How do you fold a letter for a #10 envelope?

Fold the 8½ × 11 sheet in thirds: bring the bottom third up and crease, then fold the top third down over it and crease again. The folded letter is about 3⅔ × 8½ inches, which slides into a #10 envelope (4⅛ × 9½ in) with room to spare.

Step by step (standard tri-fold)

1. Lay the letter face up with the top edge away from you. For several pages, stack them in order, square the edges and fold them together as one piece.
2. Fold the bottom edge up so the bottom third covers the middle third. Line up the sides and crease.
3. Fold the top third down over the folded part and crease. The Gregg Reference Manual's version brings the top edge to within ⅜″ of the first crease, so the inner panel doesn't jam into the fold.
4. Insert the letter with the last crease (the one made in step 3) going into the envelope first, press it flat and seal.

With this fold the printed side is inside. When the reader lifts the top panel, the letterhead and greeting appear first.

Why it fits

Envelope maker Tension lists the largest insert for a #10 as 3⅞ × 9¼ in, so a letter in thirds is well inside that. The panels don't have to be perfect; a printer's letter fold makes the inside panel slightly narrower (one guide gives 3.6875″ panels) so it tucks in neatly. Uneven folds are fine as long as the folded height stays under about 3⅞″.

Window envelope? Use a Z-fold

A regular tri-fold hides the address. For a #10 window envelope, print the address in the top third, fold the bottom third up, then turn the sheet over and fold the top third back the other way so the address faces out (the folded edge looks like a Z). Slide it in address-side toward the window and tap the envelope on each side to check that no part of the address slips out of view. USPS Publication 25 sets the window rules, including keeping at least ⅛″ of clear space around the address inside the window.

Other papers and folds

Postage note

A folded letter in a #10 mails as a regular letter as long as the envelope stays flat, flexible and no more than ¼″ thick. USPS says one Forever stamp covers 1 oz, which is about 4 sheets of 8½ × 11 paper. As of October 26, 2026 (Notice 123, effective October 4, 2026), that stamp is $0.82, and each extra ounce up to 3.5 oz adds $0.29 ($1.11 for 2 oz). A rigid or lumpy insert makes it nonmachinable, which adds a $0.49 surcharge. Check any envelope with the USPS letter size checker.

FAQ

Which edge goes into the envelope first? The last crease you made, per the Gregg Reference Manual.

Should I fold several pages separately? No. Fold the stack as one piece so the pages nest, stay in order and make one even thickness.

My letter doesn't fit. What now? Make sure the folded height is under about 3⅞″, or use a larger business envelope like a #11 or #12; see the envelope size chart.