Actual Size Online Ruler
The "image" you see below is technically not an image. It is dynamic and adjusts/calibrates itself based on the options you select below.
Is your display different? If so, Select your screen dimension:
Why You Should Enter Your Screen Dimensions (Diagonal Screen Size)
In order to display a ruler in its actual size, two details should be known: screen resolution and monitor size.
Your browser determines and exposes your screen resolutions,
but there is no way to be very sure about your monitor size without explicitly selecting it.
So, when you select your monitor size,
this page recalibrates the above ruler and the result is an accurate ruler in its actual size.
For more information about lenght units, see the official w3.org spec sheet.