Find your exact ring size in seconds
Ring Sizer measures any ring or finger on a screen that calibrates itself to your iPhone, converts between six international sizing systems, and shows you how a ring looks on your own hand before you buy.
Free on iPhone and iPad · iOS 17.0 or later · Auto-calibrated to your device’s PPI
A ring sizer that knows what screen it is drawing on
Most on-screen sizers assume a generic pixel density and are wrong on most phones. Ring Sizer reads your device first, then measures.
Measure a ring on screen
Place a ring on the display and match the adjustable circle, or use two-point tap to mark opposite inner edges for a tighter reading.
Measure a finger, no ring needed
Slide the on-screen gauge until it sits snug at the base of the finger. No printing, no string, no household ruler.
Scan hands-free with the camera
Point the camera at a ring or a hand and let the app take the measurement for you.
Calibrated before it draws anything
Ring Sizer reads your device model and sets its scale from that screen’s known pixels-per-inch. Fine-tune with a coin or a credit card if you want to check it.
Six sizing systems, offline
Convert between US/CA, UK/AU, EU/ISO, Japan, China and Russia — and read the diameter and circumference in millimetres.
See it on your own hand
Photograph your hand once, then try on any of 30+ rings from the gallery with AI virtual try-on.
Measure, convert, browse and try on
Six screens from the App Store listing — the measuring tool, the international converter, the ring gallery and AI virtual try-on.
Three steps to a size you can order against
Let it calibrate itself
Open the app and it identifies your iPhone, then sets its on-screen scale from that model’s known pixel density. You’ll see the device name and PPI on screen. Want to check it? Hold a credit card against the fine-tune screen — every card is exactly 85.60 mm wide.
Measure a ring, or measure your finger
Place a ring you already own on the display and match the circle to its inner edge — the most accurate method there is, because a ring is rigid. No ring? Use the on-screen finger gauge, or scan with the camera.
Convert, save and try it on
Read your size in US, UK, EU/ISO, Japanese, Chinese or Russian, plus the diameter and circumference in millimetres. Save it with a date and a note, then see how a ring looks on your own hand.
Ring size converter
Convert between US, UK, EU/ISO, Japanese, Chinese and Russian sizes — and read the diameter and circumference in millimetres.
International ring size chart
The same conversion data Ring Sizer uses in the app, so a size you measure and a size you look up here will always agree.
| US / CA | UK / AU | EU / ISO | Japan | China | Russia | Diameter |
|---|---|---|---|---|---|---|
| 3 | F | 44.0 | 4 | 4 | 44.0 | 14.0 |
| 3.5 | G½ | 45.5 | 5 | 5 | 45.5 | 14.5 |
| 4 | H | 46.8 | 7 | 7 | 46.8 | 14.9 |
| 4.5 | I½ | 48.0 | 8 | 8 | 48.0 | 15.3 |
| 5 | J½ | 49.3 | 9 | 9 | 49.3 | 15.7 |
| 5.5 | K½ | 50.6 | 10 | 10 | 50.6 | 16.1 |
| 6 | L½ | 51.9 | 12 | 12 | 51.9 | 16.5 |
| 6.5 | M½ | 53.1 | 13 | 13 | 53.1 | 16.9 |
| 7 | N½ | 54.4 | 14 | 14 | 54.4 | 17.3 |
| 7.5 | O½ | 55.7 | 15 | 15 | 55.7 | 17.7 |
| 8 | P½ | 57.0 | 16 | 16 | 57.0 | 18.1 |
| 8.5 | Q½ | 58.3 | 17 | 17 | 58.3 | 18.6 |
| 9 | R½ | 59.5 | 18 | 18 | 59.5 | 18.9 |
| 9.5 | S½ | 60.8 | 19 | 19 | 60.8 | 19.4 |
| 10 | T½ | 62.1 | 20 | 20 | 62.1 | 19.8 |
| 10.5 | U½ | 63.4 | 22 | 22 | 63.4 | 20.2 |
| 11 | V½ | 64.6 | 23 | 23 | 64.6 | 20.6 |
| 11.5 | W½ | 65.9 | 24 | 24 | 65.9 | 21.0 |
| 12 | Y | 67.2 | 25 | 25 | 67.2 | 21.4 |
| 12.5 | Z | 68.5 | 26 | 26 | 68.5 | 21.8 |
| 13 | Z+1 | 69.7 | 27 | 27 | 69.7 | 22.2 |
Ring sizing guides
Sixteen step-by-step guides covering every way to find a ring size — and every way it usually goes wrong.
