Envelopes for Paper Sizes: Which Envelope Fits A4, Letter, Legal and Cards
Start with the paper you are sending and how you will fold it, and the envelope follows. This guide matches every common sheet, card and photo to its envelope, shows the folds that make it fit, and gives the address positions that line up with window envelopes.
US Letter (8.5 × 11 in) folded in thirds fits a #10 envelope (4 1/8 × 9 1/2 in), and flat it fits a 9 × 12 in envelope. A4 (210 × 297 mm) folded in thirds fits a DL (110 × 220 mm), folded in half fits a C5 (162 × 229 mm) and flat fits a C4 (229 × 324 mm).
- Letter, in thirds
- #10 envelope
- Letter, flat
- 9 × 12 in
- A4, in thirds
- DL 110 × 220 mm
- A4, flat
- C4 229 × 324 mm
On this page
Envelope for each paper size
Most mail goes in one of three ways: flat, folded once (half fold) or folded in thirds (letter fold). The table gives the envelope that fits each with normal clearance. Sizes in brackets are the folded packet.
| Paper | Flat | Folded once | Folded in thirds |
|---|---|---|---|
| US Letter 8.5 × 11 in | 9 × 12 in | 6 × 9 in or A9 (5.5 × 8.5 in) | #10 or #9 (8.5 × 3 2/3 in) |
| US Legal 8.5 × 14 in | 10 × 15 or 11 1/2 × 14 1/2 in | 9 × 12 in (8.5 × 7 in) | #14, or #12 snug (8.5 × 4 2/3 in) |
| Half Letter 5.5 × 8.5 in | 6 × 9 in or A9 | A2 (4.25 × 5.5 in) | #6 3/4 (5.5 × 2.83 in) |
| Monarch / Executive 7.25 × 10.5 in | 9 × 12 in | 6 × 9 in | #7 3/4 Monarch (7.25 × 3.5 in) |
| A4 210 × 297 mm | C4 (229 × 324 mm) | C5 (148 × 210 mm) | DL or C6/C5 (210 × 99 mm) |
| A5 148 × 210 mm | C5 | C6 (105 × 148 mm) | C6/C5 (148 × 70 mm) |
| A3 297 × 420 mm | C3 (324 × 458 mm) | C4 (210 × 297 mm) | – |
| Tabloid 11 × 17 in | 12 × 18 in | 9 × 12 in (8.5 × 11 in) | – |
A4 and Letter cross over well: A4 folded in thirds fits a #10 with plenty of room, A4 fits a 9 × 12 in envelope flat, and Letter fits a C4 flat. The tight pairing is Letter in a DL: the folded packet is 215.9 mm long and the DL is 220 mm, which leaves too little play for anything thicker than two or three sheets. For every envelope size in one chart, see the envelope size chart.
How to fold paper for an envelope
Letter fold (C-fold) for #10 and DL
- Fold the bottom third up. For Letter the panel is 3 2/3 in (93 mm); for A4 it is 99 mm. Most printed letters have a faint fold mark at the left edge to guide you.
- Fold the top third down over it, so the letterhead ends up on the outside flap.
- Insert the open edge down into the envelope, with the top flap facing the back of the envelope. The reader opens the envelope and the letter unfolds the right way up.
Z-fold for window envelopes
For a window envelope the address must face out. Fold the bottom third back (away from you) and the top third forward, so the sheet forms a Z with the address panel on the outside. Insert it with the address facing the window.
Half fold and quarter fold
A single fold suits C5 and 6 × 9 in envelopes and keeps documents flatter, which matters for forms that will be scanned. A quarter fold (in half, then in half again) turns Letter into a 4.25 × 5.5 in card for an A2 envelope, the usual way to make a folded greeting card from a single sheet.
Card stock of roughly 170 gsm (65 lb cover) and heavier tends to crack along the fold unless it is scored first. Run a blunt point along a ruler on the fold line, then fold toward the scored side.
Address position for window envelopes
US #10 window envelopes
The most common #10 window is 1 1/8 × 4 1/2 in, starting 7/8 in from the left edge and 1/2 in from the bottom. With a Letter sheet folded in thirds, the top panel is 3 2/3 in tall, so that window shows the part of the letter about 2 to 3 1/8 in down from the top edge.
- The folded letter can ride up to about 1/2 in higher than that (the envelope is 4 1/8 in tall), and slide up to 1 in sideways. The band that stays visible in every position is roughly 2 1/2 to 3 1/8 in from the top of the sheet.
- Set the recipient's address in that band, starting about 1 in from the left edge, and keep it to four lines of 10 to 12 pt text and under about 3 1/2 in wide.
- Window positions vary between manufacturers. Print one page on plain paper, fold it and hold it behind the envelope before a large run.
