Envelope Size Guide

Match paper to the right envelope

Envelope sizing trips people up because the labels (DL, C5, #10) don't obviously map to paper. This guide lists the common envelope sizes in millimetres and inches and shows which paper size each one fits and how to fold it. Whether you're sending a letter, a card, or an unfolded document, you'll find the right envelope here.

The C-series and DL

European envelopes follow the C-series, designed around A-series paper. A C-size envelope holds the matching A-size sheet unfolded, or a larger sheet folded down. DL is the everyday business envelope sized for an A4 sheet folded in three.

• DL (110x220 mm): A4 folded in three; standard business letters.
• C6 (114x162 mm): A4 folded twice, or an A6 card.
• C5 (162x229 mm): A4 folded once, or an A5 sheet flat.
• C4 (229x324 mm): A4 unfolded; documents and brochures.

US envelope sizes

• #10 (4.125x9.5 in): the standard US business envelope; Letter folded in three.
• A7 (5.25x7.25 in): invitations and 5x7 cards.
• 6x9 in: documents folded once or small catalogues.
• 9x12 in: Letter-size documents unfolded.

How to use this guide

1. Decide whether you'll send the paper flat or folded.
2. Find the envelope that fits. Flat needs a same-or-larger size, folded uses a smaller one.
3. Follow the folding note (in three for DL/#10, in half for C5).
4. Leave a few millimetres of clearance so the contents slide in easily.

Notes and limitations

• Envelopes are a touch larger than the paper they're named for, to allow easy insertion.
• Thick contents (several sheets or card) need a slightly larger envelope than a single sheet.
• Window envelopes have a fixed address position. Check your letter's layout lines up before printing a batch.

The folding arithmetic

Most envelope confusion is really a folding question, and the C-series makes it easy: each size holds the next paper size down, unfolded.

• C4 — A4 flat, no folding. For documents that must not be creased.
• C5 — A4 folded once (in half), or A5 flat.
• C6 — A4 folded twice (into quarters), or A6 flat.
• DL — A4 folded into three. The standard business envelope, and the one that fits a window envelope's address panel.

DL is the odd one out: it is not part of the C-series and it takes the trifold, which is why business letters are laid out with the address block positioned to appear in the window after that specific fold.

Window envelopes and where the address goes

A window envelope only works if the address lands behind the window after folding, and that depends on the layout of the page rather than the envelope.

For a DL window envelope with A4 trifolded, the recipient's address block normally sits roughly 45–90mm from the top of the page and inset from the left margin. Most word processors' letter templates already position it correctly — the trouble usually comes from a custom layout or an added logo pushing everything down.

Always fold and check one before printing a run. It costs one sheet and saves discovering that two hundred letters show a blank window.

Note that window position varies between manufacturers, so a layout that works with one brand may not with another.

Weight, thickness and postage

Choosing the envelope is only half of it — postage is usually priced on a combination of size, thickness and weight, and the thickness limit catches people out more often than the weight.

A few sheets in a DL envelope is typically the cheapest category. Add a folded booklet or a small item and the same envelope can move into a more expensive bracket by thickness alone, even though it is well under any weight limit.

Rigid items are a separate case: many carriers treat anything that will not flex as a parcel regardless of dimensions.

Because these brackets differ by country and change over time, weigh and measure a made-up sample and check your own postal operator's current table rather than assuming.

Choosing the right style

Beyond size, three practical choices affect whether the envelope does its job.

Seal. Self-seal is quicker for a run of a few; peel-and-seal is more secure and worth it for anything important. Gummed is cheapest and slowest.

Opening edge. Wallet envelopes open on the long edge, pocket envelopes on the short edge. For C4 and larger, pocket style is easier to fill with a thick document.

Paper weight. 80–90gsm is standard; 100gsm or more resists tearing and stops dark print showing through, which matters for anything confidential.

For posting photographs or card, a board-backed envelope prevents bending and is cheaper than replacing the contents.