Ring sizer: measure a ring on your screen

How to use the ring sizer

1. Set the page zoom to its normal size and keep it there: zooming changes the size of everything on the screen and undoes the calibration.
2. Hold a bank card flat against the screen with its long side across. Put a corner into the top left corner of the box and move the slider until the box is exactly as tall as the card. Use the Smaller and Larger buttons for the last pixel, then press “The card matches”.
3. Lay a ring that fits the finger well on the disc. Move the slider until the disc fills the inside of the ring up to its inner edge, with no gap and no disc showing on the ring itself.
4. Press “Show ring size”. The result gives the inner diameter, the inner circumference and the US, ISO and Japanese size, and how far one pixel of your screen moves the result.

Nothing is stored: the sizer asks for the card again on every visit. It needs JavaScript; the ring size chart works without it.

Where the scale and the sizes come from

A web page cannot know how large its pixels are. CSS defines an inch as 96 CSS pixels, but on screens that inch is tied to a reference pixel, not to a physical inch, so it differs from device to device. The sizer therefore starts at the CSS inch and asks for something of known size.

Bank cards follow ISO/IEC 7810 format ID-1: 85.60 × 53.98 mm. The sizer uses the short side, 53.98 mm, which fits on the narrowest phone screen. When the box matches the card, the box height in pixels divided by 53.98 is the number of screen pixels per millimetre.

The disc is drawn at that scale. Its diameter is the inner diameter of the ring. The sizes use the same rules as the ring size chart: the ISO size is the inner circumference in millimetres, a Japanese size adds one third of a millimetre of diameter per size from 13 mm for size 1, and the US scale follows the gauge dimensions reported by the US National Bureau of Standards in 1921.

How accurate it is

The limit is the screen: the disc can only grow or shrink by whole pixels, and you judge the edges by eye. The table shows how far one pixel moves the result. On a screen with 6 pixels per millimetre, one pixel of diameter is 0.17 mm, which is 0.2 of a US size or half a Japanese size.

A calibration that is one pixel off over the 53.98 mm of the card changes a 17 mm ring by less than 0.1 mm, much less than the error of placing the ring. Expect the result to be right to about half a US size. For an engagement ring or a ring that cannot be resized, have the finger measured by a jeweller.

The sizer does not give UK and Australian letter sizes: their standard is withdrawn, its text was not available, and published charts disagree.

Worked example: calibrating a phone

On a phone, the box matches the card at 330 pixels. That is 330 ÷ 53.98 = 6.11 pixels per millimetre.

• A ring fills the disc at 17.3 mm, 17.3 × 6.11 = 106 pixels across.
• Inner circumference: 17.3 × π = 54.3 mm, so ISO 54.
• Japan: (17.3 − 13) × 3 + 1 = 13.9, so size 14.
• US: 17.3 mm is 0.681 in; the gauge line gives 6.96, so size 7.
• One pixel is 1 ÷ 6.11 = 0.16 mm, about 0.2 of a US size.

Common mistakes

• Skipping the calibration. Without it the disc is drawn at the CSS inch, which can be far from the real size on a phone.
• Zooming the page after calibrating. Calibrate again after any zoom.
• Matching the outside of the ring. The size is the inside: the disc must fill the hole, not cover the metal.
• Measuring a ring that is worn on another finger or on the other hand. Fingers differ; use a ring from the finger you are buying for.
• Holding the card at an angle or the ring above the glass. Keep both flat on the screen.

Sources and checks

The values in the chart are calculated from the definitions in the sources below. They are not copied from another chart. Last checked against the sources on September 29, 2026.

The sizer calibrates the screen with the short side of a bank card, 53.98 mm, and converts the matched inner diameter with the same formulas as the ring size chart of this site.

The table shows how far one CSS pixel moves the result at a given number of pixels per millimetre: one pixel of diameter changes the circumference by pi pixels.

UK letter sizes are left out, as on the ring size chart: the defining standard is withdrawn and published charts disagree.

• IEC Webstore, ISO/IEC 7810:2003 Identification cards: physical characteristics (abstract)

Used for: The ID-1 card format, used for bank cards, is 85.60 mm by 53.98 mm with a nominal thickness of 0.76 mm; the 2003 edition was replaced by ISO/IEC 7810:2019 with the same formats.

• W3C, CSS Values and Units Module Level 4, absolute lengths

Used for: On screens a CSS inch of 96 CSS pixels is anchored to a reference pixel, not to a physical inch, so a web page cannot know the real size of its pixels without calibration.

• ISO 8653:2016, Jewellery: ring-sizes, definition, measurement and designation (official preview at iTeh Standards)

Used for: The ring size is the inner circumference in millimetres; sizes 41 to 76; diameters of the measuring stick for sizes 41, 51, 61, 71 and 76.

• Platinum Guild International Japan, ring size column

Used for: Japanese size 1 has an inner diameter of 13.0 mm and each size adds about one third of a millimetre; sizes 10 and 20 are 16.0 mm and 19 1/3 mm.

• National Bureau of Standards, Circular 43, second edition, 1921 (NIST digital archive)

Used for: No official dimensions of US ring sizes were found; gauge makers reported 0.485 and 0.491 inch for size 1 and 0.877 inch for size 13.

• GIA, How to determine ring size (ring size chart)

Used for: Inside diameters in millimetres for US sizes 3 to 13.5, used to check the calculated US column.