A4 vs US Letter: Paper Sizes, Scaling Percentages and Paper Weights

A4 is 210 by 297 mm, US Letter 8.5 by 11 inches. Why neither fits the other, the copier percentages that make them fit, and what gsm and pound paper weights really mean.

Introduction

Two sheets of paper dominate the world, and they are almost but not quite the same size. A4 is the standard across Europe, Asia, Africa, Australia and South America. US Letter is standard in the United States, Canada and Mexico. Put one on top of the other and the mismatch is obvious: neither is a scaled version of the other, which is why a document laid out for one so often prints badly on the other.

The Core Dimensions

So Letter is 5.9 mm wider and A4 is 17.6 mm taller. Each is bigger in one direction and smaller in the other, and that is precisely why no single scaling factor converts one into the other cleanly.

By area, A4 covers 623.7 cm² against Letter's 603.2 cm², so A4 is about 3.4 percent larger overall. In imperial terms that is 96.7 square inches against 93.5.

Why A4 Has That Strange Ratio

A4 is not an arbitrary size. Every sheet in the ISO 216 A series has an aspect ratio of exactly 1:√2, or about 1:1.4142, and that single choice makes the whole system work.

A rectangle with that ratio has a useful property: cut it in half across the long side and both halves keep the same proportions. Nothing else does this. So A5 is exactly half an A4, A3 is exactly two A4s, and every size in the series is the same shape.

The series is anchored at A0, defined as one square metre, which rounds to 841 × 1189 mm. Each step down halves the area:

The idea goes back to a 1786 letter by the physicist Georg Christoph Lichtenberg and was standardised as the German DIN 476 in 1922, which ISO 216 later adopted.

US Letter has a ratio of 11 ÷ 8.5, which is about 1.294. Halve it and you get 8.5 × 5.5 inches, a ratio of 1.545. A different shape. That is the practical difference between the two systems: A-series sizes nest, and US sizes do not.

The Scaling Percentages That Actually Matter

This is the part worth keeping. When you fit one size onto another, the limiting dimension is whichever one has less room, and the scale factor comes from that dimension alone.

The 71 and 141 figures are the two buttons on every photocopier in a country that uses A4, and now you know where they come from. They are not rounded conveniences; they are the square root of two in both directions.

Why a Letter Document Prints Badly on A4

The failure is asymmetric, and knowing which way it breaks tells you what to look for.

A Letter document printed on A4 gains 17.6 mm of height it was not designed for, so content sits high with an odd gap at the bottom, and it loses 5.9 mm of width, which can clip the right edge of a wide table or force a line to wrap.

An A4 document printed on Letter is the more damaging direction. The page is 17.6 mm shorter, so a full page of text loses its last line or two entirely, and a footer can disappear off the bottom without any warning on screen.

The fix is to set the paper size in the document before writing rather than at the print dialogue afterwards, since changing it later reflows every page break.

Paper Weight: gsm and Pounds

The second conversion trap on the same sheet. Metric paper is sold by gsm, grams per square metre, which is a straightforward density. Because A0 is exactly one square metre and A4 is one sixteenth of it, an A4 sheet of 80 gsm paper weighs 5 grams, or 4.99 g once the rounded millimetre dimensions are taken into account. That is a genuinely handy fact when weighing an envelope for postage.

US paper is sold by basis weight in pounds, which is the weight of 500 sheets at an uncut parent size. The catch is that the parent size differs by paper type, so the pound figures are not comparable across types:

Note that 65 lb cover is far heavier than 32 lb bond despite the smaller-sounding difference, because cover stock is measured at a different parent size. When comparing papers, convert to gsm first: it is the only figure that means the same thing for every stock.

Envelopes and Photo Prints

The C series exists to hold the A series, which makes envelope choice mechanical. A C4 envelope takes an unfolded A4 sheet, a C5 takes A4 folded once, and the common DL envelope at 110 × 220 mm takes A4 folded in thirds.

Photo prints follow neither system. A 6 × 4 inch print is 152 × 102 mm with a ratio of 1.5, matching a camera sensor rather than a paper standard. Since A4 is 1.414, printing a standard photo onto A4 paper always means either a border or a crop.

Quick Reference Conversion Table

Common Mistakes Worth Avoiding

Assuming one scale factor works both ways

It does not. A4 onto Letter is 94 percent and Letter onto A4 is 97 percent, because a different dimension limits each direction.

Comparing paper by pound weight across types

Bond, text and cover stocks use different parent sizes, so their pound figures are not on the same scale. Convert to gsm before comparing.

Changing paper size after the document is written

Page breaks, headers and floating images all reflow. Set the size first.

Treating "fit to page" as harmless

It silently rescales, so a drawing printed at a stated scale is no longer at that scale. For anything dimensioned, print at 100 percent and accept the clipping instead.

Conclusion

A4 is 210 by 297 mm and US Letter is 8.5 by 11 inches, and neither fits inside the other without scaling. Remember 94 percent going from A4 to Letter, 97 percent coming back, and 71 and 141 percent for moving up and down the A series. And when comparing paper stocks, convert the weight to gsm before you trust the number.

Try our millimeters to inches converter for quick and accurate conversions.

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