Tool
Envelope finder
Pick your paper size and how you plan to fold it. The tool works out the finished dimensions and lists every standard envelope that takes it, with the clearance you get in each.
Finished size
210 × 99 mm
A4 folded in thirds
Comfortable fit
At least 8 mm spare in both directions.
- C4324 × 229 mm+114 × +130 mm spare
- C5229 × 162 mm+19 × +63 mm spare
- C6/C5229 × 114 mm+19 × +15 mm spare
- DL220 × 110 mm+10 × +11 mm spare
Tight fit
It closes, but insertion is fiddly and thick contents may not go in.
- US #10104.8 × 241.3 mm+6 × +31 mm spare
Figures are external envelope dimensions; the usable interior is 1–2 mm smaller because of the seam and glue line. Envelope reference pages: DL, C5, C6, US #10.
Window envelopes
If the envelope has a window, fitting the paper is only half the problem — the address has to land behind the opening once the sheet is folded. These are the positions the stationery trade quotes, measured from the envelope’s own edges.
| Envelope | Convention | Window | Position |
|---|---|---|---|
| DL | Continental Europe (DIN 680) | 90 × 45 mm | 20 mm from the left edge, 15 mm up from the bottom |
| DL | United Kingdom | 89 × 39 mm | 20 mm from the left, 53 mm up from the bottom — higher than the DIN position |
| C5 | Europe | 100 × 35 mm | 22 mm from the left, 52 mm up from the bottom |
| US #10 | North America | 4½ × 1⅛ in (114 × 29 mm) | ⅞ in from the left, ½ in up from the bottom |
Test one before running the batch
The tolerance is a few millimetres, and two things routinely eat it: a letterhead laid out for the other market’s window position, and a print dialog silently shrinking the page by 3–6%. Print one sheet, fold it as you intend to fold the rest, and put it in a real envelope. Our calibration test rules out the scaling half of that in one page.
Frequently asked questions
Where does the window sit on a DL envelope?
It depends on the market, which is the trap. The continental DIN 680 position places a 90 × 45 mm window 20 mm from the left edge and 15 mm up from the bottom; the common UK position uses an 89 × 39 mm window sitting noticeably higher, 53 mm up. A letterhead laid out for one convention shows an empty window in the other, so the address block, not the envelope, is usually what needs moving.
How do I make sure the address lands in the window?
Print one sheet, fold it exactly as you intend to fold the batch, and slide it into a real envelope. That single test catches everything the measurements cannot — fold accuracy, printer scaling, a letterhead built for another market. It matters more than usual here because the tolerance is a few millimetres and a misaligned window ruins the whole run rather than one sheet.
Does printing at the wrong scale affect window alignment?
Yes, and it is the most common cause of a batch that nearly works. Fit to page shrinks the sheet by 3–6%, which moves an address block sitting 60 mm down the page by two to four millimetres — enough to clip it against the window edge. Print at Actual size and verify with our calibration test before committing to a mailing.
How much bigger than the paper should an envelope be?
Around 6–10 mm larger in each direction is the comfortable range: enough to slide the sheet in without bending a corner, tight enough that the contents do not shift in the post. Below about 2 mm of clearance you are fighting the paper, and machine-inserting becomes impossible. This tool marks anything with 8 mm or more as a comfortable fit and flags the tight ones separately.
Which envelope takes an A4 letter folded in thirds?
DL — 220 × 110 mm — which is why it is the default business envelope across Europe and most of the world. The tri-folded sheet measures 210 × 99 mm, leaving roughly 10 mm each way. C6/C5 (229 × 114 mm) takes the same fold with more room, and is the usual choice when the letter includes an insert or a reply envelope.
And for a US Letter folded in thirds?
US #10 (104.8 × 241.3 mm), the American business standard. The folded sheet is 215.9 × 93.1 mm, so it goes in sideways with about 12 mm of clearance on the short dimension. A #9 envelope holds the same fold more tightly and is the traditional reply envelope tucked inside a #10.
Can I send an A4 sheet without folding it at all?
Yes — C4 (324 × 229 mm) holds A4 flat, which is what you want for certificates, contracts that will be scanned, photographs, or anything that should not carry fold lines. Expect to pay a large-letter postage rate rather than the standard letter rate in most countries, and consider a board-backed envelope if the contents must stay rigid.
Does the fold direction matter?
For the sizes here, folds are assumed parallel to the short edge — the way a letter is normally folded, reducing the height and keeping the width. Folding the other way gives different dimensions and usually a worse envelope match, because standard envelopes are proportioned for the conventional fold.
Why is my envelope listed as fitting but marked tight?
Because the numbers here are external envelope dimensions, and the usable interior is a little smaller — the seam and the glue line eat a millimetre or two. A match with 3 mm of nominal clearance will physically close, but inserting is fiddly and thick contents may not fit at all. When you have a choice, take the comfortable option.