Measure a ring size on screen
1. Calibrate the screen with a bank card
Hold any bank, ID or loyalty card flat against the screen and drag the slider until the outline matches the card exactly. Every card in the world is the same size — ISO/IEC 7810 ID-1, 85.60 × 53.98 mm.
Not calibrated yet — sizes below use the browser default and can be out by a tenth.
2. Lay the ring on the circle
Put a ring that fits the right finger over the circle and resize the circle until the inside edge of the ring and the circle line up. Read the size underneath.
Sizes are for guidance, following BS 6820 and ISO 8653 as checked on 2026-10-23. Brands cut sizes differently: try the ring on or ask your jeweller before you buy. Disclaimer
Calibrate the screen with any bank card first — then a ring you already own becomes the measuring tool.
Why the screen has to be calibrated
A browser has no idea how big your screen really is. It works in CSS pixels, and the number of real millimetres one of those covers depends on the panel, its resolution and the zoom level — a 13-inch laptop and a 27-inch monitor at the same browser zoom draw a “10 mm” line at two different widths.
The fix is to show you something whose real size is fixed by international standard and let you match it. Every bank card, ID card, driving licence and loyalty card is ISO/IEC 7810 ID-1: 85.6 × 53.98 mm, with no tolerance you could see by eye. Drag the slider until the outline matches the card in your hand and the page knows exactly how many pixels a millimetre is.
The calibration is saved in your browser on this device, so you only do it once. It never leaves your device.
Measuring the ring
Measure the inside of the band, not the outside. On a 6 mm band the outside diameter is more than a size larger than the inside — that single mistake is why people order rings that will not go on.
- Calibrate with a card, as above.
- Take a ring that fits the finger you are buying for, and the hand you are buying for.
- Lay it flat on the screen over the circle.
- Make the circle bigger or smaller until its line sits exactly under the inside edge of the band and just disappears.
- Read the size under the circle.
How accurate is it?
Once calibrated, the limit is your eye and the thickness of the drawn line, which works out at a few tenths of a millimetre — around a quarter of a size. That is as good as a cheap plastic sizer and better than a badly printed paper one.
The weak point is screen size. On a phone, the ring covers a large part of the picture and small adjustments are hard to see. On a laptop or a desktop monitor it is comfortable. If you only have a phone, the printable sizer or a paper strip will serve you better.
If you do not have a ring to measure
Then the screen cannot help — there is nothing to lay on it. Use string or paper round the finger, or print the paper sizer. Both measure the finger directly, which is what you actually want.
Questions people ask
Why do I need a bank card?
Because it is the one object almost everyone has whose size is fixed worldwide: ISO/IEC 7810 ID-1, 85.6 × 53.98 mm. Matching the screen to it turns the screen into a ruler. Any card of that shape works — bank, ID, library, gym.
Is my calibration sent anywhere?
No. It is one number stored in your browser’s local storage on this device. Clearing your browser data removes it; nothing is uploaded.
Can I do this on a phone?
Yes, and the calibration works the same way, but a ring takes up much of a phone screen and fine adjustment is awkward. A laptop screen is easier. The printable sizer is the better choice on a phone.
Do I measure the inside or the outside of the ring?
The inside, always. Ring sizes describe the hole, not the band. On a wide ring the outside can be a size and a half bigger than the inside.
The circle will not go small enough — what now?
Lower your browser zoom and calibrate again, or use the circle chart on the printable sheet, which is not limited by the screen.