Understand the simple measurement calculation
At the initial settings, 216 mm divided by eight columns gives a nominal 27 mm cell width. Dividing 279 mm by twelve rows gives a nominal 23.25 mm height, so the cells are rectangular.
These values describe the selected canvas at its listed paper size. They do not measure your monitor, the browser’s displayed preview, or objects inside a photograph.
A calculated grid still needs print calibration
A printer or document application may scale the exported image to fit within margins. That changes the physical spacing even though the file still contains the same row and column counts.
If a measurement task depends on exact intervals, measure several neighboring cells after printing and adjust the print size accordingly. This tool is a visual planning aid, not a calibrated measuring instrument or technical drawing exporter.