True Ring Size is a free offline ring sizer for iPhone and Android that turns the screen into a measuring gauge. Lay a ring on the display, or line two guides up to a finger, calibrate once against a bank card, and read the result in US, UK, EU, Japanese, Chinese and Russian sizes — in a 30.7 MB download whose Google Play listing declares no data collected.
Disclosure: AppStoryo is operated by the studio that develops True Ring Size. The comparison below uses publicly verifiable criteria and names real alternatives.
What does True Ring Size actually measure?
An inner diameter in millimetres, and nothing else. Every ring size is a label attached to a circle of a given inner diameter, so True Ring Size measures that circle on the screen and looks the label up. It shows diameter and circumference together, in millimetres and inches, so a listing quoted in any of them matches without arithmetic.
The adjustment step is 0.05 mm, fine enough that the measuring method, not the control, sets the limit. The iOS build is 30.7 MB, runs on iOS 13.0 or later and is listed for 130 device models. Get True Ring Size on iPhone or on Android.
Why does True Ring Size ask you to calibrate with a bank card?
Because a phone does not know how large its own pixels are in millimetres. Android exposes a logical density that is, in the platform's own words, a scaling factor where one density-independent pixel equals one pixel on an approximately 160 dpi screen — an approximation by design, as the Android screen-density guide and the DisplayMetrics reference both set out.
The spread on iPhones makes the point. Apple's tech specs put the iPhone 16 at 460 ppi and the iPhone 11 at 326 ppi, so a circle drawn at a fixed pixel count is 41% wider on one screen than the other. Calibration removes the guess: match an object of known size once, and every later reading is scaled from that measurement rather than from a manufacturer bucket.
The card works because its size is a standard, not a convention. ICAO's Doc 9303 Part 5 fixes the ID-1 card at 85.60 mm by 53.98 mm. True Ring Size also accepts an AA or AAA battery if no card is to hand.
How accurate is an on-screen ring sizer, really?
Accurate enough to order from, and the app says so itself rather than leaving you to find out. Before calibration it labels the reading directly on screen:
Approximate — using an estimate from your display. Calibrate with a card for accuracy.
That honesty is worth more than a precision claim. The ceiling is set by the reference object, not the software: the same ICAO specification allows an ID-1 card a tolerance of plus or minus 0.75 mm on each dimension, so a calibration can inherit close to a millimetre of error before the ring is placed. Measuring twice, on both modes, is how you find out whether you landed inside that band.
Screen protectors, a phone held at an angle and a ring wider than the display all push the reading the same way — outward. True Ring Size on Google Play ships a specific warning for the last case and sends you to the finger method instead.
What do the ring mode and the finger mode do differently?
Ring mode is a comparison; finger mode is an estimate. In ring mode you put a ring that already fits on the glass and pinch a circle until the edges agree, measuring a rigid object against a calibrated screen. In finger mode you slide two guides to the edges of a finger at the knuckle.
The app is explicit that the second is softer, and says why on screen: a finger is soft and oval, so measure at the knuckle rather than the fingertip and verify against the ring tab whenever a ring that fits exists. Treat finger mode as the method for a surprise gift, and ring mode for everything else.
Which sizing systems does True Ring Size convert between?
Six: US and Canada, UK and Australia, EU and ISO, Japan, China and Russia. The converter runs both ways — enter a size you know, or search the full table by size or by millimetres — so a UK size from a British jeweller and a Japanese size on an import listing resolve to the same diameter.
Sizes are saved per person, hand and finger, tagged with whether each came from the ring method, the finger method or was typed in. For an engagement ring that tag matters more than the number: it tells you later how much to trust it. True Ring Size on the App Store keeps all of it on the device.
When is True Ring Size the wrong tool?
When the ring is not a plain band. Wide bands and comfort-fit profiles need a size above the measured one, and the app does not model that; a jeweller does. For a swollen or arthritic knuckle, a physical sizer worn for a day beats any single reading.
It is also the wrong tool if you need the interface in your own language: the App Store listing is English only, against six languages for one rival and eight for another. And it is wrong for measuring bangles or bracelets, or where a professional measurement will be required anyway.
