What this is
RingSizerChart is a free, browser-based sizing reference for jewellery, watches and glasses. It turns your screen into a calibrated 1:1 measuring surface: lay a ring on a circle to read its size, see a diamond at its true millimetre size on a finger, compare chain widths, match a watch case or a bangle, and read your pupillary distance. Then it gives the answer in every sizing system: US, UK, EU, Japanese and Swiss ring sizes, carat dimensions, necklace drops and frame markings.
Everything runs in your browser. There are no accounts, no uploads and no server-side processing of anything you measure.
How 1:1 works
Browsers report CSS pixels, not millimetres, and every screen packs them differently. That is why shops warn that screen images are not actual size. Here you calibrate once: match an on-screen outline to a coin or a standard bank card (ISO/IEC 7810 ID-1, 85.60 × 53.98 mm), or pick your device from a list of 114. From that the site knows how many pixels make a millimetre on your display and draws every sizer at true size.
- A card calibration is good to roughly ±0.25%; a coin to roughly ±0.8%. On a 17 mm ring that is a few hundredths of a millimetre.
- Browser zoom is corrected automatically; pinch-zoom breaks the scale, so the header warns you when it changes.
- The printable sizers need no calibration, because millimetres print true at 100% scale.
Where the numbers come from
Every table is computed from a stated rule or a published table, never typed in by hand, and unit-tested before each release:
- Ring sizes: the US trade scale (11.63 mm + 0.8128 mm per size), ISO 8653:2016, BS 6820:1987 letters, Swiss = ISO − 40 and a published Japanese (JCS) table.
- Diamonds: the round-brilliant estimation formula often credited to GIA and published shape factors for fancy cuts, at stated proportions; gem densities from standard references.
- Eyewear: the ISO 8624:2020 boxed lens system; PD ranges and the near-PD rule of thumb from optical retailers' guides.
- Necklaces, bracelets and watches: retailers' published size guides, and a geometric drape model that is described on the page.
- Smart rings: the makers' own size ranges and sizing-kit guidance. We publish no conversions, because the makers don't.
What it is not
A screen sizer is a fast, accurate way to find a size, not a fitting. Fingers change through the day, wide bands fit tighter, and each maker's sizing can differ a little. For rings that cannot be resized, smart rings and prescription glasses, confirm with the seller's gauge, sizing kit or your optician.
Who runs it
RingSizerChart is an independent reference by the makers of ScrewBoltChart. Found a number that disagrees with your gauge or a maker's guide? Write tocontact@ringsizerchart.com or use the contact page, and include the source so the correction can be checked.