For an ordinary US Letter sheet folded into thirds, start with a #10 envelope, measuring 4⅛ × 9½ inches. For a flat 5 × 7-inch card, start with a US A7 envelope, measuring 5¼ × 7¼ inches. These are size matches to investigate, not guarantees that a thick stack or decorated card will slide inside.
Choose the envelope around the finished contents. Measure a card when closed, a letter after folding, and an invitation with every insert included. Then check usable space, closure and mailing requirements separately. A name on a packet cannot answer all three questions.
Common US envelope sizes
The dimensions below are nominal envelope sizes, with the shorter edge first. They are not measurements of the usable interior. The named sizes match the Envelopes.com size reference; the examples explain how you might shortlist them.
| Envelope | Size in inches | Starting point to test |
|---|---|---|
| #6¾ | 3⅝ × 6½ | Small folded correspondence |
| #9 | 3⅞ × 8⅞ | Reply enclosure inside a larger mailing |
| #10 | 4⅛ × 9½ | US Letter folded into thirds |
| #11 | 4½ × 10⅜ | Longer enclosure needing more room than #10 |
| #12 | 4¾ × 11 | Long, narrow paperwork |
| #14 | 5 × 11½ | Larger narrow enclosure |
| A2 | 4⅜ × 5¾ | 4¼ × 5½-inch closed card |
| A6 | 4¾ × 6½ | 4½ × 6¼-inch closed card |
| A7 | 5¼ × 7¼ | 5 × 7-inch closed card |
| A8 | 5½ × 8⅛ | 5¼ × 7⅞-inch flat insert |
| A9 | 5¾ × 8¾ | 5½ × 8½-inch closed card |
Numbers in these names are identifiers, not inch measurements. A #10 is not ten inches long. Also check how a seller orders the dimensions: an opening on the short edge behaves differently from an opening on the long edge, even when the rectangle has the same size.
Work out the folded size before choosing
A US Letter sheet is 8½ × 11 inches. Folding its 11-inch direction into three equal sections creates a rectangle about 3⅔ × 8½ inches. Against a #10 envelope’s nominal exterior, that leaves roughly 0.458 inch in the short direction and one inch in the long direction. Those differences explain the common pairing, but seams and paper thickness reduce practical clearance.
Folding the same sheet in half instead produces 5½ × 8½ inches. A 6 × 9-inch envelope is a possible starting point for that folded letter. Keeping the entire 8½ × 11-inch sheet flat calls for a larger candidate, such as 9 × 12 inches. Neither comparison accounts for a document sleeve, backing board or several pages, so measure the assembled contents again.
When folding by hand, aim for neat folds without forcing an exact mathematical third. The panel tucked inside may need to be slightly shorter so it does not buckle. Measure the result rather than assuming every fold lands in the same place.
Allow room for cards, inserts and thickness
A 5 × 7-inch card and a 5¼ × 7¼-inch A7 envelope differ by ¼ inch in each overall dimension. If centered, that is only ⅛ inch per opposite edge before accounting for construction. It is not an extra ¼ inch on every side, and it is not a measured internal allowance.
Try the card with its liner, reply card and any other enclosure. Multiple layers occupy depth as well as width and height. The envelope must curve around the stack, which uses some of the space that looked available when everything lay flat. There is no universal allowance that makes every paper weight and flap design fit.
Use a sample envelope. Contents should enter and come out without curled corners, snagging or pressure on the seams. Close the flap normally and inspect for bulging. If it pulls tight, choose a roomier size or packaging designed for that thickness rather than compressing the contents to make the flap reach.
C4, C5, C6 and DL envelopes
For A-series paper, the following common metric envelope sizes are verified in Blake’s envelope-size guide. DL is a separate designation; do not treat it as another C-number. The content column describes the intended paper arrangement, subject to the same thickness and sample-fit checks.
| Envelope | Nominal size in millimeters | Paper arrangement |
|---|---|---|
| C4 | 229 × 324 | A4 kept flat |
| C5 | 162 × 229 | A5 flat, or A4 folded once |
| C6 | 114 × 162 | A6 flat, or A4 folded into quarters |
| DL | 110 × 220 | A4 folded into thirds |
An A4 sheet folded into exact thirds measures 99 × 210 mm, compared with DL’s 110 × 220 mm exterior. The differences are 11 mm and 10 mm overall, not on each side. Actual folds and envelope construction still matter.
US A7 envelopes and ISO A7 paper use different naming systems. A US A7 envelope is 5¼ × 7¼ inches; ISO A7 paper is 74 × 105 mm. For the paper dimensions behind these comparisons, see the A4 and A-series paper reference. Order by the numeric dimensions as well as the name.
A good fit still needs a postal check
USPS classification considers the completed mailpiece, including its dimensions, weight, shape, flexibility and thickness. An envelope that holds a card comfortably does not automatically qualify for ordinary letter postage. Square shapes, rigid contents and protruding decorations can change handling or price.
Check the current USPS letter and envelope instructions before mailing. For an unusual or bulky assembly, take a finished sample to the counter before preparing a batch. Keep rigid objects out of ordinary paper letter envelopes; ask about suitable package options when protection is needed.
If shopping, browse mailing envelopes on Amazon with your measured dimensions beside you. Check the opening, stated capacity and quantity before ordering. One successfully packed sample is more useful than a large box chosen only by its size name.
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