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The envelope problem
You have a letter, a card or a folded brochure, and a drawer of envelopes. The question is simple: what is the smallest envelope it fits in? Guessing wastes envelopes and postage, and forcing a tight fit creases the contents. Envelopes follow standard series, mainly the C series, where each size is designed to hold a paper size folded a set number of times. This matcher turns your paper size and fold into the smallest fitting envelope.
How envelope sizes work
The C series is built around the A paper series. C4 holds an unfolded A4 sheet, C5 holds an A5 sheet or an A4 sheet folded once, and C6 holds an A6 sheet or an A4 sheet folded twice. DL is the long, narrow envelope for business letters: it holds an A4 sheet folded into thirds, the classic letter fold. Each step up roughly halves the area, so the series forms a neat ladder from C4 down to C6, with DL sitting beside it for tri-folded letters.
How to use this matcher
Pick your paper size, choose how it will be folded, and set a small clearance so the contents slide in without force. The tool folds the paper on paper, so to speak: a half fold halves the long side, a C-fold or Z-fold divides it into thirds, and a roll fold into quarters. It then checks each envelope in the series, in both orientations, and reports every envelope that fits plus the smallest one. Three millimetres of clearance is a good default; use more for thick or multi-page contents.
C-fold versus Z-fold
Both folds divide the sheet into three panels, so both fit the same envelopes, but they open differently. A C-fold, also called a letter fold, wraps one panel inside the other like the letter C, and it is the standard for business letters because it opens naturally from the top. A Z-fold zigzags like the letter Z, so each panel opens in turn, which suits brochures and leaflets where the reader unfolds step by step. For envelope size they are identical; for reading experience they are not, so choose by how the piece will be opened.
Thickness changes the answer
This matcher works with flat dimensions, but real contents have thickness. A single sheet slides into a snug envelope; a ten-page letter, a card with an insert, or a brochure on heavy stock needs more room, and the fold itself gets bulkier at the spine. If the fit is exact to the millimetre, go one envelope size up. Postage also rises with thickness and weight in many postal systems, so a larger envelope that still fits the same postage band is often the better deal.
Paper sizes beyond A4
US Letter is slightly wider and shorter than A4, so it folds to slightly different dimensions and sometimes lands in a different envelope. The matcher includes US Letter for that reason. Small cards and invitations often fit C6 without folding, while large posters and certificates need C4 unfolded. When in doubt between two sizes, test with a real envelope and a real folded sheet: paper, unlike this calculator, has thickness, and a physical test settles borderline cases in seconds.
Common mistakes
The most common mistake is folding against the grain or with uneven panels, which makes the folded sheet wider than planned and breaks the fit. Another is forgetting clearance and ending with an envelope that needs force. A third is using a DL envelope for a half-folded A4, which is too short. Fold evenly, allow clearance, and match the fold to the envelope series, and the letter will slide in first time.
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