Pocket workshop tool
Mobile ruler
Turn your phone screen into a short ruler. First match the calibration line to a real UK 1p coin. Screen pixels alone cannot tell us a physical length.
01 / Set the scale
Match a 1p coin
The Royal Mint specifies a 1p coin diameter of 20.3 mm. Place the coin flat against your screen, away from a case lip. Adjust the line below until its ends match the coin’s widest width.
Use the slider or the buttons for one-pixel changes. Check both ends from directly above the screen.
Not calibrated. The ruler is hidden until you set the scale.
02 / Measure
Read from zero
Hold the object along the scale with one end at the zero line. Each short tick is 1 mm; numbered lines are 10 mm apart.
This is a quick estimate for small, flat items. Recheck the coin before measuring. Use a physical rule or calliper for drill bits, fixings, blade fit or any safety-critical size.
Why calibration matters
A CSS millimetre does not reliably equal a physical millimetre on a phone screen. The W3C explains this screen-unit limitation. This tool calculates its scale from the coin length you set, saves that choice only in this browser, and asks you to check again when the screen size, orientation or browser zoom changes.
The ruler is limited by the exposed screen area and the width of the coin alignment. Screen protectors, viewing angle and finger placement can add error. No camera or account is needed.
Source check: Royal Mint coin specification and W3C screen-unit guidance, 4 October 2026.