This page explains exactly how our sizers turn a circle on your screen into a ring or bangle size, which standards the numbers come from, and how accurate you can expect the result to be.
1. Screen calibration
Screens have different pixel densities, so a circle drawn “50 pixels wide” is a different real size on every phone. To draw true-size circles we need the number of pixels per millimetre on your screen. Every credit, debit and ID card is made to the ISO/IEC 7810 ID-1 standard: 85.60 mm wide and 53.98 mm tall. When you drag the slider until the outline matches your card, the tool divides the outline width in pixels by 85.60 and stores the result in your browser (local storage key rbs_ppm_v1). Nothing is sent to our server.
Without calibration the tool falls back to the CSS standard of 96 pixels per inch (3.78 px/mm) and marks the result as approximate, because most phones do not report their real screen size to the browser.
2. Ring sizes
ISO 8653 defines a ring size as the inner circumference in millimetres. We store the inner diameter for each US size from 3 to 15 in half steps (for example US 7 = 17.3 mm) and calculate everything else from it:
- Circumference = diameter × π (US 7: 17.3 × 3.1416 = 54.3 mm).
- EU / ISO size = circumference rounded to the nearest mm (54).
- Swiss size = EU size − 40 (14).
- India / Japan size = (diameter − 13 mm) × 3 + 1, rounded (14). Each step is one third of a millimetre.
- UK / Australian letters are taken from the standard US-to-UK conversion table, because the letter scale does not follow a single formula.
The full table is on the ring size chart and the ring size in mm page.
3. Bangle sizes
Indian and Pakistani bangle sizes are written as inches and sixteenths of an inch of inner diameter. “2.4” means 2 + 4/16 = 2.25 inches; “2.10” means 2 + 10/16 = 2.625 inches. We convert to millimetres with 1 inch = 25.4 mm, so 2.4 = 57.2 mm and 2.10 = 66.7 mm. Adult sizes step by 2/16 inch (3.2 mm).
For the hand method, the calculator divides your hand circumference (measured over the knuckles with the thumb tucked in) by π to get the smallest opening your hand can pass through, then picks the first bangle size that is at least that wide. Rounding up is deliberate: a rigid bangle that is 1 mm too small will not go on at all.
4. Bracelet lengths
Bracelet length = wrist circumference + an ease allowance that depends on the style: ½ to 1¼ inch for chain bracelets, ¼ to ¾ inch for open cuffs. The allowances match the ranges most jewellers publish for snug, comfortable and loose fits.
5. How accurate is it?
After calibration the circle is drawn to the nearest pixel. On a typical phone (about 16 pixels per mm) that is better than 0.1 mm, far finer than the 0.4 mm between half ring sizes or the 3.2 mm between bangle sizes. In practice the limit is lining the ring up by eye. Our guidance is therefore:
- Rings: expect ±¼ US size. Measure twice, with two rings if you can.
- Bangles: the result is reliable to one size; if you are between sizes, choose the larger for a rigid bangle.
6. Real-world accuracy test
We are testing the sizer on several phones and laptops against rings and bangles measured with a digital vernier caliper. The results, with photos and the size each device reported, will be published in this section once testing is complete. Until then, treat every on-screen result as a strong estimate and confirm it with a jeweller or the seller’s own chart before buying an expensive piece.
7. Known limits
- Some phones apply display scaling or zoom that changes after calibration. If you zoom the page, calibrate again.
- Thick or comfort-fit bands fit tighter than their inner diameter suggests; go up ¼–½ size for bands wider than 6 mm.
- Bangle sizes vary slightly between makers. Glass bangles are often made a little smaller than metal ones of the same number.
Found a problem? Please contact us.
Frequently asked questions
Why do I need a credit card to calibrate?
All bank cards are exactly 85.60 mm wide, so matching one on the screen tells the tool how many pixels make a millimetre on your device. You can use any ID-1 card, including a library card or ID card.
Is my calibration or measurement uploaded?
No. The calibration number is stored only in your browser and the sizing happens on your device. No measurements are sent to our server.
How accurate is an on-screen ring sizer?
After calibration the circle is accurate to about 0.1 mm. Lining up the ring by eye limits the result to about a quarter US size, similar to a plastic sizer set.
Which standard do you use for ring sizes?
ISO 8653, which defines ring size as the inner circumference in millimetres. Other systems are converted from the inner diameter.