Paper Sizes Explained: A4 vs Letter, and Every Size in mm, Inches and Pixels

A4 is 210 × 297 mm (8.27 × 11.69 in); US Letter is 215.9 × 279.4 mm (8.5 × 11 in). They look interchangeable but they are not — Letter is about 6 mm wider and 18 mm shorter than A4 — which is why a document laid out for one clips at the edge or leaves uneven margins on the other. Everything else follows two simple ideas: ISO sizes (A, B, C) all share a 1:√2 ratio so halving a sheet gives the next size down, while US, ANSI and Architectural sizes are their own fixed set. Pixels are never a fixed number — they are inches × DPI.

A4 vs US Letter: the difference that trips people up

The rest of the world uses A4 (210 × 297 mm); the US and Canada use Letter (8.5 × 11 in, or 215.9 × 279.4 mm). The gap is small but real:

	Width	Height	Area
A4	210 mm	297 mm	623.7 cm²
US Letter	215.9 mm	279.4 mm	603.2 cm²
Difference	Letter +5.9 mm	Letter −17.6 mm	A4 is ~3% larger

Because Letter is wider and shorter, there is no scale factor that maps one onto the other — printing an A4 PDF on Letter paper (or vice versa) either crops content or adds white space. When it matters, set your document’s page size explicitly rather than trusting “Fit to page.” The paper size converter shows both side by side in every unit.

The ISO system: one ratio, endless sizes

Every ISO 216 A and B size shares the same shape: a width-to-height ratio of 1:√2 (about 1:1.414). That single choice has a neat consequence — cut any sheet in half across its long edge and the two halves keep the exact same ratio, so they are simply the next size down:

• A0 841 × 1189 mm → A1 594 × 841 → A2 420 × 594 → A3 297 × 420 → A4 210 × 297 → A5 148 × 210 …

A0 is defined as having an area of one square metre, which is why its sides are those slightly odd numbers. The B series (B0 = 1000 × 1414 mm) sits between the A sizes and is used for posters and books, while the C series (C4, C5, C6, DL) sizes envelopes to hold folded A-series letters — a C5 envelope fits an A5 sheet, or an A4 folded once.

mm, inches, points and pixels

A paper size is a physical thing (millimetres), but you often need it in other units:

Unit	How it’s derived	A4	US Letter
Millimetres	the standard	210 × 297 mm	215.9 × 279.4 mm
Inches	mm ÷ 25.4	8.27 × 11.69 in	8.5 × 11 in
Points	inches × 72	595 × 842 pt	612 × 792 pt
Pixels @ 96 DPI	inches × 96	794 × 1123 px	816 × 1056 px
Pixels @ 300 DPI	inches × 300	2480 × 3508 px	2550 × 3300 px

The one that catches people out is pixels. There is no single “A4 in pixels” — it is inches × DPI. Use 96 DPI for on-screen or CSS work, 150 DPI for drafts, and 300 DPI for print-quality artwork. Set the DPI in the paper size converter and it computes the exact pixel canvas for you.

US large-format: ANSI and Architectural

For engineering and architecture, the US uses two large-format families. ANSI runs A to E, where ANSI A is exactly US Letter and each step up roughly doubles the area (ANSI B = 11 × 17 in, the “Tabloid” size). Architectural (Arch A–E) is a parallel set with slightly different proportions favoured by architects. Both are in the converter’s chart alongside the ISO and US sizes.

Quick answers

• Landscape vs portrait: just swap width and height — A4 landscape is 297 × 210 mm.
• Business documents: A4 (or Letter in North America) for letters; A5 for booklets; DL or C5 envelopes.
• Print artwork: design at 300 DPI so the pixel canvas matches the physical size — an A4 flyer is 2480 × 3508 px.

Pick any size, orientation and DPI in the Paper Size Converter and read it back in millimetres, centimetres, inches, points and pixels — all in your browser, nothing uploaded.