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Online Ring Sizer

An on-screen ring sizer draws a circle at a true diameter on your screen so you can match a ring to it. First calibrate by matching the dashed box to a bank card (85.60 mm wide), then lay a ring that fits on the circle and move the slider until the circle just fills the inside of the band. A 17.3 mm circle, for example, is US 7, UK N½, EU 54.5.

Step 1: calibrate with a bank card

Turn a phone sideways (the card’s 85.6 mm long side is wider than a phone screen held upright). Lay any payment or ID card on the screen over the dashed box and move the slider until the box is exactly as wide as the card.

3 pixels per mm assumed until you calibrate.

Step 2: match a ring that fits

Lay the ring on the circle and move the slider until the circle just fills the inside of the band. Then read the size.

Your size

17.3 mm across · US 7 · UK N½ · EU 54.5 · Japan 14

  • US / Canada7
  • UK / AustraliaN½
  • EU / ISO54.5
  • Japan14
  • Inner circumference54.3 mm

Browser zoom must stay at 100% after calibrating. A phone laid flat and turned sideways is easier than a laptop screen; a tablet is easiest.

How does the sizer get the size right?

Pixels per mm = the dashed box’s width in pixels ÷ 85.60. Circle width in pixels = chosen diameter in mm × pixels per mm. The sizes come from the diameter: US = (diameter − 11.63) ÷ 0.8128; circumference = π × diameter for UK, EU and Japan.

Worked example. If the box matches your card at 300 pixels wide, the screen is 300 ÷ 85.6 = 3.5 pixels per mm. A ring whose inside the circle fills at 17.3 mm is drawn 17.3 × 3.5 = 60.6 pixels across. (17.3 − 11.63) ÷ 0.8128 = 6.98, so US 7; π × 17.3 = 54.3 mm around, so UK N½, EU 54.5, Japan 14.

Getting a good match

Look straight down at the ring; at an angle the inside edge looks narrower than it is. Match the circle to the inside of the band where it touches the screen. If the ring is a comfort-fit band (curved inside), the inner edge you see is slightly wider than the part that grips the finger, so take a quarter size less.

Between two sizes

The readout shows how far the diameter is from the nearest size on each scale. If it says “+0.3 mm” for US 7, the ring is leaning towards a 7¼; a quarter size is 0.64 mm of circumference, 0.2 mm of diameter. For a thin band, round down; for a wide one, round up.

Checking the result

Lay the ring on the chart page’s finder figure: an inner diameter of 17.3 mm should give the same sizes. Or print the printable sizer and lay the ring on the paper circles; the two methods should agree to within a quarter size.

Guides

Questions about the on-screen sizer

How accurate is an on-screen ring sizer?

As accurate as the calibration. A half-millimetre error on the 85.6 mm card is 0.6%, which is about 0.1 mm on a 17 mm circle, under an eighth of a US size. Keep the browser zoom at 100% after calibrating, and lay the phone flat.

Why calibrate with a bank card?

Because every payment, ID and loyalty card is made to one size, 85.60 × 53.98 mm (ISO/IEC 7810 ID-1), and almost everyone has one. Matching the box to the card tells the page how many pixels your screen uses per millimetre; screens differ, so a sizer that skips this step is guessing.

Which ring should I use?

One that fits the finger you are buying for, worn on the same finger and hand. Lay it flat on the screen, look straight down at it, and match the circle to the inside edge of the band, not the outside.

Can I use it on a laptop?

Yes, but a phone or tablet laid flat is easier because you can look down on the ring. A phone must be turned sideways: the card’s long side, 85.6 mm, is wider than any phone screen held upright. On a laptop, hold the ring against the screen and read the circle through it.

What if I have no ring that fits?

Use the string or paper method instead: wrap a strip round the finger, measure it, type in the length. The printable sizer gives a paper strip marked in sizes.