How to use
- Pick an object: choose something with a known size, like a credit or debit card, ID, coin, or battery.
- Place it on screen: lay it flat in the center, face up.
- Drag the frame: drag the corner handles so the frame matches the object’s edges exactly (aspect locked).
- Finish: tap “Done” and the ruler shows at true scale.
Tip: keep the phone steady while reading.
Why calibrate
Most on-screen rulers assume a default pixel size, but your screen’s real pixel density may differ — its “1 cm” is often not a true 1 cm. This tool uses a real object (a credit or debit card is 85.6 mm wide) as a reference to compute your screen’s true pixel density, then draws the ruler at true scale.
Measure accurately
Accuracy depends on display pixel density and system zoom. After calibrating with a real object, the ruler shows at true size; just hold the object next to the on-screen scale and read it off. Rotate the phone to measure longer objects.
Unit conversion
Fill any one box and the others convert automatically. 1 in = 2.54 cm, 1 cm = 10 mm.
FAQ
Is the on-screen ruler accurate?
After calibrating with a real object it shows true size for the current screen and zoom level; before calibration it is only approximate, because pixel densities differ across screens.
Why do different screens measure differently?
Each screen has a different physical pixel size, and system zoom (for example Windows 125% or 150%) also changes the effective size, so calibration is per device and zoom level.
Do I need to look up my screen’s PPI / DPI?
No. Aligning with a known object avoids the common mistake of entering the wrong DPI.
Why use a phone ruler
Your phone is the one measuring tool you always have with you. This page turns the screen into a ruler, protractor and triangle ruler — nothing to install, no account, and it remembers your calibration for the next visit.
What people measure with a phone ruler
- Parcels and letters — check a package against size limits before you queue at the post office.
- Small hardware — screw and nail lengths, bolt diameters, drill bit sizes.
- Furniture and gaps — whether a gap is wide enough, or a shelf deep enough.
- Screens and bezels — a display diagonal, or the width of a bezel.
- Jewellery and crafts — ring diameters, bead spacing, template layouts.
Getting better accuracy on a phone
- Lay the phone flat and put the object directly on the screen when you can.
- Rotate to landscape for anything longer than the short side — you get a longer measuring span in one go.
- Keep the browser zoom at 100%. Changing zoom after calibration invalidates the scale.
- Prefer a hard, flat, standard-sized object such as a credit or debit card (85.6 mm). Avoid coins and soft or curved items.
- Recalibrate after switching devices or display settings — calibration is per screen and zoom level.
Phone ruler or something else?
- Phone ruler — best when you are away from a desk and the object fits on the screen.
- Desktop on-screen ruler — a larger screen means a longer measuring span in one go.
- Printable ruler — a physical strip you can cut out; better for paper, homework, and anything you would rather not put on a screen.
Is a phone ruler accurate enough for real work?
For everyday tasks — parcels, gaps, small parts, crafts — yes, once you calibrate with a known object. It is not a substitute for calipers when a drawing carries a real tolerance such as ±0.1 mm.
Do I need to install an app?
No. It runs in the browser. Add it to your home screen if you want a shortcut.