A macOS ruler app. Put a physical object against the screen and read its dimensions off the ruler. Resize the window to whatever you need; click the green button to go fullscreen for measuring larger things.
brew install xcodegen # one-time, if you don't have it
xcodegen generate
open RulerApp.xcodeproj
Then ⌘R in Xcode.
- U — cycle unit: cm → inches → both
- G — toggle 1cm grid
- C — recalibrate the display the window is currently on
The same actions are available as buttons in the bottom-right corner.
On first launch you'll be walked through calibration for any uncalibrated displays. Hold a credit card (long edge, 85.6 mm) flush against the on-screen line and drag the slider until they match. Calibration is stored per display (keyed by CGDisplayCreateUUIDFromDisplayID) and picked up automatically when you move the window between displays. Hit Calibrate in the controls strip (or C) any time to redo it.