How to measure your ring size at home
Five ways to measure ring size at home, ranked by accuracy — plus the calibration step that decides whether any of them actually works.
Read the guideGuideRing size chart — US, UK, EU, Japan, China and Russia
The complete international ring size chart with millimetre diameters and circumferences, plus how to read it and which column your seller actually means.
Read the guideGuideRing size converter
Convert ring sizes between US, UK, EU/ISO, Japanese, Chinese and Russian systems instantly — plus the millimetre diameter every jeweller can work from.
Read the guideGuideHow to measure ring size without a ring sizer
The string and paper methods explained properly — including the error each one introduces, and the fix that makes a phone screen more accurate than either.
Read the guideGuideHow to find her ring size without her knowing
Eight ways to get an engagement ring size secretly, ranked by how accurate and how risky each one is — and what to order when you still are not sure.
Read the guideGuideHow to measure ring size using a ring you already own
The most accurate at-home method: measure the inner diameter of a ring that fits. Here is how to do it on screen, on paper, and with a caliper.
Read the guideGuideHow to measure your finger for a ring size
Measure a finger accurately when you have no ring to copy — the on-screen gauge method, the knuckle test, and when to take the reading.
Read the guideGuideVirtual ring try-on — see a ring on your own hand
How AI virtual try-on works, what it can and cannot tell you about a ring, and how to shoot a hand photo that produces a genuinely useful preview.
Read the guideGuideAverage ring size for women and men
The most common ring sizes for women and men, what the distribution looks like, and how to use an average safely when you cannot measure.
Read the guideGuidemm to ring size — diameter and circumference converted
Turn a millimetre measurement into a ring size. Full diameter and circumference chart, plus how to tell which of the two your number actually is.
Read the guideGuidePrintable ring sizers — and why they so often print wrong
How to use a printable ring sizer without the scaling error that ruins most of them, plus the credit-card check that verifies any printed or on-screen scale.
Read the guideGuideMen’s ring size chart and how to measure
The men’s ring size chart from US 8 to 13 with millimetres, why men’s bands usually need a size up, and how to measure a man’s finger properly.
Read the guideGuideWide band ring sizing — when to size up, and by how much
Wide bands fit tighter at the same size. Here is the adjustment table by band width, why it happens, and how comfort fit changes the answer.
Read the guideGuideRing sizing when your knuckle is bigger than your finger
Measure both the knuckle and the base, pick a size between the two, and stop the ring spinning — the complete fix for large-knuckle ring sizing.
Read the guideGuideEngagement ring size guide
How to get an engagement ring size right when you only get one attempt — secret measuring, what to order if you cannot, and the resize questions to ask first.
Read the guideGuideUS to UK ring size conversion
Convert US ring size numbers to UK letters and back. Full chart from US 3 to 13, plus the half-letter rule that trips most conversions up.
Read the guideReading on rings, fit and sizing
Twenty articles on the details that decide whether a ring fits — timing, temperature, knuckles, resizing and buying online.
The best time of day to measure your ring size
Your fingers change size by up to half a ring size between morning and night. Here is when to measure, and why the evening reading is the one to order against.
Read the articleAccuracyNine ring sizing mistakes that cost people a resize
The nine errors behind most badly fitting rings — from measuring the outside of a band to trusting an unscaled printout — and how to avoid each one.
Read the articleTechnologyHow screen calibration turns a phone into a real ring gauge
Why most on-screen ring sizers are wrong, what pixels-per-inch has to do with it, and how to verify any screen scale with a coin or a credit card.
Read the articleAccuracyString, paper, printout or app: every ring sizing method ranked
Six ways to measure a ring size compared on accuracy, cost and what each one actually gets wrong. The honest ranking, including where apps lose.
Read the articleConversionWhat ring size is 17 mm?
A 17 mm inner diameter is a US 6½. Here is the full decoder for every millimetre measurement between 14 and 23 mm, in every sizing system.
Read the articleProposalsA seven-day plan to get her ring size secretly
A day-by-day plan for finding an engagement ring size without giving anything away, with a fallback for each step that fails.
Read the article- Why the UK uses letters and Europe uses millimetres
- Can you resize a ring?
