Envelope Size Guide

The labels do not tell you anything

DL. C5. Number 10. None of those names says what paper goes inside, which is why the wrong envelope gets bought roughly every time.

This is the chart. Sizes in millimetres and inches, what fits each one flat, and what fits folded.

The C-series is easier than it looks

One rule covers it: a C-size envelope holds the matching A-size sheet unfolded, or a larger sheet folded down to fit.

• C4, 229 × 324 mm. A4 flat, no crease anywhere, which is what you need for a certificate, a photograph, or anything that will be scanned at the other end.
• C5, 162 × 229 mm. A4 folded once, or A5 flat.
• C6, 114 × 162 mm. Quarters, or an A6 card.
• DL, 110 × 220 mm. A4 folded into three. The everyday business envelope.

DL is the odd one out and worth understanding, because it is not part of the C-series at all. It takes the trifold, and the trifold is why business letters put the address block where they do: it has to land behind the window after that specific fold and no other.

US sizes

• #10, 4.125 × 9.5 in. The standard business envelope. Letter folded in three, the direct equivalent of DL.
• A7, 5.25 × 7.25 in. Invitations, and 5 × 7 cards.
• 6 × 9 in. Letter folded once, or a small catalogue.
• 9 × 12 in. Letter flat, unfolded, uncreased.

Every envelope is a few millimetres larger than the paper it is named for, because otherwise nothing would go in. Add a second or third sheet and that clearance disappears faster than you expect, so a document that just fits as one sheet will fight you as four.

Window envelopes, and the sheet you should waste first

A window envelope works only if the address lands behind the window after folding, and that is a property of your page layout rather than of the envelope.

For DL with A4 trifolded, the recipient's address usually needs to sit roughly 45 to 90 mm down the page and inset from the left margin. Word processor letter templates get this right on their own; the trouble comes from a custom layout, or from a logo that pushed everything down by fifteen millimetres.

Fold one and check it before printing a run. It costs a single sheet, and the alternative is discovering that two hundred letters have a blank window and a name hidden behind card.

Window position also varies between manufacturers, so a layout that worked with last year's boxes may not work with this year's.

Thickness costs more than weight

Postage is priced on some combination of size and thickness and weight, and it is nearly always the thickness that catches people out.

A few sheets in a DL envelope sits in the cheapest bracket. Add a thin folded booklet and the same envelope can jump a price band on thickness alone while remaining comfortably under any weight limit, which is a surprise at the counter and a worse one on a mailing of three hundred. Rigid items are a separate problem again: many carriers treat anything that will not flex as a parcel whatever its dimensions.

Brackets differ by country and change. Make up a sample, weigh it, measure its thickness, and check your own postal operator's current table rather than trusting a number you remember.

Seal, opening edge, paper weight

Three choices beyond size, and each of them decides whether the envelope does its job.

Seal. Self-seal is quickest for a handful. Peel-and-seal is more secure and worth the money for anything that matters. Gummed is the cheapest and the slowest, and unpleasant in any quantity.

Opening edge. Wallet envelopes open along the long edge, pocket envelopes along the short one. At C4 and above, pocket is much easier to fill with a thick document.

Paper weight. 80 to 90gsm is standard. 100gsm and up resists tearing and stops dark print showing through from inside, which matters for anything confidential. For photographs or card, a board-backed envelope costs less than replacing what it was protecting.