How to measure your ring size at home โ accurately, before you order that surprise ring
Measuring on screen, step by step
- On the Measure on screen tab pick a real object to calibrate with: a credit card (85.6 mm), or a 500, 100 or 10 won coin (26.5, 24 and 18 mm).
- Hold the object against the red outline on the screen and adjust with the slider, the โ10 ยท โ1 ยท +1 ยท +10 buttons or a two-finger pinch until the outline matches its edges. The line underneath shows the current ratio, for example "85.6mm = 320px".
- Press It matches. The pixels-per-millimeter value is saved in your browser and the page moves to step 2; next time it skips straight there.
- Set your ring on the circle and adjust with the slider, the ยฑ0.1 ยท ยฑ0.5 buttons or a pinch until the circle touches the ring's inner edge. The results update as you go.
- Read the six result cards โ Korea/Japan size, inner diameter, circumference, US, EU/ISO and UK โ then use Copy result or Share result. Recalibrate takes you back to step 1 on a different device.
Three ways to find your ring size
- Measure a ring on screen โ when you already own a ring that fits. First hold a credit card or coin against the display and match the outline; this calibrates screen pixels to real millimeters (every phone has a different pixel density, so this step is essential). Then place the ring on the circle and fit it to the inner edge to read the size. Accuracy is about ยฑhalf a size.
- From circumference โ when you have no ring. Wrap string or a paper strip around the finger. The ring has to pass over the knuckle, so measure that too and go with the larger number. Fingers swell in the evening and in heat and shrink in the morning and cold, so measure two or three times and average.
- Size chart โ convert between US, EU/ISO, UK and Korean/Japanese sizes when shopping abroad.
Why calibrate first
A browser does not know how many millimeters wide your screen is or how many pixels make up an inch. All it can tell is roughly whether the display is high-resolution, so the page starts with a guess โ about 3.5 pixels per millimeter on high-density screens, 3.8 otherwise โ and draws the card outline at that scale. The moment you match the outline to a real card or coin, the exact ratio for this device is known and every circle drawn afterwards is a trustworthy millimeter measurement. The ratio is stored locally, so you calibrate once per device; changing the browser zoom level or opening the page on another device or browser means calibrating again. The circumference tab does not depend on screen size at all and needs no calibration.
How ring sizes are defined
Korean and Japanese sizes are based on inner diameter: size 1 = 13.0 mm, growing about 0.33 mm per size (about 1.05 mm in circumference), so inner diameter = 12.67 + size รท 3. US sizes follow inner diameter = 11.63 + 0.81 ร US size, and EU/ISO sizes are simply the circumference in millimeters. Women typically fall in KR 7โ13 (US 4โ7), men in KR 15โ22 (US 7โ10), but hands vary a lot, so treat those as rough guides.
UK sizes are letters: an inner diameter of 12.04 mm is A, each letter adds 0.4 mm and each half size (ยฝ) 0.2 mm. The Korea/Japan result is rounded to the nearest whole size, and when the calculated value sits 0.3 or more away from a whole number the card adds "between 11 and 12". On the circumference tab you enter either the circumference or the diameter and the other is filled in; values outside an 8โ30 mm inner diameter are ignored. A 52 mm circumference, for instance, gives a 16.6 mm diameter, KR/JP between 11 and 12, US 6 and EU 52.
Conversion chart
| KR/JP | Inner ร mm | Circumference mm | US | EU/ISO | UK |
|---|---|---|---|---|---|
| 3 | 13.7 | 42.9 | 2.5 | 43 | E |
| 5 | 14.3 | 45.0 | 3.3 | 45 | Fยฝ |
| 7 | 15.0 | 47.1 | 4.2 | 47 | Hยฝ |
| 9 | 15.7 | 49.2 | 5.0 | 49 | J |
| 11 | 16.3 | 51.3 | 5.8 | 51 | Kยฝ |
| 13 | 17.0 | 53.4 | 6.6 | 53 | Mยฝ |
| 15 | 17.7 | 55.5 | 7.4 | 56 | O |
| 17 | 18.3 | 57.6 | 8.3 | 58 | Pยฝ |
| 19 | 19.0 | 59.7 | 9.1 | 60 | Rยฝ |
| 21 | 19.7 | 61.8 | 9.9 | 62 | T |
| 23 | 20.3 | 63.9 | 10.7 | 64 | Uยฝ |
The Size chart tab lists every size from 1 to 30. Because these values come from the diameter formula, a brand's own printed chart may differ by about half a size at the boundaries.
Tips for choosing
Wide bands (5 mm and up) feel tighter at the same size, so go up a half or full size. If you are between sizes, choose the smaller one in winter and the larger one in summer. Most jewelers resize once or twice for free after purchase, so when in doubt, slightly larger is the safer pick. Your measurement is stored only in this browser and never sent anywhere.
Frequently asked questions
Can I measure on a desktop monitor?
Yes โ calibrating with a card against the monitor works the same way. Just keep the browser zoom fixed: changing it alters the pixel size and invalidates the calibration, so set your zoom first, calibrate, and measure without touching it.
The result says "between 11 and 12" โ which do I pick?
Your inner diameter falls roughly midway between two sizes. Weigh the season and the band width as described above; if the jeweler offers free resizing, the larger size is the cheaper mistake. Brands that make half sizes can simply supply an 11.5.
Another chart shows a different size
This page uses inner diameter = 12.67 + size รท 3 and charts round at different decimal places, so boundary values can differ by about half a size. Comparing by inner diameter in millimeters avoids the discrepancy.
I pressed "It matches" too early
Press Recalibrate next to the step 2 instructions to return to step 1. Match the outline again and confirm; the new ratio replaces the old one.
Related tools
- Unit Converter โ turn an inch measurement from a shop listing into millimeters for the circumference tab
- Day Counter & Date Calculator โ count the days to the anniversary so the ring arrives in time