Envelope finder

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.

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.

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.

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.

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.

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.