A4, Letter, Legal and A5

The four sizes

Dimensions are width by height, upright, in millimetres.

Paper	Width (mm)	Height (mm)
A4	210	297
A5	148	210
Letter	215.9	279.4
Legal	215.9	355.6

Legal shares Letter’s width of 215.9 mm and is the tallest of the four at 355.6 mm. A5 is the smallest, and its height, 210 mm, is exactly A4’s width.

Why A4 and Letter are the awkward pair

A4 is 210 mm wide and 297 mm tall. Letter is 215.9 mm wide and 279.4 mm tall. A4 is the narrower of the two and the taller; Letter is the wider and the shorter.

That is the whole problem. Neither fits inside the other. Lay A4 on Letter and it overhangs at the foot; lay Letter on A4 and it overhangs at the side. There is no orientation in which one sits inside the other, so a print dialogue asked to fit one onto the other has only one move available to it, which is to shrink the page until it does fit — and the ruling shrinks with it.

The shrink is not hypothetical. An A4 sheet fitted onto Letter comes out at 94.1 per cent of its drawn size, and a Letter sheet fitted onto A4 at 97.3 per cent. A 5 mm spacing becomes 4.7 mm in the first case and 4.86 mm in the second. The way round it is not to argue with the dialogue but to draw the sheet on the paper that is actually in the tray, which the press will do either way.

Which paper each sheet is drawn on

Of the twenty-five published sheets, twenty-four start on A4. One — squared paper at 5 mm — is also published on Letter, at 215.9 by 279.4 mm. One lined sheet is drawn landscape on A4, which is 297 mm wide by 210 mm tall.

The starting paper is only a starting point. Every published sheet can be set to any of the four on the page itself, and the sheet is redrawn from the new dimensions. The cell counts change when it does, because the paper is one of the numbers the drawing is made from: squared paper at 5 mm holds 38 columns by 55 rows on A4, which is 2,090 squares, and 39 columns by 51 rows on Letter, which is 1,989. Same ruling, same 5 mm, a different sheet.