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Guide

Paper sizes: A4, US Letter and printing across both

A4 and US Letter are close enough to look interchangeable and different enough to ruin a print run. A4 is narrower and taller; Letter is wider and shorter. If you have ever had a header vanish or a footer creep onto a second page, this mismatch is usually why.

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Two systems built on different logic

The A series is defined by a ratio. Every sheet has sides in the proportion 1 to the square root of two, which means cutting one in half across the long side produces the next size down with the same proportions. A4 is 210 × 297 mm, A3 is exactly twice its area, and A5 exactly half. In inches, A4 is 8.27 × 11.69.

US Letter comes from tradition rather than mathematics. At 8.5 × 11 inches — 216 × 279 mm — it has no consistent ratio with its neighbours, so Legal and Tabloid do not scale cleanly from it the way A sizes do.

The practical consequence is a 6 mm width difference and an 18 mm height difference. Printing an A4 document on Letter loses just over a centimetre and a half from the bottom unless your driver scales to fit; going the other way leaves a wider margin at the foot. Setting the page size before you start writing avoids the problem entirely.

Key facts

A4 dimensions

210 × 297 mm, which is 8.27 × 11.69 inches.

US Letter dimensions

8.5 × 11 inches, which is 216 × 279 mm.

The difference

Letter is 6 mm wider and 18 mm shorter than A4.

The A-series ratio

Sides sit at 1:√2, so halving any sheet gives the next size with identical proportions.

A3 and A5

A3 is 297 × 420 mm; A5 is 148 × 210 mm.

Bleed allowance

Commercial printing needs 3 mm of bleed beyond the trim on every edge.

How to get it right

1

Set the size first

Choose A4 or Letter in your document setup before writing, not at print time.

2

Match the destination

Use the standard of the country the document is going to.

3

Convert when specifying

Multiply millimetres by 0.03937 for inches, or inches by 25.4 for millimetres.

4

Scale rather than crop

If you must print across standards, enable scale-to-fit so nothing is lost.

Questions people ask

8.27 × 11.69 inches, from 210 × 297 mm. It is often rounded to 8.3 × 11.7.

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