Is an on-screen ring sizer accurate?

Only after calibration. Screens have very different pixel densities, so a circle drawn at "17 mm" without calibration can be off by 30% or more. Matching a bank card first tells the page exactly how many pixels make a millimetre on your screen. After that, it is as accurate as your matching: usually within half a size.

Credit, debit and most ID cards are made to one international standard (ISO/IEC 7810 ID-1): 85.60 × 53.98 mm. Almost everyone has one within reach, so it makes a precise ruler that needs no printing.

Does it work on iPhone and Android?

Yes. Calibrate on the device you are using, because the result is different for every screen. If you zoom the page in or out afterwards, calibrate again. The page warns you when that happens.

Can I measure my finger on screen?

Not directly: a finger is soft and not flat. This tool matches a ring you already own. To measure a finger, use the printable strip, or wrap a strip of paper and enter the length in the ring size converter.

Is my card number used or seen?

No. You only hold the card against the glass to compare its size. Nothing is photographed, typed or sent. The page has no camera access at all.

Actual-size ring sizer on your screen

Match a ring you already own to an actual-size circle on your screen. One minute of calibration with a bank card makes the circles true to size on any phone, tablet or monitor.

Runs in your browser. Nothing is uploaded.

Step 1

Set your screen to true size

Not calibrated

Hold any bank card or ID card flat against the screen, inside the box. Make the box exactly as wide as the card's short side.

Step 2

Match a ring you own

Lay the ring flat on the circle. Adjust until the circle's outer edge meets the inside edge of the ring all the way round, with no screen showing in between.

US & Canada: –
UK & Australia: –
EU / ISO: –
Diameter (mm): –
Italy, Spain, Swiss: –
Japan & China: –
Inside diameter / circumference: –

Calibrate first (step 1). Until then, circles are drawn at a guessed size and may be wrong.

Actual-size ring chart

Every half size from US 3 to 13, at true size once calibrated.

How the on-screen ring sizer works

A screen has no idea how big it is. A web page asks for "17 millimetres" and gets a number of pixels that depends on the device: on one laptop that is 17 mm, on a phone it may be 12 mm, on a large monitor 21 mm. That is why most "actual size" ring charts online are not actual size at all.

Calibration fixes it. You match something whose size is known exactly. A bank card is 85.60 × 53.98 mm on every card in the world, so when the box matches your card, the page knows how many pixels make one millimetre on your screen. Every circle after that is drawn from the same ring size chart the rest of this site uses.

Getting an accurate match

Look straight down at the screen, not from an angle. Parallax makes the ring look smaller than the circle.
Match the inside edge of the ring, not the outside. The metal's thickness is not part of your size.
Use a ring that fits the same finger, and the same hand. Your dominant hand is often slightly larger.
Between two sizes? Choose the larger. A ring you can turn is fine; one stuck at the knuckle is not.
Wide bands (6 mm and up) fit tighter. Many jewellers suggest a quarter to half size up.

No ring to match?

Use the printable ring sizer: its finger strip measures your finger directly, and it checks your printer the same way this page checks your screen. Or wrap a thin strip of paper around your finger, mark where it overlaps, and enter the length in the ring size converter.

Common questions

Is an on-screen ring sizer accurate?: Only after calibration. Screens have very different pixel densities, so a circle drawn at "17 mm" without calibration can be off by 30% or more. Matching a bank card first tells the page exactly how many pixels make a millimetre on your screen. After that, it is as accurate as your matching: usually within half a size.
Why a bank card?: Credit, debit and most ID cards are made to one international standard (ISO/IEC 7810 ID-1): 85.60 × 53.98 mm. Almost everyone has one within reach, so it makes a precise ruler that needs no printing.
Does it work on iPhone and Android?: Yes. Calibrate on the device you are using, because the result is different for every screen. If you zoom the page in or out afterwards, calibrate again. The page warns you when that happens.
Can I measure my finger on screen?: Not directly: a finger is soft and not flat. This tool matches a ring you already own. To measure a finger, use the printable strip, or wrap a strip of paper and enter the length in the ring size converter.
Is my card number used or seen?: No. You only hold the card against the glass to compare its size. Nothing is photographed, typed or sent. The page has no camera access at all.