Which envelope fits A4 paper?

If you want to post a sheet of A4 without folding it, you need a C4 envelope. Fold it in half and a C5 will do, and fold it in three, the way most business letters are folded, and it goes in a DL.

That's all you need to know to buy the right envelopes. If you're posting in the UK, though, it's worth reading on before you seal one, because the envelope you choose also decides which stamp you need, and folding a single sheet of A4 can save you £1.50 on a first class stamp.

Why the sizes fit so neatly

The fit is deliberate: the C sizes were designed to go with the A sizes, so for every A sheet there's a C envelope with the same number that takes it with a little room to spare. A4 is 210 × 297 mm and a C4 envelope is 229 × 324 mm, which leaves enough space to slip the letter in easily without it rattling about once it's inside. Fold the sheet in half and you have A5, which fits a C5 in exactly the same way, and fold it again and A6 goes into a C6.

The reason, as Markus Kuhn of Cambridge University explains, is that each C size sits between the A and B sizes with the same number, so an A4 letter fits a C4 envelope, which in turn fits a B4 envelope, should you ever need to post an envelope.

The one that breaks the pattern

A sheet folded in three is a different story. It comes out at 99 × 210 mm, which doesn't match any of the C sizes, so it has an envelope all of its own: the long, slim DL, which at 110 × 220 mm is the most widely used business envelope, even if its name takes some explaining. DL probably began as "DIN lang", German for "DIN long", although, as Kuhn puts it, the international standard for envelopes explains it "more diplomatically as 'Dimension Lengthwise'".

Window DLs need one extra check, because there's no international standard for where the window goes. Before you print the whole batch, fold one letter and hold it up to the envelope.

Why folding saves money

Folding also matters for the price, because Royal Mail charges by size as well as weight, and anything up to 240 × 165 mm and 5 mm thick counts as a standard Letter, as long as it weighs no more than 100 g. A folded sheet in a C5 or a DL fits inside those limits. A C4 doesn't, because at 324 mm long it's always a Large Letter, even with only one sheet inside.

Since April 2026, that has meant paying £3.30 for a first class stamp instead of £1.80, or £1.55 instead of 91p for second class. So if your document doesn't mind a crease, folding it saves you £1.50 first class, or 64p second.

How many sheets you can send

Folding only saves money up to a point, of course, because enough paper will take any envelope over the Letter limits. The sums are easier than you might expect, thanks to the way A sizes are designed. A0 is one square metre and A4 is a sixteenth of it, so a sheet of ordinary 80 gsm office paper weighs 80 ÷ 16, or 5 g. As for thickness, Navigator's 80 gsm paper is 0.11 mm a sheet.

In a DL, each sheet is folded into three layers, so it's the thickness rather than the weight that catches you out. Allow for the envelope itself and you reach the 5 mm limit at about 14 sheets, which only weigh 70 g between them. In a C5, where each sheet is two layers, the 100 g limit comes first, and allowing 10 g for the envelope leaves room for about 18 sheets.

Those are Royal Mail's limits rather than the envelope's, and in practice a DL gives up long before then. PaperDirect says its #10 envelope, which is much the same size as a DL, holds up to six folded sheets. So for a letter of a few pages, fold it and save the money. For anything longer, or anything you'd rather not crease, use a C4 and pay for a Large Letter. If you allow 20 g for the envelope, as Kuhn does, it will take about 16 sheets before it passes 100 g and costs more again.

Posting to America

Write to someone in the US and you'll find they do things differently. Americans write on US Letter, and the most common business envelope there is the #10, 4⅛ × 9½ inches, which takes a sheet of Letter folded in three.

A4 and US Letter are different shapes, but fold either one in three and it will fit the other country's envelope. A4 slips easily into a #10, and US Letter fits a DL with just 4 mm to spare, as long as you fold it straight. The two sides of the Atlantic have never agreed on paper, but their envelopes get on fine.