Paper Size Reference: Dimensions and Ratios

This page is the reference: dimensions, area and aspect ratio for each standard, and the maths behind why the A-series halves the way it does. If you want a sheet’s weight as well, the paper size converter takes a GSM figure alongside the dimensions and covers the C-series envelopes that pair with each A size.

Paper sizes follow two main international standards: the ISO 216 series (used in most of the world) and the ANSI/ASME series (used primarily in the United States and Canada). Understanding how they relate helps when printing, scanning, or designing documents for international distribution.

ISO 216 formula (the elegant math behind A-sizes): Each A-size is exactly half the next larger size. The ratio of long side to short side is always √2 ≈ 1.41421: Width × √2 = Length A(n) dimensions = A(n−1) dimensions ÷ √2

This means A4 folded in half becomes A5. A3 folded in half becomes A4. The area halves at each step while the proportions stay identical, which is the whole point: scaling between A sizes never crops or letterboxes anything.

Common sizes:

The B series sits between the A sizes and is easy to forget. B5 (176 × 250 mm) is the common book and notebook size, and B4 (250 × 353 mm) turns up in Japanese printing. Each B size is the geometric mean of the A sizes either side of it, so B4 falls between A4 and A3. It keeps the same 1:√2 shape.

Key differences: A4 vs. US Letter:

• A4 is taller and narrower; US Letter is shorter and wider.
• A4: 8.27 × 11.69 inches. Letter: 8.5 × 11 inches.
• Printing a US Letter document on A4: content may be cut off at the bottom unless scaled.
• Printing an A4 document on US Letter: small white bands appear on sides.

Worked example (scaling): You have an A4 layout (210 × 297 mm) and need to print on US Letter (216 × 279 mm). Scale to fit: width scale = 216 ÷ 210 = 102.9%, height scale = 279 ÷ 297 = 93.9%. Height is the limiting factor, so scale to 93.9% and nothing gets clipped.

Tip: Design in A4 if your audience is international. It is the default for business documents, academic papers, and legal correspondence everywhere outside North America.