USPS envelope size limits for letters, large envelopes and packages

USPS envelope size limits at a glance

USPS sorts mail by shape. The size, thickness and stiffness of your envelope decide whether it travels as a letter, a large envelope (USPS calls these flats) or a package, and each shape has its own price list. The table shows the limits for each shape as the Domestic Mail Manual and the USPS letters page state them.

A piece that breaks any limit in its row moves to the next shape and pays that price. A letter that is too thick becomes a flat, and a flat that is rigid, lumpy or too big becomes a package.

Prices on this page come from USPS Notice 123, effective July 12, 2026. Prices change from time to time, so check Notice 123 again every January and July.

USPS letter size requirements

To pay letter prices, an envelope must be rectangular and made of paper. It can be as large as 11-1/2" long x 6-1/8" high and must stay no more than 1/4" thick. For comparison, a standard No. 10 business envelope is 9-1/2" long x 4-1/8" high, so it fits with room to spare.

The smallest letter USPS accepts is 5 inches long, 3-1/2 inches high, and 0.007-inch thick. The weight limit for a letter is 3.5 ounces. A heavier letter-size piece pays large envelope prices even though it is letter-sized.

Letter postage rises by weight. A stamped letter costs $0.82 for the first ounce, $1.11 at two ounces, $1.40 at three ounces and $1.69 at the letter weight limit.

Where you drop it off matters too. If a stamped envelope weighs more than 10 oz or is thicker than 1/2", USPS says you cannot put it in a collection box. Hand it to an employee at a Post Office counter instead.

The USPS nonmachinable surcharge: what makes a letter nonmachinable

USPS sorts letters on high-speed machines. A letter that cannot run through them is nonmachinable and pays a $0.49 surcharge on top of its postage. Notice 123 lists the same $0.49 surcharge for retail letters. In plain terms, USPS says a letter is nonmachinable if it is rigid, lumpy or has clasps, string, or buttons, and that square or vertical (taller than they are wide) envelopes pay more.

The Domestic Mail Manual lists the exact criteria. A letter-size piece is nonmachinable if it has any one of these:

• An aspect ratio (length divided by height) of less than 1.3 or more than 2.5. Example: a square greeting card in a square envelope has a ratio of 1, so it pays the surcharge. Very long, narrow envelopes fail the same test.
• clasps, strings, buttons, or similar closure devices. Example: a string-and-button envelope for a hand-bound zine.
• Contents such as pens, pencils, keys, or coins that make the thickness uneven. Example: a button badge or a thick sticker pack bunched in one corner. A wax seal on the outside makes the envelope lumpy in the same way.
• An envelope that is too rigid: it does not bend under a belt tension of 40 pounds around an 11-inch diameter turn. Example: a print taped to stiff chipboard inside a letter envelope.
• A piece that is polybagged or otherwise enclosed in any plastic material, or has a non-paper outside.
• A delivery address written parallel to the shorter dimension, which is what happens when you address a letter vertically.

For decorated envelopes, USPS suggests two options. If you use wax seals, ribbons or string on the outside, put them inside another envelope. You can also pay the surcharge and ask a counter clerk to hand-cancel the envelopes so they skip the machines.

USPS large envelope rates and size

A large envelope, or flat, is anything bigger than a letter in any one dimension, up to 15 inches long, 12 inches high, 3/4 inch thick. It must be flexible and evenly thick, with no more than 1/4-inch variance in thickness. Flats suit unfolded paper goods: art prints, certificates, pattern sheets, calendars and thin booklets.

Flats are priced by weight up to the First-Class Mail limit of 13 ounces. These are the retail prices for stamped or counter-paid flats.

The nonmachinable surcharge applies to letters. A large envelope that fails the flexibility or thickness rules is considered parcels and must pay the applicable parcel prices.

Which maker goods cannot travel as letters or flats

Paper goods are what letters and flats are built for. USPS says stickers, photos, trading cards are usually fine as long as the envelope stays flat and is not lumpy. Small orders of stickers, washi samples, postcards, bookmarks and thin prints can go as a letter or a flat.

Hard and three-dimensional goods are the problem. USPS warns against putting flash drives, coins, keys, hard plastic card cases loose in unpadded paper envelopes, because they can tear through the envelope or jam a machine. For these, USPS recommends a padded envelope or small box and sending it as a package.

The same rule decides whether a large envelope still counts as a flat. If it is nonrectangular, rigid (can't bend), or lumpy (not uniformly thick), you pay the package price. Boxes are never flats, even thin ones.

• Enamel pins and button badges: rigid and lumpy, so send them as a package.
• Keychains and acrylic charms: rigid and uneven, so send them as a package.
• Earrings on a backing card in a small gift box: a box is never a flat.
• Sticker packs in a rigid top-loader or hard sleeve: rigid, so a package. Loose stickers in a card sleeve that bends can stay a letter or flat.
• Prints backed with stiff board: rigid, so a package. A print in a flexible card mailer can stay a flat if it bends and stays evenly thick.

Why USPS letters and flats have no tracking

USPS states it directly: Tracking is not available for First-Class Mail items. That covers letters and large envelopes, whether you use stamps or buy postage at the counter. Certified Mail, an add-on that costs extra, gives proof that you mailed your item and that the recipient signed for it.

For an online shop this matters more than the postage. If a buyer says an order never arrived, tracking is the record that shows it was delivered. If an order is worth more than you can afford to refund, send it as a package with tracking, even when it fits in an envelope.

When to move to a USPS Ground Advantage envelope

USPS Ground Advantage is a package service. Sent this way, a padded or poly envelope counts as a package, so lumps and rigid items are fine. Ground Advantage packages arrive in 2-5 days, USPS Tracking ® service is included in the price, and $100 of insurance is included.

Prices go in weight tiers: up to 4 oz; up to 8 oz; up to 12 oz; and up to 15.999 oz. At a Post Office, a package in the lightest tier costs $7.90 to $9.45 depending on distance. Shipping labels bought online use lower Commercial Rates: the same tier costs $6.93 to $8.40.

Move to a Ground Advantage envelope when the item is rigid or lumpy, when a buyer or marketplace expects tracking, or when the order is worth more than you could afford to refund. Keep letters and flats for flexible, low-value paper goods, such as a free sticker sent to a past customer.

How to choose between a letter, a flat and a package

Run each order through these checks before you buy postage.

1. Check stiffness and lumps

   Bend the filled envelope gently. If it will not bend or has a bump, it is a package. Pins, charms, keychains and boxed items stop here.

2. Measure the envelope

   Up to 11-1/2" long x 6-1/8" high and no more than 1/4" thick is a letter. Larger, up to 15 inches long, 12 inches high, 3/4 inch thick, is a flat. Anything bigger is a package.

3. Check the shape of a letter

   A square or vertical envelope, a clasp or string closure, or a wax seal adds the $0.49 nonmachinable surcharge. A rectangular envelope with the address along the long side avoids the shape and address triggers.

4. Weigh it

   Letters stop at 3.5 ounces and flats at 13 ounces. Heavier pieces pay the next shape's price.

5. Decide on tracking

   Letters and flats have no tracking. If you need proof of delivery, use a Ground Advantage envelope and buy the label online for commercial prices.

Where Nation fits

Questions about USPS envelope sizes