How to fold a letter for an envelope

Which fold fits which envelope

Pick the fold from the envelope you have. The insert sizes below come from envelope manufacturers’ charts, which list the largest card or folded sheet each envelope is made to hold; exact sizes vary slightly by maker.

• Tension: common envelope size charts ↗

How to fold a letter into thirds

The tri-fold, also called a letter fold, is the standard way to fit a full sheet into a No. 10 business envelope. Tension, an envelope manufacturer, lists the largest insert for a No. 10 as 3⅞ × 9¼ inches, so a sheet folded in thirds fits with a little room to spare.

1. Lay the letter face up with the top edge away from you. If there are several pages, stack them in order and square the edges.
2. Fold the bottom edge up until it sits about 3⅔ inches below the top edge, so the bottom third covers the middle third. Line up the sides and crease.
3. Fold the top third down over the folded section, line its edge up with the bottom fold, and crease again.
4. Slide the folded letter into the envelope, press it flat, and seal it.

Which side faces out on a tri-fold letter

With this fold, the printed side is inside and the back of the page is outside. When the recipient lifts the top panel, the letterhead and greeting are the first thing they see, followed by the rest of the page as they unfold the bottom panel.

Three equal panels make a neat result, but they don’t need to be perfect. What matters is that the folded letter is narrower than the envelope’s opening and lies flat, so the envelope stays an even thickness.

For a letter of several pages, fold the stack as one piece instead of folding each page separately. The pages nest together, stay in order, and make one even fold that is easier to slide into the envelope. Run a fingernail or the edge of a ruler along each crease so the letter doesn’t spring open and push against the envelope.

How to fold a letter for a window envelope

A window envelope needs the delivery address on the outside of the folded letter, which a regular tri-fold doesn’t give you. Use a Z-fold, which folds the two outer thirds in opposite directions.

1. Print the address block in the top third of the page, positioned to match your envelope’s window.
2. Lay the letter face up and fold the bottom third up over the middle third. Crease.
3. Turn the folded sheet over and fold the top third back, so the address faces out. Crease.
4. Slide the letter in with the address facing the window, then tap the envelope on each side to check that no part of the address hides behind the paper edge.

• USPS Publication 25: Designing Letter and Reply Mail (PDF) ↗

USPS rules for addresses in a window

USPS Publication 25 sets the window standards. Every character of the delivery address must stay completely visible through the window throughout the full range of movement of the insert. There must be at least 1/8 inch of clear space between the address block and every edge of the window, even when the letter is pushed as far as it can go in each direction.

The address must be printed on white or very light paper, and on a letter-size envelope the window must run parallel to the long edge. If a window envelope came with a form, such as a bill, use the fold the sender intended so the address lands in place.

Folding a letter in half for smaller envelopes

Greeting cards, invitations, and notes usually go in A-style announcement envelopes, which open on the long side. Leader Paper Products, an envelope maker, lists the A2 for inserts up to 4¼ × 5½ inches, a half sheet folded in half; the A7 for a 5 × 7 inch card; and the A9 for inserts up to 5⅝ × 8⅝ inches, which takes a full sheet folded once.

The Envelope Manufacturers Association notes that a 6 × 9 booklet envelope is often used for 8½ × 11 sheets folded in half, so they mail at the lower letter price instead of the large-envelope price. That works because a 6 × 9 envelope is within USPS letter dimensions, as long as it stays no more than 1/4 inch thick and has no clasp.

How many sheets fit in one envelope?

Weight decides the postage. USPS says about four sheets of printer paper in a business-size envelope weigh 1 ounce, the most one Forever stamp covers. At USPS retail prices effective July 12, 2026, unchanged in the October 4, 2026 Notice 123, a Forever stamp is $0.82 and a 2-ounce letter needs $1.11. Letters over 3.5 ounces pay large-envelope prices.

Thickness matters too. A letter can be at most 1/4 inch thick; anything thicker, or bigger than 11.5 × 6.125 inches, is a large envelope (flat). A thick stack folded into a No. 10 can bulge, so weigh and measure it before you choose stamps.

• How much it costs to mail a letter, by weight ↗

Keep the folded letter flat and machinable

USPS sorts letters on machines, and the Domestic Mail Manual lists what makes a letter nonmachinable. Any one of these adds a $0.49 surcharge:

A paper window envelope doesn’t make a letter nonmachinable. Loose rigid items are a bigger problem: USPS warns that keys, coins, or flash drives loose in a paper envelope can tear out, jam sorting machines, or get lost, and recommends a padded envelope or small box sent as a package.

• Clasps, strings, buttons, or similar closures.
• Items such as pens, keys, or coins that make the thickness uneven.
• Contents too rigid to bend easily.
• A length divided by height under 1.3 or over 2.5, such as a square envelope.
• An address parallel to the short side of the envelope.

• USPS Domestic Mail Manual 101: Physical Standards ↗

When not to fold: mailing pages flat

If you don’t want creases, mail the pages flat in a 9 × 12 envelope, which takes an unfolded 8½ × 11 sheet. USPS treats it as a large envelope (flat): postage starts at $1.69 for 1 ounce, and the envelope must bend, stay evenly thick, and be rectangular, or it pays package prices. Flats can be up to 15 × 12 inches and 3/4 inch thick, and a stamped flat over 10 ounces or 1/2 inch thick must be handed to a Post Office employee.

IRIS can handle the folding for ordinary letters. You upload a US Letter-size PDF, and IRIS prints it, puts it in an envelope, and mails it by USPS First-Class Mail to a US address. A letter can be up to 60 single-sided or 120 double-sided pages, or up to six sheets with Certified Mail. IRIS doesn’t mail large (flat) envelopes and doesn’t enclose physical items, so it isn’t the right choice if a document must arrive unfolded or if you need to include an original you hold.

• Mail a PDF letter ↗
• Letter and postcard prices ↗