If you’ve ever ordered envelopes in bulk and realised the documents inside don’t quite fit, or the flap won’t close properly because the paper has been folded too many times, you’ll understand how much a seemingly simple detail can disrupt an otherwise smooth administrative process.
For schools managing high-volume correspondence, councils distributing official communications, businesses handling regular invoicing and reporting, childcare centres sending out notices and newsletters, or not-for-profit organisations coordinating community programs, getting envelope sizes right isn’t a minor consideration. It’s a practical necessity.
This guide covers every envelope size used in Australian offices, schools, and institutions, explains the dimensions and what each size is designed to hold, and helps procurement teams and administrators make better-informed choices. It also explores how choosing recycled envelope stock supports your organisation’s broader sustainability goals without changing a single thing about how you mail.
Before getting into the sizes themselves, it’s worth understanding why this decision is worth thinking through carefully.
Ordering the wrong envelope size at scale wastes budget, creates operational delays, and often results in either overstuffed envelopes that seal poorly or documents folded more times than they should be, which affects legibility and presentation. For government communications, school reports, formal notices, or grant-related correspondence, presentation reflects on the professionalism of the organisation sending it.
Envelope size also affects postage. Australia Post applies different rates based on envelope thickness, weight, and format. Choosing the right size from the outset reduces the risk of oversized or overweight mail that attracts higher postage charges across large mail runs.
And increasingly, envelope procurement is being reviewed through the same sustainability lens as paper, packaging, and stationery. Choosing recycled envelope stock is one of the easiest, lowest-disruption improvements an organisation can make to its environmental footprint.
Australian envelope sizes follow the ISO 216 international standard, which is the same system used for A-series paper. The relationship between paper and envelope sizing is straightforward once you understand the logic behind it.
C-series envelopes are designed specifically to hold A-series paper. A C4 envelope holds an A4 sheet unfolded, a C5 envelope holds an A4 sheet folded once (or an A5 sheet unfolded), and a C6 envelope holds an A4 sheet folded twice (or an A6 sheet unfolded). The pattern continues down the range.
DL envelopes (short for Dimension Lengthways) are sized to hold an A4 sheet folded into thirds, making them the standard business correspondence envelope across Australia.
B-series envelopes are slightly larger than their C-series counterparts and are used when additional thickness or a tighter fit around the A-series document is required.
Square envelopes sit outside the standard ISO letter system and are used primarily for greeting cards, invitations, and event communications.
With that foundation in place, here is every size you’re likely to encounter when ordering envelopes for institutional or business use.
The DL is the most widely used envelope in Australian business and institutional settings. Its dimensions are 110mm x 220mm, and it is designed to hold an A4 document folded into thirds lengthways.
This is the envelope your organisation most likely uses for:
The DL is available with or without a window. The window version, which features a clear panel on the front, allows the recipient’s address printed on the document inside to show through without any need for separately addressed envelopes. This makes windowed DL envelopes particularly useful for high-volume administrative mail runs, where addressing each envelope individually would be impractical.
For organisations sending regular correspondence at scale, the DL is the workhorse of the envelope range. It fits standard letter trays, is compatible with most franking and stuffing equipment, and is accepted by Australia Post within standard letter dimensions.
Buyecogreen stocks DL envelopes in recycled stock, including the popular eco brown kraft finish, which signals environmental responsibility in every piece of correspondence your organisation sends.
The C6 measures 114mm x 162mm and holds an A4 sheet folded into quarters, an A5 sheet folded in half, or an unfolded A6 card. It is a compact envelope suited to lighter correspondence and card-based communications.
Common uses in institutional settings include:
The C6 is not suited to multi-page documents, but it is an appropriate size when the communication itself is concise and the priority is presentation over volume. For schools sending home event invitations or thank-you cards to volunteers and sponsors, the C6 offers a neater, more considered format than folding a full A4 sheet into a DL envelope.
Buyecogreen’s recycled C6 envelopes in eco brown kraft are a popular choice for schools, childcare centres, and not-for-profit organisations that want their communications to reflect their values from the outside in.
The C5 is sized at 162mm x 229mm and is designed to hold an A4 sheet folded in half, or an A5 sheet unfolded. It sits between the DL and C4 in terms of capacity and is particularly useful when a document needs to be sent with minimal folding.
Institutions use C5 envelopes for:
One of the practical advantages of the C5 is that it reduces the number of folds required compared to a DL. For documents that will be read immediately upon opening, fewer fold lines makes the content easier to handle and review. For certificates or recognition letters, a C5 avoids the creasing of a DL fold.
The C4 is one of the most important envelopes for institutional settings that regularly send or receive formal documentation. At 229mm x 324mm, it holds a full A4 sheet with no folding required.
This is the envelope of choice when:
For procurement managers and school administrators who regularly despatch formal documentation, having C4 envelopes consistently in stock prevents the temptation to fold documents into smaller envelopes in a pinch, which risks damaging professionally printed or signed materials.
The C4 is also commonly used as the outer envelope when a DL reply envelope is included inside for the recipient to use.
The B4 is slightly larger than the C4 at 250mm x 353mm and is used when additional clearance around an A4 document is needed, or when thicker document packs require a roomier fit.
B4 envelopes are well suited to:
While the B4 is less commonly stocked than C4, it is the practical choice when document bulk is the key consideration rather than simply sheet size.
Square envelopes sit outside the standard letter-size system and are used specifically for cards, invitations, and occasion-based communications. They are not intended for standard correspondence and will attract different postage rates through Australia Post due to their non-standard format.
105mm square envelopes are designed for small gift cards, compliment slips, and compact greeting cards. They are commonly used by schools and community organisations when distributing small acknowledgement cards or seasonal messages.
130mm square envelopes are a versatile mid-range square format suited to standard greeting cards, thank-you cards, and event invitations. This is the most commonly ordered square size for institutional gifting and community communications.
150mm square envelopes offer a more generous format for larger greeting cards, formal invitations, and premium presentation pieces. Not-for-profit organisations and schools running fundraising events or recognition ceremonies often use this size when the quality of presentation reflects the significance of the occasion.
Buyecogreen stocks square envelopes in eco brown kraft recycled stock across all three sizes, giving organisations a sustainable option for every card-based communication in their annual calendar.
Learn more: https://www.buyecogreen.com.au/envelope-sizes-guide/
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