Browser zoom
Browser zoom scales page content so that text and controls appear larger or smaller. Because the ruler is a web page element, its CSS geometry participates in that scaling. A physical calibration made at one zoom level should therefore not be assumed valid at another zoom level.
For a reproducible calibration, return to 100% zoom before measuring. In Chromium-based browsers this is commonly Ctrl+0; on macOS it is Cmd+0.
Operating-system display scaling
Desktop operating systems can scale interface content so applications remain readable on high-density displays. This can affect the browser's CSS viewport and the relationship between CSS pixels and physical pixels.
The key point is that display scaling is not a property the ruler should “correct twice.” A robust system chooses one final scale value for rendering and derives all measurement output from it.
Rotation and display changes
On a phone or tablet, portrait and landscape orientations expose different runtime geometry. Connecting or disconnecting a monitor can also change the active display context. The safe response is to notice the change and require a fresh estimate or verification rather than silently carrying forward an unrelated physical calibration.
A practical checklist
Use 100% browser zoom, keep the device in the intended orientation, do not switch displays after calibration, and verify a known reference before relying on the ruler for a physical measurement.
When the environment changes, treat the old calibration as stale until it is verified again. This is more conservative, but it prevents a previously correct measurement scale from being applied to a different rendering context.
For the complete physical procedure, read How to calibrate an online ruler.