When a screen or printed ruler does not match

First, identify the ruler

On a screen, a browser cannot infer the physical size of a display. The screen ruler needs a reference match and reports the state and error band. A printed ruler is drawn for a physical sheet; the issue is whether the print dialog and printer preserved its size. Its check is on the paper, and does not calibrate the display.

Screen: match the card, then try another reference

1. On the screen ruler choose a clean, flat, undamaged, smooth-edged ID-1 card and the edge that fits. Hold it just above the screen without touching; look square-on. Do not press or slide the card on the glass.
2. Adjust the outline to the selected edge and confirm the match. The screen now has a measured scale; the stated error band is conditional on the reference and alignment assumptions. Gap, tilt, screen curvature and other unmeasured errors are outside that band.
3. If a sheet of A4 fits your display, use the ruler's second-reference controls and visually match its long edge in the same way, without touching the screen. The readout reports how far the two measurements differ. The site calls them cross-checked when they agree within 1%.
4. If they disagree, repeat both alignments, verify that the card and paper are the stated objects, and check the displayed state. A disagreement above 1% does not identify which reference is wrong; both measurements have uncertainty. Do not use either as a precise result until you understand the difference.

A card long-edge match carries a stated ±1.2% band under the site's assumptions; the A4 long-edge check carries ±1.0%. The full derivation and limits are in calibration and reference objects. A successful check shows agreement; it cannot establish that unknown errors cancel.

Worked arithmetic: what one card match means

Suppose the card's 85.60 mm long edge matches an outline 340 CSS pixels wide. This is an illustration, not a reading from your device: 340 ÷ 85.60 = approximately 3.972 CSS pixels per millimeter. A 100 mm span would be drawn at about 397.2 CSS pixels at that scale. The underlying reference and alignment uncertainty still apply; more displayed digits do not make the physical match more precise.

If the same scale is checked against the A4 long edge, 297 mm would correspond to roughly 1,179.7 CSS pixels. A second physical match close to that value is a useful comparison only if the sheet fits and can be aligned without touching or flexing against the display. The tool's comparison uses both measurements, not a guessed pixel value.

If it changes after zoom or a display move

Read the calibration state again after changing browser zoom, operating-system scaling or displays. Chrome, Edge and Firefox often expose page-zoom changes through the pixel ratio; Safari and macOS scaled displays can report different signals. The ruler uses available signals and may ask for a new match, but it cannot detect every monitor swap, mirroring or magnifier. A new screen can have the same reported resolution and a different physical size. When in doubt, match again and repeat the second check.

Paper: a 100 mm bar that measures 97.4 mm

The printable-ruler page documents a test in which a Letter sheet sent to A4 came out with the 100 mm bar at 97.4 mm. That is 97.4% of the intended length, or 2.6% short. This is an example of an observed print setting, not a prediction for every printer.

Choose the paper size shown on the sheet and set the print dialog to Actual Size or 100%, not Fit or Shrink. Print a fresh sheet, then use its card vernier, or measure its 100 mm bar with a trusted physical ruler. If the check fails, correct the paper or scaling settings and print again before using the ruler. A screen calibration cannot repair a scaled print. See the printable ruler and its checks.