Borrow a ring they already wear and measure its inside on a calibrated phone screen. The covert methods that work, and the shortcuts that cost a size.
To find someone's ring size without them knowing, borrow a ring they already wear on that finger and measure its inner circumference in millimetres — a band 54 mm around the inside is ISO size 54. True Ring Size does this on a calibrated phone screen, offline and free on iPhone and Android, and saves a size per person.
Because it is a fixed object and a hand is not. A band has one inner circumference that does not change while you carry it to the next room, whereas the finger it came off is warmer in the evening and thinner in January. Comparing hands or judging from a photograph produces an estimate with no measurement in it.
Note which finger the ring came from, because sizes are not transferable between them. One person wears several sizes across a single hand, and left and right rarely match, so a band from a right middle finger tells you little about a left ring finger.
Calibrate once, then lay the band on the glass. In ring mode you place the borrowed band on the display and resize the on-screen circle until its edge sits inside the metal, and the app reads the inner diameter and circumference together. It takes under a minute, which matters when the ring is meant to be back in its dish before anyone notices.
Calibration is not optional, because a screen has no idea how large a millimetre is. The W3C's
So give the screen one object whose size is published. Any bank card works, because ICAO's
Record the inside, not your impression of it. Stand the band on paper and trace the inner edge with a sharp pencil, or press it into a bar of soap; the circle can then be measured once the ring is back.
Understand what a slip costs. Consecutive whole US sizes are 0.032 in apart — 0.8128 mm of inner diameter — so about 2.55 mm of circumference separates one size from the next. Because circumference is pi times diameter, a 0.3 mm error in a traced diameter arrives as nearly 1 mm of circumference, and two of them in the same direction cost a full size. Sliding the band onto your own finger is the weakest fallback of all.
Move the evidence off the shared surfaces. Take the ring to another room, return it in the same orientation to the same dish, and keep the number somewhere they will not open — which argues for a sizing app, not the household notes file. True Ring Size stores a size per person under a label you choose, and its listing declares no account, no tracking and no data collection.
Then watch the ordering trail rather than the measuring. A confirmation email gives away more than a ring lifted for forty seconds, so order to work or to a collection point.
Convert from circumference, and keep the raw number. The international standard for ring sizes, ISO 8653, expresses size as the inner circumference of the band in millimetres, and 54 mm around the inside is a 17.19 mm inner diameter once you divide by pi. Britain uses letters, Japan and China use their own ladders, and US charts quote diameter, which is how one ring collects four labels.
Write the millimetres down before the size, because a converted label cannot be converted back without knowing which chart produced it.
When the band cannot be resized. Tungsten carbide and titanium cannot be cut and rejoined the way a gold shank can, and an eternity band set all the way round has no plain metal to take out, so a half-size error becomes a returned parcel. Buy those from a jeweller who exchanges unworn pieces.
It is also wrong when the hand 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. Buying large and having it taken in beats buying small, because a ring that will not pass the knuckle cannot be worn once — and where the question can be asked, asking beats anything here, ours included.
All four were free to download on the US App Store on 27 September 2026; the versions, dates and ratings below come from those listings.
Its listing describes four methods, including a camera measurement of a hand on a flat surface — the one approach needing no borrowed band.
49 ratings at a 4.7 average from Ivan Frolov; 4.6 MB, version 1.0, last updated 1 December 2023.
The listing states $4.99 per week for full access.
Version 1.2.18, updated 1 September 2026 — the most recently maintained here.
177 ratings at a 4.6 average from Vipul Prajapati; 26.1 MB, iOS 15.6 or later.
Its listing names Australian, US, UK, European, Japanese and Chinese conversions.
607 ratings at a 4.7 average from Appcano LLC — the largest sample here.
Version 1.0.7, updated 22 January 2026; 71.4 MB, iOS 15.0 or later.
The listing leads on finger measurement, and says full access needs an annual auto-renewing subscription.
Free, with no advertising, subscription or in-app purchases, on both
Ring-on-screen and finger-on-screen modes, calibrated against a bank card, with no network permission.
Saves a size per person and converts between six national scales.
Version 1.0, released 31 July 2026, with a review base of one — the opposite trade-off to the older apps.
The surprise-gift case is the one the App Store listing for True Ring Size: Sizer & Gauge puts in its own feature list:
Save sizes for the whole family — perfect for surprise gifts and engagement rings
That is the method in one line: measure a real ring, store the millimetres, convert at the till, and buy something resizable.
Borrow a ring she already wears on that finger and measure its inner circumference, on a calibrated phone screen or by tracing the inside on paper. Note which finger it came from and store the millimetres rather than a converted label.
Yes, and that is the accurate way round. A band off the finger has a fixed inner diameter you can measure repeatedly, while a finger changes with temperature and time of day. Measure the metal, convert with pi, and the number still holds tomorrow.
Accurate to better than a size once calibrated against a bank card, because the ICAO ID-1 format fixes that card at 85.60 mm wide. Uncalibrated, the app is guessing at pixel density — Apple lists the iPhone 16 at 460 pixels per inch.
Then buy something you can change: a plain shank a jeweller can resize, or a band sold with free exchange. Go up rather than down, because a large ring can be taken in while one that will not pass the knuckle is unwearable.
About 0.8128 mm of inner diameter between consecutive whole US sizes, which is roughly 2.55 mm of circumference. That is why a thick pencil line around a traced ring, or a chart shrunk by a print dialog, can each move the answer a full size.
It is a last-resort bracket, not a measurement. Marking where a borrowed band stops on your own finger says which half of the chart to shop in and little more, because your finger tapers differently. Trace the inner edge on paper instead.
No, and neither can an app. A photograph has no scale unless a known object sits in the same plane as the hand, and hands photograph at whatever size lens and distance decide. Bring a borrowed ring or a measured circumference instead.
Neither. The listing declares no account, no tracking and no data collection, and the app works with no network permission, so the conversion tables and saved sizes stay on the device. The iOS download is 30.7 MB and needs iOS 13.0 or later.