How to measure your ring size at home with your phone

Can I measure ring size with my phone?

Yes, but only once you give the phone a real reference. A ring size boils down to one number — the inside diameter of the band, in millimetres — and a screen only draws an honest millimetre when it knows exactly how many pixels fit into one on your device.

The app starts from a lookup table of known screen densities. If a device isn't on it, Calibration guesses, and the guess can be corrected by eye against a steel ruler. That's really the whole trick: compare the on-screen marks to a physical ruler, nudge a slider until they agree, and every circle you draw afterward gets converted into real millimetres.

Step 1: calibrate the screen

1. On the home screen, open Calibration.
2. Choose Millimeters (mm) or Inches (in) to match the ruler you have.
3. Lay a physical ruler flat on the screen, with its zero on the on-screen zero.
4. Move the slider until the on-screen marks line up with the ruler's marks along the whole length. The value labelled Factor changes as you go (it starts at 1.00 and can go from 0.50 to 1.50).
5. If you get lost, tap Reset to Default and start again. The Dark Mode switch on this screen can make the marks easier to see against the ruler.

That factor sticks. Every tool that draws in millimetres reuses it, so the only time you'll see this screen again is on a new phone.

Step 2: measure a ring you already own

1. Open Ring Sizer and place a ring that fits the target finger flat on the screen.
2. Drag the slider (or pinch) until the drawn circle sits exactly on the inner edge of the band.
3. Read the top of the screen: diameter, circumference, US, UK, Swiss/EU and Asian size.
4. Tap Save Measurement to keep the result in History.

Leaving Use Fixed Ring Sizes off lets the circle drift past the edge of the band without warning. Switching it on under Settings snaps the slider to real table sizes instead of drifting freely; there's also Ring width for a thinner circle line and Grid Size mm for the background grid, which most people leave at the default.

Step 2 (no ring): measure your finger

No ring handy? Open Finger Sizer instead, lay the finger flat on the screen and widen the two bars until they just graze both sides at the base, where the ring will actually sit. Same readout, same Save Measurement button, just a finger instead of metal.

Worked example: a ring with an 18.5 mm inside diameter

Is a ring sizer app accurate?

Within about half a size, once calibrated. Both things that decide it are in your hands, not the app's: how carefully you calibrated, and whether you matched the inside edge rather than the outside. The app has no way to grade either step.

• Match the inside edge — the outside adds the band's thickness twice and inflates the size.
• Look straight down at the screen, not from an angle.
• Use a round, unbent ring, ideally close to the width of the one you're buying.
• Measure in the evening at room temperature.
• Measure twice on different days and compare what you saved.

Questions and answers

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