Envelope Sizes Table
| USPS class | Min (L × H × T) | Max (L × H × T) | Max weight |
|---|---|---|---|
| Postcard | 5 × 3.5 × 0.007 in | 6 × 4.25 × 0.016 in | — |
| Letter | 5 × 3.5 × 0.007 in * | 11.5 × 6.125 × 0.25 in | 3.5 oz |
| Flat (large envelope) | exceeds any letter max | 15 × 12 × 0.75 in | 15.996 oz |
*A letter longer than 6.125 in or taller than 4.25 in needs a 0.009 in minimum thickness. Length is the side parallel to the address line.
| Envelope | Size | Metric | Carries |
|---|---|---|---|
| DL | 4.3 × 8.7 in | 110 × 220 mm | A4 folded twice (thirds) |
| C6 | 4.5 × 6.4 in | 114 × 162 mm | A6 flat / A4 in quarters |
| C6/C5 | 4.5 × 9 in | 114 × 229 mm | A4 folded in thirds |
| C5 | 6.4 × 9 in | 162 × 229 mm | A4 folded once |
| C4 | 9 × 12.75 in | 229 × 324 mm | A4 flat |
| C3 | 12.75 × 18 in | 324 × 458 mm | A3 flat |
| #10 | 4.125 × 9.5 in | 105 × 241 mm | US letter folded in thirds |
| A2 | 4.375 × 5.75 in | 111 × 146 mm | 4.25 × 5.5 in card |
| A6 | 4.75 × 6.5 in | 121 × 165 mm | 4.5 × 6.25 in card |
| A7 | 5.25 × 7.25 in | 133 × 184 mm | 5 × 7 in card |
Envelope size is a postage decision before it is a stationery decision: the United States Postal Service sorts by machine, and a piece only stays at letter rate while it fits the letter window - no more than 11.5 inches long, 6.125 inches high and 0.25 inch thick, and no heavier than 3.5 ounces. Cross any one of those lines and the same sheet of paper rides as a flat, at a higher rate, in the flat window (up to 15 x 12 x 0.75 inches). The chart below carries the whole USPS retail ladder from postcard to flat with the exact DMM 101 limits, then the ISO 269 C-series that wraps A-series paper without folding, and the US stationery names that never quite match either system.
The two systems answer different questions. USPS sizes are a shipping classification: they tell you what an envelope costs to send, and the cheapest bands are surprisingly tight - a #10 business envelope (4.125 x 9.5 inches) fits the letter window with room to spare, but a square invitation is nonmachinable on sight because its aspect ratio is 1.0, below the 1.3 minimum the sorters require. ISO sizes are a folding decision: C4 takes an A4 sheet flat, C5 takes A4 folded once, DL takes A4 folded twice in thirds - the C-series is literally sized as the air gap around each A-series sheet. US A-sizes (A2, A6, A7) ignore both ladders; they exist for cards and invitations and are named in the order they were stocked, not any geometric rule.
How to use
- Classify before you buy postage: measure length (the side parallel to the address), height, and thickness at the thickest point - a single bulky liner or wax seal can push a letter-sized envelope over the 0.25 inch line and into flat pricing.
- Check the aspect ratio too: length divided by height must fall between 1.3 and 2.5 or the piece is nonmachinable (extra postage). Square envelopes fail automatically at 1.0; so does a narrow tall piece.
- Match the envelope to the fold, not the printer: A4 goes flat into C4, once into C5, twice into DL; US letter (8.5 x 11) folds in thirds for a #10. Buying the envelope first is how letters get creased twice.
Frequently asked questions
What is the largest envelope that still ships at letter rate?
Under USPS retail standards a letter may be up to 11.5 inches long, 6.125 inches high and 0.25 inch thick, weighing no more than 3.5 ounces - and the minimum is 5 x 3.5 inches (with a 0.009 inch thickness floor once you exceed 6.125 x 4.25, because a too-thin piece jams the sorters). Every limit is independent: exceed any single one - length, height, thickness, weight, or the 1.3-2.5 aspect ratio - and the piece moves up to flat postage or a nonmachinable surcharge. That 6.125 inch height line is the one people hit most: it is exactly why #10 envelopes are 4.125 x 9.5 and legal-style envelopes are 4.125 x 9.5 too, both comfortably inside.
What is the difference between C4, C5 and DL envelopes?
It is how many times the A4 sheet is folded. C4 (229 x 324 mm, about 9 x 12.75 in) carries A4 unfolded - the flat document envelope. C5 (162 x 229 mm, about 6.4 x 9 in) carries A4 folded once. DL (110 x 220 mm, about 4.3 x 8.7 in) carries A4 folded twice into thirds - and DL is exactly a third of C5's height and half its length, which is the whole design: the C-series is sized as clearance around each A-series format, so Cn wraps An, Bn wraps Cn, and the paper and envelope ladders never touch. The US equivalent is the #10 envelope for a letter-sheet folded in thirds; there is no exact US counterpart to C4, which is why flat document mailers persist there.
What size is a #10 envelope and what fits in it?
The #10 business envelope is 4.125 x 9.5 inches (about 105 x 241 mm) - the standard bill, statement and letter envelope of the US office. It takes an 8.5 x 11 letter sheet folded in thirds, or a half-fold 8.5 x 5.5 piece, or a #9 return envelope (3.875 x 8.875) nested inside - that #10-outside, #9-insides pair is the classic billing mailer. It sits well inside the letter window (max 11.5 x 6.125), which is one reason it became the default: it mails at the cheapest machine-sortable rate. The US A-series that confuses people - A2 (4.375 x 5.75), A6 (4.75 x 6.5), A7 (5.25 x 7.25) - are invitation sizes with no #10 relationship at all.
Why does a square envelope cost more to mail?
Because the sorting machines cannot orient it. Machinable letters need a length-to-height aspect ratio between 1.3 and 2.5 so the equipment can find the leading edge and face the address; a square invitation is 1.0 and a narrow tall piece fails the other bound, and either way the piece must be hand-cancelled - hence the nonmachinable surcharge. Square envelopes also lack a defined 'top', so the address can sit on any side, which the barcode reader will not forgive. The workarounds are geometric, not expensive: put the square inside a slightly larger rectangular envelope, or design the piece at 5 x 7 (aspect 1.4, machinable) instead of 5 x 5. The chart's aspect column exists so you can check this before the post office does.