Three ways to measure ring size at home — a calibrated phone screen, a paper strip and a printed sizer — plus the 1 mm accuracy that decides the size.
To measure your ring size at home you need one number: the inner circumference of a band that fits, in millimetres. Three methods get you there — a ring laid on a calibrated screen in True Ring Size, a paper strip around the finger, or a printed sizer. Accuracy inside 1 mm decides the size you order.
A circumference in millimetres. The international standard for ring sizes, ISO 8653, expresses size as the inner circumference of the band in millimetres: a band measuring 54 mm around the inside is ISO size 54. A 1 mm error is therefore not a rounding detail — it is one whole size.
Diameter is the same fact viewed differently, and the conversion is unforgiving: circumference is pi times diameter, so 1 mm of circumference is only 0.32 mm of diameter. Every method below is judged on whether it can hold a third of a millimetre. The scale a shop quotes changes nothing: measure once, convert after. True Ring Size moves one reading between the US, UK, EU, Japanese, Chinese and Russian scales.
Two ways, and both start with calibration. In ring mode you lay a band you already wear on the display and resize the on-screen circle until its edge sits inside the band; in finger mode you line two guides up to the finger. Ring mode is more reliable: a metal band holds its shape, while a fingertip pressed to glass flattens and reads large.
Without calibration the screen has no idea how large a millimetre is. The W3C's
So give the screen one real measurement. Any bank card works, because its size is a standard: ICAO's
Cut a strip about 6 mm wide — paper, not string, because string stretches and a stretched loop reads small. Wrap it around the base of the finger until it is snug but still slides, mark the overlap with a sharp pencil, then lay it against a ruler. The millimetres between edge and mark are the circumference, and therefore the ISO size.
It is free and the easiest method to get wrong, because three small errors all point the same way: pulling the strip tight costs perhaps 0.5 mm, a thick pen mark another 0.5 mm, and buckled paper a little more — together, a full size too small. Wrap three times, take the middle reading.
Because the page is rarely printed at the size it was drawn. A print dialog set to shrink a page to fit the margins shrinks the sizer with it, and the CSS conversion of 1in to 96px only survives if the output device honours physical units. A 4% scale error on a 54 mm circumference is 2.2 mm — nearly two sizes.
The test needs no app: lay a bank card against the ruler most printable sizers include. If the card does not span 85.60 mm there, the sheet is the wrong size and so is every circle on it. Reprint at 100% scaling, or use the screen method.
Warm, and late in the day. Fingers are smaller when cold and larger after heat, exercise or a salty meal, so a cold morning reading and an after-dinner reading can differ by most of a size. Measure on three separate days and treat a single reading as a first draft.
Then measure twice on the finger. The band must pass the knuckle but sit at the base, so take both circumferences and order the larger. A wide band grips along its whole width and feels tighter than a narrow one at the same nominal size, so above roughly 6 mm consider the next half size up.
When the ring cannot be resized afterwards. Tungsten carbide and titanium cannot be cut and rejoined the way gold can, and an eternity band set all the way round has nowhere to take metal out, so the first order has to be right. Take those to a jeweller with steel gauge rings rather than to any app, ours included.
It is also wrong when the finger is not a stable shape: swollen joints, pregnancy and significant weight change all move the number, and a knuckle much larger than the finger base needs a hinged band rather than a better measurement. With no card, coin or ruler to calibrate against, an on-screen sizer is guessing — use the paper strip.
All four were free on the US App Store on 24 September 2026; versions and dates come from those listings.
6,817 ratings at a 4.5 average from Uniqo Lab. — the largest review base in the category.
Version 4.0.6, updated 13 August 2026 — the pick if a long track record outweighs everything else.
A 4.8 average across 459 ratings from AppLiks Sp. z O.O. — the highest average here.
Version 2.0.7, updated 15 May 2026, on a much smaller sample than the leader.
2,399 ratings at a 4.2 average from LightApps OU.
Version 2.3.5, last updated 5 December 2025 — the oldest update date here, worth knowing on a new phone.
Free, no advertising and no in-app purchases, on both
Works with no network at all; the listing declares no data collected.
Converts one reading between six national scales, diameter and circumference side by side.
Version 1.0, released 31 July 2026, with a review base of one — the opposite trade-off to the leader.
No app beats the physics of an uncalibrated screen, which the App Store listing for True Ring Size: Sizer & Gauge admits in its own closing line:
Note: on-screen measuring depends on your screen; calibrate with a card and measure twice for best results.
That holds for every method here: calibrate against something whose size is published, measure three times, order the larger candidate, and check whether the band can be resized.
Yes, if you calibrate first and measure a ring rather than a finger. Calibration against a bank card gives the screen a known 85.60 mm reference, which removes the pixel-density guesswork. Measuring a band you already wear is more repeatable than pressing a finger to glass, because the band keeps its shape.
Wrap a 6 mm strip of paper around the base of the finger, mark the overlap, and measure its length against a ruler in millimetres. That length is the inner circumference, which is the ISO size. Repeat on three different days and take the middle reading.
Both are used, which is why listings disagree. ISO 8653 uses inner circumference in millimetres; many US charts quote inner diameter. They convert with pi: a 54 mm circumference is a 17.2 mm diameter. Record what your app shows and convert when ordering.
About 2.5 mm of circumference, or 0.8 mm of diameter, between consecutive whole US sizes. That is small enough that a paper strip pulled tight, a thick pencil mark, or a printout scaled to 96% can each cost you a size on its own.
The evening, with warm hands. Fingers are smaller when cold and after a night's rest, so a morning measurement tends to read low and produces a ring that will not pass the knuckle later in the day. Three readings across different days beat one careful one.
Because printing changes their scale: if the print dialog shrinks the page to fit the margins, every circle shrinks with it. Check the printout by laying a bank card against its ruler — the card must span 85.60 mm, or the sheet is unusable.
Only from a ring they already wear on that finger. Borrow it, measure the inside on a calibrated screen or against a ruler, and return it. Guessing from a photograph or hand size is not measurement, and an unresizable band ordered on a guess cannot be fixed.
It is free with no advertising and no in-app purchases, and it works entirely offline — the conversion tables and the calibration live on the device. The iOS download is 30.7 MB and needs iOS 13.0 or later; the Android build is on Google Play.