First-Class Mail Size and Weight Limits for Letters and Large Envelopes

A First-Class letter can be up to 11.5 inches long, 6.125 inches high and 1/4 inch thick, and it can weigh up to 3.5 oz. An envelope that goes past any one of those sizes, such as a 9x12, mails as a large envelope, which USPS calls a flat. A large envelope can be up to 15 inches long, 12 inches high, 3/4 inch thick and 13 oz.

The category and the weight set the price. With stamps, a 1 oz letter costs $0.82 and each extra ounce adds $0.29. A large envelope starts at $1.69. A letter-size envelope that weighs more than 3.5 oz pays large-envelope prices.

Measure and weigh the envelope with everything inside it. USPS uses the longest side as the length when it decides whether a piece is a letter or a large envelope, whichever way the address runs.

Letter and large-envelope limits at a glance#

Letters must also be at least 5 inches long and 3.5 inches high. One oversize measurement is enough: a 12-by-6-inch envelope mails as a large envelope on its length alone.

Which business envelopes mail as letters#

Here is how the envelopes most offices use are priced when addressed along the long side:

A window envelope costs the same as a plain one: USPS says a window in a paper envelope doesn't add the $0.49 surcharge. A plastic envelope does.

How many pages fit in one ounce, and what each extra ounce costs#

USPS's rule of thumb is that four sheets of printer paper in a business-size envelope weigh about 1 oz, which one $0.82 stamp covers. Printing on both sides halves the sheet count, so an eight-page document printed double-sided is four sheets. A fifth sheet, a reply envelope or heavier paper can push it over 1 oz, so weigh one finished envelope before you stamp a whole batch.

Postage goes up in 1 oz steps. A letter that weighs 1.2 oz pays the 2 oz price of $1.11, and each ounce after that adds $0.29, up to $1.69 at the 3.5 oz limit. Past 3.5 oz, even a #10 pays the large-envelope price for its weight.

With a postage meter, a 1 oz letter costs $0.78 instead of $0.82. The First-Class Mail rates guide lists every letter and large-envelope weight, and how much it costs to mail a letter shows how many stamps to use.

Folded in a #10 or flat in a 9x12: what the choice costs#

Folding is the cheaper choice whenever the folded pages still mail as a letter. Here is the same document both ways, in plain paper envelopes without clasps. Folded into a #10, it is a 1.4 oz letter: $1.11 with stamps. Unfolded in a 9x12, the bigger envelope brings it to 1.8 oz, still under 2 oz, but it mails as a large envelope: $1.11 + $0.87 = $1.98.

That difference repeats on every envelope in a monthly batch, so fold whenever the document allows it. Keep it unfolded when the recipient needs an original without fold lines, such as a signed contract or a certificate.

When you send a letter online with Postmarkr, sheet count sets the envelope: up to six sheets, including the address page it adds, are folded into a #10 double-window envelope, and seven or more go unfolded in a 9x12, which adds $2.50. That six-sheet cutoff is Postmarkr's packing rule, not a USPS limit, and you can choose the 9x12 for a shorter document. Postage is included in the price.

What adds $0.49 to a letter: the nonmachinable surcharge#

USPS adds a $0.49 nonmachinable surcharge to a letter its sorting machines can't handle. Any one of these triggers it:

• A square or nearly square shape, or a very long, narrow one: length divided by height under 1.3 or over 2.5
• A clasp, string, button or similar closure
• A pen, key, coin or other item that makes the thickness uneven
• An envelope too stiff to bend
• An address that runs parallel to the short side
• A plastic envelope or wrap

A 6-by-6-inch square envelope weighing 1 oz costs $0.82 + $0.49 = $1.31. The surcharge applies only to letters. A large envelope that fails its own rules pays package prices instead.

When a large envelope pays package prices#

A 9x12 gets large-envelope prices only if it is rectangular, bends and is evenly thick. USPS treats one that fails any of these as a package, and it pays USPS Ground Advantage package prices. The same happens to an envelope bigger than 15 by 12 inches or thicker than 3/4 inch, and anything over 13 oz can't go First-Class Mail at all. At a Post Office, Ground Advantage starts at $7.90, compared with $1.69 for a 1 oz large envelope.

In a 9x12 of documents, the usual causes are a cardboard stiffener, a rigid sleeve or folder, a bulky binder clip, or pages bunched at one end. Take out the stiffener and lay the pages flat so the envelope bends and lies evenly. If a certificate or photo needs a stiff backing, plan on the package price.

Check one envelope before you buy postage#

Run these four checks on one finished envelope:

1. Measure it with everything inside and compare it with the letter limits.
2. Weigh it and round up to the next ounce; for a letter, 3.5 oz is the last step.
3. Flex it. A stiff letter pays the $0.49 surcharge, and a stiff large envelope pays package prices.
4. Check its shape and address direction against the surcharge list above.

Underpaying is worse than overpaying. USPS may deliver a short-paid letter postage due, which means your customer pays the difference to accept your invoice. Or USPS may return it to you marked "Returned for Additional Postage." A short-paid letter with nonmachinable features, such as a square envelope mailed without the $0.49 surcharge, comes back to you.

Heavy or thick packets have one more rule. A stamped envelope that weighs more than 10 oz or is more than 1/2 inch thick can't go in a blue collection box or be left for your carrier to pick up; hand it to an employee at a Post Office. Metered mail doesn't have this limit.