Ring size chart & 1:1 on-screen ring sizer

Calibrate once, then trust the circle

Screens report their size to the browser in CSS pixels, and the number of those pixels in a real inch changes from one phone, tablet or monitor to the next. That is why most ring sites say their charts are not actual size. Here you match a coin or a bank card against an outline once; the site then knows how many pixels make a millimetre on that screen and remembers it for that screen only.

A bank card (ISO/IEC 7810 ID-1, 85.60 mm wide) is the most precise reference because it is long: matching it to one device pixel is about ±0.25% of its width. A coin is quicker and good to about ±0.8%. On a ring of 17 mm, the card leaves you within about 0.05 mm of the true size.

Match the inside edge

Lay the ring flat on the circle and look straight down. Grow or shrink the circle until the red line just disappears under the inside of the band all the way round. The gold ring drawn outside it is the band wall: do not match the outer edge, which would read two or three sizes too big.

On a high-density phone screen one pixel is about 0.06 mm, so a careful match lands within about ±0.15 mm, under a quarter size. On a standard desktop monitor one pixel is about 0.25 mm, so allow ±0.25 mm there. If the ring is slightly oval, match the average of the two directions.

Six systems, one measurement

Every ring size system measures the same thing: the inside of the band. The US and Canadian scale starts at 11.63 mm inside diameter for size 0 and adds 0.8128 mm (0.032") per whole size, so each quarter size is about 0.2 mm. There is no ASTM or ANSI standard behind it; it is a trade scale that ring mandrels and sizing gauges follow.

ISO 8653, used across continental Europe, simply names a ring by its inside circumference in millimetres, so EU 54 is a 54 mm loop. Swiss, Italian and Spanish sizes are that number minus 40. The UK, Ireland and Australia use letters; since BS 6820 (1987) each letter is 1.25 mm of circumference, with C at 40 mm, and half letters in between. Japan uses numbers 1 to 30 that step about a third of a millimetre of diameter.

US inside Ø (mm) = 11.63 + 0.8128 × size · circumference = π × Ø · EU = circumference rounded · Swiss = EU − 40

Why string gives the wrong answer

A US size is 2.55 mm of circumference. String stretches as you pull it snug and the pen mark has width, so a 2 mm error is easy, and that is most of a whole size. A paper strip about 5 mm wide does not stretch; if you have no ring to measure, use the paper-strip mode and read the strip against the 1:1 ruler.

Fingers are smallest in the morning and in the cold and swell in heat, after exercise and late in the day. Measure at room temperature in the afternoon, and make sure a ring will still pass over the knuckle, which is often the larger dimension.