences are all manufactured to ISO/IEC 7810 ID-1: 85.60 mm by 53.98 mm, everywhere in the world, to a tolerance far tighter than you can see. That makes one an accurate ruler you already carry.
Do I need to calibrate again next time?
No. Your calibration is stored in your browser and reused automatically. It is re-confirmed after 90 days, and you can recalibrate at any point from the banner above the sizer.
Is measuring a ring more accurate than measuring my finger?
Yes, clearly. A ring has one fixed inside diameter. A finger changes width through the day, flattens when pressed against glass, and has to pass a knuckle that is usually wider than the base. Measure an existing ring when you have one.
What if I am between two sizes?
Go up. A ring slightly too large can be resized or fitted with a sizing bead; one that will not pass the knuckle cannot be worn at all while you wait.
When during the day should I measure?
Late afternoon or evening, at room temperature. Fingers are at their smallest when cold and first thing in the morning, and swell in heat and after exercise. Measuring at your largest means the ring fits all day rather than only some of it.
Does the width of the band change my size?
It does. A wide band presses against more of your finger and feels tighter at the same nominal size. Add half a size from about 5 mm of width, and a full size from about 9 mm. The band-width control in the calculator applies this for you.
Can I use this for an Oura or Galaxy smart ring?
Use it to get your diameter, then check that against the manufacturer’s own chart, since smart rings use their own numbering. For a smart ring, go up rather than down if you are between sizes: a loose sensor gives bad data, but a ring you cannot get on gives none.
How accurate is this compared with a jeweller?
A card-calibrated measurement lands within about ±0.15 mm of inside diameter — roughly a fifth of a US size — which is enough to order online with confidence. A jeweller with a steel mandrel and a set of test rings is still the reference standard, and worth visiting for an expensive ring. The accuracy and methodology page shows how each tolerance is derived.
The site
Who runs this, and on what terms
About us
Why the sizer calibrates your screen first, and where every number on the site comes from.
Contact us
One inbox, read by a person. Corrections to a chart and bugs in the sizer answered first.
Privacy policy
No accounts, no cookies, no tracking. Your measurement never leaves your browser.
Terms & conditions
What the calculator may be used for, and the limits of a screen-based measurement.