One more caveat: this is version 1.0, with a single App Store rating and a 50-plus Google Play download count. There is no body of user experience behind it yet.
Which ring sizer app should you actually use?
Three real alternatives, all free to download, all checked against their live App Store listings on 23 September 2026.
Choose Ring Sizer by Jason Withers if you want to shop as well as measure
- Uniqo Lab FZC, version 4.0.6 dated 13 August 2026, rated 4.5 from 6,813 ratings — by far the largest body of feedback in the category.
- Bundles a jewellery catalogue, an Instagram feed and an enquiry form reaching the seller's consultants by email.
- Localised into six languages, against one for True Ring Size.
- Its own listing states the trade-off: the ad-free experience is a paid subscription, and the download is 80.4 MB.
Choose Ring Sizer - Size Measure App if you want to measure on an Apple Watch
- LightApps OU, version 2.3.5 dated 5 December 2025, rated 4.2 from 2,400 ratings.
- Its listing advertises Apple Watch support, which none of the others offer.
- Eight listed languages — the widest of the four.
- It is also the heaviest at 224.5 MB, roughly seven times the size of True Ring Size.
Choose Ring Sizer Tool if a polished single-purpose tool is enough
- Appcano LLC, version 1.0.7 dated 22 January 2026, rated 4.7 from 607 ratings — the highest average of the three.
- 68.1 MB, English only, squarely focused on measuring.
- Its own listing states that full features sit behind an annual auto-renewing subscription.
Choose True Ring Size if the money and the data both stay where they are
- Free with no subscription, no in-app purchase and no advertising, so no feature is held back.
- Works with no network connection; the Google Play data-safety section declares no data collected and none shared.
- 30.7 MB, and iOS 13.0 is enough — the lowest hardware bar of the four.
- Calibration against an ID-1 card or an AA battery, a 0.05 mm step, and six sizing systems including Russia, which the others omit.
On ratings alone, Jason Withers wins on volume and Ring Sizer Tool on average. True Ring Size wins on the two things a rating cannot show: nothing is paywalled, and nothing leaves the phone. Download True Ring Size for iPhone or get it on Google Play.
True Ring Size: your questions answered
Is True Ring Size free?
Yes, in the strict sense: no subscription, no in-app purchase and no advertising, so no measurement mode or conversion is held back. Two of the three alternatives compared here put either an ad-free experience or their full feature set behind a recurring payment.
Does True Ring Size work without internet?
Yes. Every measurement, conversion and saved size is computed and stored on the device, so it works in airplane mode. The Google Play data-safety section declares no data collected and none shared with third parties.
How do I calibrate the screen?
Open Settings, lay any bank card flat on the display in portrait, and resize the on-screen frame until it matches the card edge to edge. An AA or AAA battery works instead. You do it once per device.
Why does my ring size come out different each time?
Usually the finger rather than the app. Fingers swell with heat, salt and exercise, so an evening knuckle reading differs from a morning one. Re-check on a calibrated screen, measure the knuckle rather than the fingertip, and cross-check with ring mode.
Can I measure without having a ring?
Yes — that is what finger mode is for. Slide the two guides to the edges of the finger at the knuckle and read the size. The app itself warns that a finger is soft and oval, so the method is approximate.
Which country sizing systems does it convert between?
US and Canada, UK and Australia, EU and ISO, Japan, China and Russia, in both directions. Search the full table by a size you know or by a diameter in millimetres, and set a default system so every reading appears in the one you order in.
Is a phone ring sizer accurate enough to order an engagement ring?
For a plain band, on a calibrated screen, cross-checked against a ring that already fits, it is a reasonable basis for an online order. For a wide or comfort-fit band, have a jeweller confirm it — a resize costs less than a return.
Which phones does True Ring Size run on?
On iOS it requires version 13.0 or later and is listed for 130 device models, covering iPhones several generations old. The Android build is on Google Play. Any device works better after calibration, because screen density varies enormously between models.
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