DIN 5008 letters for DL and C5 window envelopes
European business letters on A4 follow DIN 5008, which places the address field so it lines up with standard DL and C5 window envelopes (window sizes and positions are set in DIN 680).
| Measurement | Form A | Form B |
|---|---|---|
| Letterhead height | 27 mm | 45 mm |
| Address field (85 × 45 mm) starts | 27 mm from top | 45 mm from top |
| Address field from left edge | 20 mm | 20 mm |
| First fold mark | 87 mm from top | 105 mm from top |
| Second fold mark | 192 mm from top | 210 mm from top |
| Hole-punch mark | 148.5 mm from top | 148.5 mm from top |
Form B is the usual choice for business letters because the taller letterhead leaves room for a logo. Folding at the marks gives panels of 105, 105 and 87 mm (Form B) or 87, 105 and 105 mm (Form A), which fit a DL window envelope with the address field behind the window. The hole-punch mark sits at the midpoint of the sheet's height, matching the two-hole filing pattern described in the A4 paper guide.
Envelopes for cards, invitations and photos
Cards and photos should go in unfolded, so the envelope is chosen from the item's size plus about 1/8 in (3 mm) in width and 1/4 in (6 mm) in length.
| Item | Size | Envelope |
|---|---|---|
| RSVP card | 3.5 × 5 in | A1 or 4 Bar (3 5/8 × 5 1/8 in), snug |
| Thank-you / note card | 4.25 × 5.5 in | A2 (4 3/8 × 5 3/4 in) |
| 4 × 6 photo or postcard | 4 × 6 in | A6 (4 3/4 × 6 1/2 in) |
| 5 × 7 card or photo | 5 × 7 in | A7 (5 1/4 × 7 1/4 in) |
| Folded Letter card | 5.5 × 8.5 in | A9 (5 3/4 × 8 3/4 in) |
| 8 × 10 photo | 8 × 10 in | 9 × 12 in with a card stiffener |
| A6 card (ISO) | 105 × 148 mm | C6 (114 × 162 mm) |
| A5 card (ISO) | 148 × 210 mm | C5 (162 × 229 mm) |
| Square card | 5.75 × 5.75 in or 140 × 140 mm | 6 × 6 in (152 mm) square |
For the print sizes themselves and the frames they go in, see photo print sizes.
How much paper fits before postage goes up
The envelope size decides the price category, and the folded thickness and weight decide whether you stay in it.
- US First-Class letter, one stamp (1 oz): a sheet of 20 lb Letter paper weighs about 4.5 g and a #10 envelope about 5 to 6 g. Four sheets stay under 28.35 g; five is borderline. The letter category tops out at 3.5 oz and 1/4 in thick.
- US flats: a 9 × 12 in envelope is a flat (large envelope) whatever it contains, so mailing Letter paper unfolded costs more than folding it into a #10.
- UK Royal Mail Letter: up to 100 g and 5 mm thick in a DL, C6 or C5. An A4 sheet of 80 gsm paper weighs 5 g, but thickness runs out first: folded in thirds, each sheet adds about 0.3 mm, so roughly 15 sheets is the practical limit.
- UK Large Letter: A4 sent flat in a C4 is a Large Letter (up to 750 g and 25 mm thick).
Full postal size limits, including the US aspect-ratio rule that penalizes square envelopes, are on the envelope sizes page. Paper dimensions for every sheet mentioned here are in the paper sizes chart, and A5 booklet formats are in the A5 size guide.
Frequently asked questions
What envelope do I need for a full sheet of paper without folding?
For Letter (8.5 × 11 in), a 9 × 12 in envelope. For A4 (210 × 297 mm), a C4 (229 × 324 mm). Both also work for the other paper: A4 fits a 9 × 12 in envelope with 8 mm to spare in length, and Letter fits a C4.
Does A4 paper fit in a #10 envelope?
Yes. A4 folded in thirds makes a 210 × 99 mm packet (8.27 × 3.9 in), which fits a #10 (105 × 241 mm) with room to spare. Letter folded in thirds also fits a DL, but only with about 4 mm of play in length.
What size envelope fits a 5 × 7 photo?
An A7 envelope (5 1/4 × 7 1/4 in) holds a single 5 × 7 print. For protection in the mail, use a 6 × 9 in envelope with a piece of card behind the photo, since an A7 leaves no room for a stiffener.
What envelope fits Legal-size paper?
Folded in thirds, an 8.5 × 14 in sheet becomes 8.5 × 4 2/3 in, which fits a #14 (5 × 11 1/2 in) comfortably and a #12 (4 3/4 × 11 in) snugly. Flat, it needs a 10 × 15 in or 11 1/2 × 14 1/2 in envelope.
How many sheets can go in a #10 envelope for one stamp?
About four sheets of 20 lb Letter paper. Each sheet weighs about 4.5 g and a #10 envelope about 5 to 6 g, so a fifth sheet brings the total to the 1 oz (28.35 g) limit of a single First-Class stamp. Weigh anything borderline.