Methodology

Ring sizing has one anchor everyone agrees on: ISO 8653 defines a ring size as the inner circumference of the band in millimetres. Every national system is a different label for that same physical measurement, so the whole table is built from millimetres out.

Rather than transcribe a chart (published tables are riddled with rounding disagreements), we generate the whole table from the standard linear formulas below, so every row is internally consistent. The formulas match the published equivalency data and are cross-checked against jeweler charts. The anchor they all share, and the one our tests assert, is that a US 7 is about 17.3 mm across (54.4 mm around), which is UK N½, EU 54, and 14 in Japan. UK letters and Japan sizes are lookup columns, flagged approximate because charts disagree by up to half a size.

North American sizing is a straight line in diameter. For a US size, the inner diameter in millimetres is:

The other systems follow from the millimetre measurement:

• UK / Australia: a letter scale where one whole letter is about 1.25 mm of circumference, with size C at 40 mm. Because it does not line up perfectly with the US numbers, we list the published letter for each size rather than compute it.
• EU / ISO: the inner circumference in millimetres, rounded to a whole number. So EU 54 is about 54 mm around.
• Japan (号): a diameter scale where 1号 is 13.0 mm across and each size adds one third of a millimetre, so 号 = 3 × diameter − 38. That places US 7 at 14号.

When you measure on screen, we round the US size to the nearest quarter, the finest step jewelers commonly cut, and snap the UK letter to the closest listed value.

A screen has no fixed real-world scale: the same 100-pixel line is a different physical length on a phone, a laptop, and a 4K monitor. To measure in millimetres, the tool needs a physical reference of known size.

We use a standard bank card, which is manufactured to ISO/IEC 7810 ID-1: 85.60 by 53.98 mm, the same everywhere in the world. You resize the outline until it matches a real card, and the tool stores how many pixels cover one millimetre on your device. A US quarter (24.26 mm) is offered as an alternative that fits on a phone screen. The stored ratio lives in your browser only.

On-screen measuring is a good estimate, not a guarantee. Honest limits:

• You match the circle by eye, so a careful reading is good to roughly a quarter or half size, no finer.
• Fingers change through the day and with temperature; the same finger can differ by half a size between a cold morning and a warm evening.
• A wide band sits tighter than a thin one at the same number, so wide bands are usually sized up.
• A cracked screen, a thick screen protector, or a zoomed browser can throw off calibration.

For a ring the wearer cannot easily resize, an engagement ring above all, treat the result as a strong starting point and confirm the final size with a jeweler.

Ring-size charts do not all round the same way, so small differences are normal:

• Many retailer charts round the UK letter to the nearest whole letter; we keep the more precise half-letter from the equivalency table.
• The Japanese 号 scale is sometimes tabulated with fractional values that blend Japan and China. We use the standard diameter-based 号 scale, which puts US 7 at a clean 14号, matching common jeweler charts.
• European sizes are occasionally shown with a decimal; we round the circumference to a whole millimetre, which is how most charts print it.

• ISO 8653:2016, Jewellery: Ring sizes , for the definition of a size as inner circumference in millimetres.
• Ring size (Wikipedia) , whose sourced equivalency table aggregates ISO 8653 and the national standards, for diameter, circumference, and UK letters.
• ISO/IEC 7810 , for the ID-1 bank-card dimensions used in calibration.
• Published jeweler conversion charts, used to cross-check the US, UK, EU, and Japan equivalents.