Trust
Accuracy and limitations
Home measurement is useful and honest — and it has limits. This page states them plainly, so you can judge when a home result is enough and when it is not.
How calculations work
Every result on this site comes from one dataset and two operations. The dataset is the unified chart (US 3–13.5 in half steps, anchored at US 3 = 14.07 mm diameter). The operations are circumference = π × diameter and diameter = circumference ÷ π. Internal values are never rounded before lookup; only the displayed diameter (two decimals) and circumference (one decimal) are rounded. Sizes are never computed from a universal formula — the lookup always goes through the validated table, because size systems are not interchangeable by formula (letters, half steps, and regional conventions break any single equation).
How screen calibration works — and why it can refuse
A screen's pixels are not millimetres, and every display differs. The sizer therefore calibrates against a standard bank card (85.60 × 53.98 mm): you resize an on-screen rectangle until it matches the card's long edge, and the tool derives pixels per millimetre from that match. Until you confirm calibration, no size is shown — because an uncalibrated circle is a guess with a picture attached, and this site does not present guesses as results. If your display changes (zoom, rotation, a different monitor), calibration is invalid and the tool asks you to recalibrate.
Why physical verification matters
Screens and prints are not rulers
A screen circle is only as good as its calibration; a printed circle is only as good as its 100% print setting. Both must be verified against a physical reference (a card, a ruler) before the result is trusted. The printable guide makes its ruler check mandatory for exactly this reason.
Fingers are not fixed numbers
Finger size moves during the day, with temperature, and across seasons. A measurement is a reading on one day, in one condition — not a permanent property of the finger. The evening, warm-hands routine exists to make that reading the most forgiving one, not to pretend variation does not exist.
No accuracy figure is claimed
No study has yet measured how close this site's home methods come to a professional reference. A protocol for such a study exists (see the accuracy research framework in the site documentation), but a protocol is not a result — so no accuracy percentage, “within half a size” claim, or certification appears anywhere on this site.
UK, Australia and India are approximate
Published charts disagree for UK letters (by a half-letter in places), Australian letters (by about 0.3 mm), and India sizes (by about ±1). Those columns are labelled approximate on every page, and for an order the unambiguous numbers are diameter and circumference in millimetres.
When to seek professional measurement
Get a professional fitting when the ring is expensive, cannot be resized, is a wide band, has a large knuckle-base gap, or when home measurements keep disagreeing. A jeweller's sizing set tests the actual width on the actual finger, which no home method can fully replicate. The professional sizing guide lists the cases and what to ask for (size in millimetres, sizer width, resizing terms). For a ring that must be right the first time, home measurement is the preparation for a fitting — not a replacement for it.