Envelope Sizes Australia: C4, C5, C6 and DL Explained

Ordering envelopes for an organisation should be one of the easier purchasing jobs, yet the letter-and-number labels trip a lot of people up. Which one holds an A4 sheet flat? Which one takes a folded letter? What does DL even stand for? This guide clears it up for the four sizes Australian schools, councils, government departments and offices use most, and shows how to match each one to what you are sending. Every size covered here is stocked in the Buyecogreen recycled envelope range.

How Australian Envelope Sizes Work

Australian envelope sizes follow the ISO 216 international standard, the same system behind A-series paper. That link is what makes the whole thing logical once you see it. The C-series is built to hold the A-series: a C4 envelope holds an A4 sheet unfolded, a C5 holds an A4 folded once (or an A5 sheet flat), and a C6 holds an A4 folded twice (or an A6 sheet flat). Each size down the range is half the one above it, and the envelopes nest the same way the paper does, so a C6 slips inside a C5 and a C5 inside a C4.

DL is the odd one out. The letters stand for Dimension Lengthways, and it is sized to hold an A4 sheet folded into thirds. It does not fit the neat halving pattern of the C-series, but it is the standard business letter envelope across Australia, which is why most correspondence goes out in one.

C4, C5, C6 and DL at a Glance

When you match contents to an envelope, allow roughly 10mm of clearance so paper slides in cleanly and the seal is not under strain.

C4 Envelopes (229 x 324mm)

A C4 holds a full A4 sheet without a single fold, which makes it the size for anything that should arrive flat and uncreased: annual reports, certificates, printed booklets, brochures and board papers. Because there is no folding, C4 is also the practical choice when you are posting several A4 pages together or including a stiff insert that should not bend. The Buyecogreen C4 recycled envelope is a pocket-style envelope (the opening is on the short edge, so A4 slides straight in) supplied in boxes of 250, and it carries Australian Forestry Standard certification on carbon neutral paper.

C5 Envelopes (162 x 229mm)

A C5 sits between the DL and the C4. It holds an A4 sheet folded once down the middle, or an A5 sheet flat. The single fold is its main advantage: a document that will be read the moment it is opened has fewer crease lines to fight, which matters for certificates, recognition letters and invitations where presentation counts. The Buyecogreen C5 recycled envelope is a wallet-style envelope in natural unbleached eco brown kraft, made in Australia from 100% post-consumer waste paper.

C6 Envelopes (114 x 162mm)