Are your ring size conversions accurate?

Yes. We compute every value from the US diameter standard and cross-check it against ISO 8653 and multiple jeweler charts. We use UK half-letters (US 7 = N½), which many charts get wrong by up to a full size.

What standard do you use for EU ring sizes?

ISO 8653, under which the EU size number equals the inner circumference of the ring in millimetres (so EU 54 = 54 mm).

How we calculate ring sizes

Accuracy is the whole point of this site, so here's exactly how every number is derived — and the common chart mistakes we deliberately avoid.

Last reviewed: June 24, 2026

One source of truth

Retailer charts round inconsistently and sometimes contradict themselves. Instead of copying a chart, we compute everything from the physical primary — the inner diameter — and label each national system from a single verified table.

The formula

The US size sets the inner diameter; everything else follows:

diameter (in) = 0.458 + 0.032 × US size
diameter (mm) = diameter (in) × 25.4
circumference (mm) = π × diameter (mm)
EU / ISO size = round(circumference) — under ISO 8653 the size is the circumference in mm
Switzerland / Italy / Spain = round(circumference) − 40
UK letters and Japan are matched from a verified lookup table (no clean formula exists)

The UK half-letter correction

US whole sizes fall between UK whole letters, so the accurate UK equivalent is normally a half letter — for example US 7 = UK N½, not N. Many charts (including some very popular ones) collapse this to whole letters and are wrong by up to a full size. We always show the half letter.

Accuracy & limitations

US, EU/ISO and millimetre values are exact; Japan, India and China are approximate lookups.
Home and on-screen measurements carry error — string overstates size, fingers swell, knuckles vary.
Our chart is standard-fit; comfort-fit and wide bands run tighter (size up).
For an engagement or non-resizable ring, confirm with a jeweler.

Use the printable ring sizer only after confirming the 85.6 mm credit-card check or the 50 mm ruler check. Use the on-screen sizer only after calibrating the screen with a real card.

Sources

ISO 8653:2016 — ring sizes (EU/ISO circumference basis)
JIS S 4700 — Japanese ring sizing
The US jeweler diameter standard (size 0 = 0.458″, +0.032″ per size)
Wikipedia “Ring size” (governing formulas) and multiple cross-checked jeweler charts

See the full sources & references, try the converter, browse the full chart, or use the printable sizer.