- Why your wedding band is not the same size as your engagement ring
- Cold hands, wrong size: how temperature changes ring fit
- How tight should a ring be?
- Ring size and pregnancy
- Does losing weight change your ring size?
- Buying a ring online: the sizing checklist
- Comfort fit vs standard fit — which size should you order?
- How to gift a ring when you cannot ask the size
- Your ring arrived and it does not fit. Now what?
- Ring size by hand shape and finger type
- How to photograph your hand for the best AI ring try-on
- Left hand vs right hand: why your ring sizes differ
Ring sizing questions, answered
How do I measure my ring size at home?
Measure the inner diameter of a ring that already fits and match it to a size chart — 17.3 mm is a US 7. If you have no ring, measure the circumference at the base of your finger instead. Either way, measure in the evening with warm hands, and make sure whatever scale you measure against is calibrated.
Is a ring sizer app actually accurate?
It depends entirely on calibration. A 17.3 mm circle is 313 pixels on a 460 PPI iPhone and 222 on a 326 PPI one, so an app that assumes a single generic screen density can be several sizes out. Ring Sizer reads your device model and sets its scale from that screen’s known PPI, and shows you which device and density it used. Measuring a ring you own, expect about ±¼ of a size.
How do I measure ring size without a ring sizer?
Wrap a non-stretchy paper strip around the base of your finger, mark the overlap and measure the marked length in millimetres — that is your inner circumference, and 54.4 mm is a US 7. Avoid string: it stretches, and two millimetres of stretch is most of a ring size.
What is the average ring size?
The average women’s ring size is a US 6 to 7 (16.5–17.3 mm diameter) and the average men’s is a US 9 to 10 (18.9–19.8 mm). Most women’s engagement rings sold fall between a US 5½ and a US 7. Use an average to sanity-check a measurement, not to replace one.
How do I convert a US ring size to UK or European?
A US 6 is a UK L½ and an EU/ISO 51.9; a US 7 is a UK N½ and an EU/ISO 54.4. European sizes are the ring’s inner circumference in millimetres, which is why they look so different from US numbers. Convert once, directly from your measurement — converting twice rounds twice.
What ring size is 17 mm?
A 17 mm inner diameter is a US 6½ — a UK M½, an EU/ISO 53.1 and a Japanese 13. If your 17 mm is a circumference rather than a diameter, it is not a ring measurement: ring circumferences run from about 44 mm to 70 mm.
How can I find out her ring size secretly?
Borrow a ring she already wears on the correct finger and measure its inner diameter — it takes under a minute and it is the only method that measures the actual target. If that is impossible, photograph the ring next to a credit card, or ask the person in her life who can keep a secret.
Do I need to print anything?
No. Printable sizers only work if they print at exactly 100% scale, and printer drivers routinely apply “fit to page” without telling you — a 2% scale error is roughly a full ring size. Ring Sizer sets its own scale from your device instead, so there is nothing to print and nothing to check.
Should I size up for a wide band?
Yes. A band 5 mm or wider generally needs a half size up, and 8 mm or wider often needs a full size, because a wide band grips more of the finger and has further to travel over the knuckle. Comfort-fit bands run the other way — about a quarter to a half size large.
What if my knuckle is bigger than my finger?
Measure both the knuckle and the base, then choose a size between the two, biased toward the knuckle — the ring has to pass the joint to be worn at all. If the gap is more than a full size, ask a jeweller about sizing beads or a hinged shank rather than compromising on the size.
When is the best time of day to measure?
The evening, at room temperature, with warm hands. Fingers are smallest first thing in the morning and after cold exposure — cold hands can read a full ring size small — and largest at the end of the day. Take three readings and use the largest.
Can I see a ring on my hand before I buy it?
Yes. Photograph your hand once — flat, fingers spread, in indirect daylight — and Ring Sizer’s AI virtual try-on will render any ring from the 30+ gallery onto it. It answers whether a setting suits your hand. It cannot measure your finger; nothing in a photograph can.
Which sizing systems does Ring Sizer support?
US/Canada, UK/Australia, EU/ISO, Japan, China and Russia, plus the inner diameter and circumference in millimetres. Millimetres are the value to send a jeweller if you are ordering somewhere whose sizing you do not recognise — every jeweller in the world can work from them.
Is Ring Sizer free?
Ring Sizer is free to download from the App Store and includes optional in-app purchases. It runs on iPhone, iPad and iPod touch with iOS 17.0 or later.