Paper Size Chart & Pixel Calculator

In short The ISO A series is built on a single elegant rule: every sheet has an aspect ratio of 1:√2, so cutting one in half across the long side gives exactly the next size down with the same proportions. A0 is defined as exactly one square metre, and everything else follows — A4 is 210×297 mm because it is A0 halved four times. That is why scaling a document between A sizes never leaves a margin mismatch, and why a photocopier's 71% reduction fits A3 onto A4 exactly. US paper sizes have no such rule. Letter is 215.9×279.4 mm — wider and shorter than A4 — and the two are close enough that a document laid out for one will usually print on the other while quietly clipping or reflowing, which is the source of most cross-border printing complaints. The pixel dimensions matter for anything you intend to print from a digital file: A4 at 300 DPI is 2480×3508 pixels, and an image smaller than that will be upscaled by the printer driver whether you want it to or not. Design at the print size and DPI from the start; scaling up at the end never recovers the detail.