Paper sizes: Letter, Legal, A4, A3

How it works

Formula: A(n+1) = A(n) folded in half

The A sizes follow the ISO 216 standard: A0 is one square meter, and each size is the previous one folded in half, rounded down to the millimeter. The ratio of the sides stays the same (√2 ≈ 1.414), so an A4 enlarged becomes an A3 with no margins. The US sizes (Letter, Legal, Tabloid) are defined in inches. For printing, pixels are inches × resolution: 8.5 inches at 300 dpi is 2,550 pixels.

• ISO 216:2007 — Writing paper and certain classes of printed matter (A and B series)

Worked examples

• An A4 sheet

  210 × 297 mm, or 8.27 × 11.69 inches.

• A4 at 300 dpi

  2,480 × 3,508 pixels.

• A4 folded in half

  An A5 of 148 × 210 mm.

Common mistakes

• Thinking Letter and A4 are the same size.

  Letter is 215.9 × 279.4 mm: wider and shorter than A4 (210 × 297 mm).

• Thinking A3 is twice as wide and twice as tall as A4.

  An A3 (297 × 420 mm) is two A4 sheets side by side: twice the area, sides × 1.41.

• Preparing a print at 72 dpi.

  For a sharp print, 300 dpi is a common target: an A4 is then 2,480 × 3,508 pixels.

What the calculation leaves out

• Sheet sizes without print margins: most printers leave a few millimeters.

Paper size dimensions

Frequently asked questions