A4 (210×297 mm) is the international standard used in most countries, while Letter (8.5″×11″, or 216×279 mm) is the standard in the US and Canada. A4 is slightly narrower and taller — about 6 mm narrower and 18 mm taller than Letter. The area difference is small: A4 is ~623.7 cm² vs Letter at ~602.6 cm² (A4 is ~3.5% larger). A4 follows the ISO 216 √2 aspect ratio (1:1.414), which allows perfect halving without changing proportions. Letter has a 1:1.292 ratio.

ISO 216 is the international standard for paper sizes, defining the A, B, and C series. The A-series starts from A0 (841×1189 mm, area = 1 m²) and halves at each step (A1 = half of A0, A2 = half of A1, etc.). All A-series sheets share the same √2 aspect ratio (1:1.4142). This unique property means folding or cutting an A-series sheet in half always produces the next size with identical proportions — perfect for scaling documents. The standard was adopted in 1975 and is now used worldwide except in North America.

Pixels depend on the DPI (dots per inch) setting. The formula is: pixels = (mm ÷ 25.4) × DPI. For example, A4 at 300 DPI: width = (210 ÷ 25.4) × 300 ≈ 2480 px, height = (297 ÷ 25.4) × 300 ≈ 3508 px. At 72 DPI (screen), A4 = 595 × 842 px. For print, 300 DPI is standard; for high-quality art prints use 600 DPI; for web/screen display use 72 or 96 DPI. Always set your document DPI before creating pixel-based designs.

Letter is 8.5″×11″ (216×279 mm), while Legal is 8.5″×14″ (216×356 mm). They share the same width (8.5″) but Legal is 3 inches (76 mm) taller. Legal paper is traditionally used for contracts, legal documents, and real estate paperwork in the US. The extra length provides more room for detailed terms and signatures. Legal size paper requires specific trays in printers — not all standard printers support it by default.

C-series sizes (defined in ISO 269) are specifically designed for envelopes. The key relationship: a Cn envelope perfectly fits an An sheet without folding. For example, a C4 envelope (229×324 mm) fits an A4 sheet (210×297 mm) with room to spare. A C5 envelope fits A5 or a folded A4. C6 fits A6 or a twice-folded A4. This makes the C-series essential for stationery and mailing. The geometric mean of A and B series with the same number gives the C size: Cn = √(An × Bn).

The US paper size system evolved independently from historical British paper sizes (like Foolscap and Quarto). Letter size (8.5″×11″) was standardized in the early 20th century by the US government and became the de facto standard. While there have been attempts to adopt ISO 216 in the US (notably in the 1970s), the entrenched infrastructure — copiers, printers, binders, filing systems — made switching prohibitively expensive. Today, the US and Canada remain the major economies using Letter, while the rest of the world uses A4.

Architectural paper sizes (Arch A through Arch E) are used primarily for architectural drawings and blueprints in the US. They have aspect ratios of 3:4 or 4:3, making them different from both ISO and ANSI standards. Common sizes: Arch A (9″×12″, 229×305 mm), Arch B (12″×18″, 305×457 mm), Arch C (18″×24″, 457×610 mm), Arch D (24″×36″, 610×914 mm), and Arch E (36″×48″, 914×1219 mm). Arch D and E are widely used for construction documents and large-format printing.