A4 is an ISO 216 size of exactly 210 × 297 mm. US Letter is based on exactly 8.5 × 11 inches, equivalent to 215.9 × 279.4 mm because one inch is exactly 25.4 mm. Letter is wider and shorter than A4, so the two formats have different proportions. PaperSize keeps those source units visible instead of presenting one as a rounded copy of the other.
Compare the physical dimensions
In portrait orientation, Letter is 5.9 mm wider than A4, while A4 is 17.6 mm taller than Letter. Rotating either sheet only swaps its own width and height; it does not remove the difference. A document designed edge-to-edge for one format may reflow, gain margins, or lose content when printed on the other.
The pixel canvases differ too
At 300 PPI, A4 is approximately 2480 × 3508 pixels after nearest-whole-pixel rounding. Letter is exactly 2550 × 3300 pixels at the same PPI because its inch dimensions multiply to whole numbers. A canvas for one is therefore not a drop-in replacement for the other. PaperSize calculates from each standard’s exact physical base before rounding once.
Choose the format required by the destination
Use the format named by the printer, form, institution, or recipient. Do not choose A4 or Letter only because it is locally familiar. Neither standard defines a universal printable area, bleed, safe zone, or borderless result; those depend on the device and provider. PaperSize compares sheet and canvas dimensions but leaves production allowances to the relevant specification.