A4 vs A5 vs A6 Envelope Sizes Compared

Envelope sizes and paper sizes don’t share names by coincidence. The international C-series envelope system was built specifically to pair with A-series paper.

A C4 envelope (229 x 324mm) holds an A4 sheet completely flat. A C5 envelope (162 x 229mm) holds that same A4 sheet folded once in half. A C6 envelope (114 x 162mm) holds it folded twice, into quarters.

Once you know that logic, picking the right envelope for any folded or unfolded paper size stops being guesswork.

The full C-series envelope size chart

DL isn’t technically part of the C-series. It comes from a separate “Dimension Lengthwise” standard.

Still, it’s grouped with C-series envelopes constantly, since it does the same job as the C6/5 at a slightly narrower width. It’s the envelope you get handed at a bank, or that a utility bill arrives in.

What the difference actually looks like in practice

A C4 is genuinely large. A full A4 sheet slides in without a single fold. That’s why C4s get used for contracts, certificates, and anything where fold creases would look wrong.

Compared side by side, a C5 runs almost exactly half the length of a C4 (229mm vs. 324mm on the long edge). That’s literally what it’s sized to hold: an A4 sheet folded once.

A C6 is half of a C5 again (162mm vs. 229mm), matching an A4 sheet folded twice — first in half, then in half again.

Laid on a table, a C6 envelope comes close to the size of a folded restaurant menu or a standard birthday card. A C5 is close to a small notepad. A C4 is close to a magazine.

The DL sits in an odd spot dimensionally. It’s narrower than a C6/5 by 9mm but shares the same overall shape. Both are designed to hold an A4 sheet folded into thirds, like a standard tri-fold letter.

In practice, DL is the one you’ll actually encounter most often. It’s the default envelope for printed business correspondence throughout Europe, the UK, and Australia.

Which envelope size suits which use

C4 (229 x 324mm) suits anything that shouldn’t be folded: certificates, legal documents, photographs, or a full A4 report where creases would look unprofessional or interfere with scanning.

It’s also used to mail an A4 document alongside smaller enclosures — a cover letter plus a folded brochure, for instance — without folding the primary document.

C5 (162 x 229mm) is the middle ground. It’s big enough to send a few A4 sheets folded once without heavy bulk. It’s also small enough to fit standard letter racks and mailboxes sized around A5-ish dimensions. It’s common for invoices, multi-page letters, or greeting cards paired with A5 stationery.

DL (110 x 220mm), the tri-fold size, is the default for single- or few-page business letters. The folded shape fits comfortably in a jacket pocket or a standard filing system. It also uses less paper-to-envelope ratio than a C5 would for the same one-page letter.

C6 (114 x 162mm) suits short notes, greeting cards, invitations, and anything naturally small rather than a folded-down large sheet. An A6 postcard or a folded A5 card sits in a C6 without extra folding.

How folded paper maps to envelope size

This is the part most people are actually trying to work out. Which envelope does my folded letter need?

• A4 sheet, no fold → C4 envelope
• A4 sheet, folded once (in half) → C5 envelope
• A4 sheet, folded twice (in quarters) → C6 envelope
• A4 sheet, folded in thirds (standard tri-fold) → DL or C6/5 envelope
• A5 sheet, no fold → C5 envelope
• A5 sheet, folded once (in half) → C6 envelope
• A6 sheet, no fold → C6 envelope.

This pattern holds because each A-series size runs exactly half the area of the one before it. A4 is half of A3, A5 is half of A4, A6 is half of A5.

Each C-series envelope gets sized with a small margin around whichever paper size it’s meant to receive at that fold count.

Common mistakes when choosing envelope sizes

Confusing the C-series with the US envelope system is the single most common error for anyone ordering internationally. The US doesn’t use A-series paper — it uses Letter, 8.5 x 11in — or C-series envelopes.

It uses its own numbered system instead. A #10 envelope (4.125 x 9.5in) is the rough equivalent of a DL, sized for a Letter sheet folded in thirds.

The two systems aren’t interchangeable. A DL envelope runs narrower and shorter than a #10, so a US Letter sheet folded for a #10 envelope won’t sit flush inside a DL.

Mixing up C6/5 and DL is another frequent mix-up, since they hold the same folded document and look nearly identical at a glance. The 9mm width difference (114mm vs. 110mm) rarely matters for the letter itself.

It matters plenty for window envelopes, though, where an address printed for one size can end up misaligned with the window cut into the other.

Ordering by “the envelope I’ve always used,” without checking the paper size inside it, causes recurring problems for anyone switching printers or letterhead.

A redesigned letterhead that’s now A5 instead of A4 needs a C6 or C5 envelope. The DL that worked for the old A4 tri-fold layout won’t fit anymore.

Frequently Asked Questions

What size is a C6 envelope compared to A4 paper?
A C6 envelope (114 x 162mm) holds an A4 sheet folded twice, into quarters. That’s a quarter of the flat A4 area once folded down. It’s why C6 is the standard “small note” or greeting card envelope size.

Does a C5 envelope fit an A4 sheet?
Yes, folded in half once. A C5 envelope (162 x 229mm) is sized specifically to receive an A4 sheet (210 x 297mm) folded along its long edge. A small margin lets the folded sheet slide in without resistance.

What’s the difference between a DL and a C6/5 envelope?
Both are designed for an A4 sheet folded into thirds. DL runs 110 x 220mm; C6/5 runs 114 x 229mm — DL is slightly narrower and shorter. DL is far more common in everyday business mail; C6/5 shows up mostly in specific printing and window-envelope contexts.

Can a C4 envelope hold an A3 sheet?
Only if the A3 sheet gets folded in half first. A3 measures 297 x 420mm, twice the area of A4. A C4, sized for flat A4, needs that A3 sheet folded once to fit — the same relationship as A4-to-C5.

What US envelope size is equivalent to a DL?
A #10 envelope, 4.125 x 9.5in or about 105 x 241mm, is the closest US equivalent. Both get designed to hold a business letter folded in thirds. They’re not identical dimensions, though. A #10 is sized around US Letter paper (8.5 x 11in), not A4 (210 x 297mm) — which runs slightly narrower and longer.

Is A6 the same size as a C6 envelope?
Not quite. A6 paper measures 105 x 148mm, and a C6 envelope measures 114 x 162mm — a few millimeters larger on each edge. That margin is intentional. An envelope needs to run slightly bigger than the paper it holds, so the sheet slides in and out without catching.

Do A5 envelopes exist, separate from the C-series?
Not really, as a standard called “A5 envelope.” The C5 envelope is what people usually mean by that phrase, since it’s sized to hold an A5 sheet flat.

Some UK stationery brands do sell envelopes labeled “A5” that are actually C5-dimensioned. That causes exactly the kind of naming confusion this comparison aims to clear up.