Printing A4 on Letter Shrinks It to 94 %

A0 is a square metre, and everything follows from that

The series was defined by two conditions: each sheet has the proportion √2, and the largest one has an area of one square metre. Solve those and A0 comes out at 841 × 1189 mm, which is 0.999949 m² once the sides are rounded to whole millimetres. Halving A0 gives A1, halving A1 gives A2, and the shape never changes — which is why an A4 page enlarges onto A3 with no white bars and why photocopiers have a 141 % button and a 71 % button rather than an arbitrary dial. It also makes paper weight trivial to reason about: a sheet of A4 is one sixteenth of a square metre, so 80 grams per square metre means an A4 sheet weighs five grams, and a 500-sheet ream weighs 2.5 kilograms.

Exporting at the right pixel size

A page has no pixels until a resolution is chosen, which is why the same A4 is 2480 × 3508 at 300 dpi, 1240 × 1754 at 150, and 595 × 842 at 72. That last pair is worth recognising: 595 × 842 is the size a PDF reports for an A4 page, because a PDF point is one seventy-second of an inch. An image intended to fill an A4 page in print therefore needs to be at least 2480 pixels across; supplying one at 1240 halves the effective resolution and the print shows it. Letter at the same 300 dpi is 2550 × 3300 — wider and shorter in pixels for the same reason it is wider and shorter in millimetres.

Frequently asked questions

How do I print an A4 document on Letter without shrinking it?

Set the scaling to 100 % — often called "actual size" — and accept that 17.6 mm will be lost from the bottom, or reformat the document for the sheet you are actually printing on. For anything where size carries meaning — a scale drawing, a cutting template, a form that must align with something physical — the second option is the only correct one, and it is worth doing before the file leaves your hands rather than after someone prints it wrong.

What is the C series for?

Envelopes. Each C size is slightly larger than the A size of the same number, so the paper goes in: C4 at 229 × 324 mm takes a flat A4, C5 at 162 × 229 takes an A4 folded once, and C6 at 114 × 162 takes it folded twice. The B series sits between the two and is used where something has to be slightly larger than an A size — book covers, some posters, and envelopes that must hold a C-size envelope.

Why is Legal so much longer?

Legal keeps Letter's width and adds three inches of height, reaching 355.6 mm — 58.6 mm taller than A4. It exists for documents where the point is to fit more onto one sheet without a second page, and it is why a contract printed on Legal and reprinted on A4 either loses its foot or reflows onto an extra page. A document that must survive both should be laid out for the shorter of the two.