USPS Packaging Guidelines: Boxes, Tape, Size and Labels

USPS packaging guidelines come down to five rules: use a box strong enough for the weight (corrugated fiberboard for loads up to 70 pounds), cushion the contents so nothing shifts, close every seam with 2-inch packing tape (not string, cellophane or masking tape), stay within the size limits (108 inches in length plus girth, or 130 inches for Ground Advantage), and put all labels on one side with old barcodes covered. These rules come from the USPS Domestic Mail Manual and USPS’s own packing instructions.

This guide covers each rule in order, from mail categories and box strength to sealing, labeling, restricted items and the packaging choices that keep postage down.

What size categories do USPS packaging guidelines use?

USPS sorts mail into letters, flats (large envelopes) and parcels, and each has its own size limits. A piece that’s too big, too thick or too rigid for its category is priced as the next one up, so a lumpy envelope can cost package postage.

Letters

A letter must be at least 5 inches long, 3-1/2 inches high and 0.007 inch thick, and no more than 11-1/2 inches long, 6-1/8 inches high and 1/4 inch thick. Letter-size First-Class Mail can weigh up to 3.5 ounces. Our guide to how much a book of stamps costs helps if you mail letters in volume.

A letter is nonmachinable, and pays a surcharge, if its length divided by its height is below 1.3 or above 2.5, if it’s square, plastic-wrapped or too rigid, or if it has clasps, strings or lumpy contents such as keys or pens.

Flats (large envelopes)

A flat is bigger than a letter in at least one dimension but no more than 15 inches long, 12 inches high and 3/4 inch thick, and First-Class flats can weigh up to 13 ounces. Flats must be flexible and uniformly thick, with bumps no more than 1/4 inch. A rigid or lumpy large envelope, or one stuffed with a small box, is treated as a parcel.

Parcels

Parcels can weigh up to 70 pounds and measure up to 108 inches in combined length and girth, or up to 130 inches for USPS Ground Advantage. A parcel must also be big enough to fit the delivery address, return address, postage and barcode on one side without folding. If you’re new to shipping, our guide on how to ship a package walks through the whole process.

Which boxes and materials meet USPS packaging guidelines?

USPS requires packaging strong enough to protect the contents and the mail around it, because your box rides alongside packages weighing up to 70 pounds. The box must keep its shape so the label stays readable, and it should be just large enough for the item plus cushioning.

Reusing a box is fine if it’s still rigid, with no crushed corners, tears or soft spots. Cover or black out every old logo, barcode, shipping label and address, because a leftover barcode can send the package to the wrong place. You can also reuse boxes across carriers in some cases, and our guide on whether you can use USPS boxes for UPS explains the rules.

Padded and poly mailers work well for soft, unbreakable goods such as clothing. Our guide to the USPS bubble mailer covers when a padded envelope is enough and when an item needs a box.

How should you cushion items inside a USPS package?

Under DMM 601, fragile items must be cushioned, contents must not shift, and heavy items must be braced so they don’t damage other mail. Loose-fill cushioning such as packing peanuts must overfill the box before you close it, so the contents can’t settle and leave empty space.

• Wrap each fragile item separately so items can’t hit each other.
• Leave room for cushioning on every side, including the top and bottom.
• Don’t pack heavy, dense items with fragile ones unless they’re separated and secured.
• Do a shake test: if you hear or feel movement, add more cushioning.

USPS also suggests putting a slip with the recipient’s name and address inside the box, so the package can still be delivered if the outside label is damaged.

How should you seal a USPS package?

Use 2-inch-wide packing tape to close and secure every seam. Pressure-sensitive packing tape, reinforced paper tape and water-activated paper tape all work, while cellophane tape and masking tape aren’t allowed. Any adhesive must hold in temperatures from -20°F to 160°F.

Taping the center seam and both edge seams on top and bottom, often called the H-taping method, keeps flaps from popping open. Don’t wrap a box in string, cord or twine, because it catches in USPS sorting equipment, and if you use strapping, it must go around the length and girth of the package, not loosely or in metal. Staples are allowed when they’re placed within 1-1/4 inches of the box ends, no more than 5 inches apart and clinched tight.

Where do labels and addresses go on a USPS package?

Put the delivery address, return address and postage on the same side of the package, parallel to its longest side, and keep every label flat on one surface without folding it over an edge. The return address goes in the top-left corner. If you write the address by hand, use a pen or permanent marker and capital letters.

A printed label is the most reliable option, and our guide to shipping labels covers formats and printing. If you use free Priority Mail packaging, the label has to be Priority Mail: USPS doesn’t allow Priority Mail boxes for Ground Advantage, and Flat Rate pricing applies only to USPS Flat Rate boxes and envelopes.

Which items need special packaging or can’t be mailed?

Hazardous, restricted and perishable items follow USPS Publication 52. Explosives and most flammable or toxic materials are prohibited, while many everyday items, such as aerosols, alcohol-based perfume, lithium batteries and some cleaning products, are allowed only by ground transportation and with specific packaging and markings. Free Priority Mail packaging can’t be used for restricted hazardous materials or live animals.

Liquids need their own setup. Mark the outer box to show it holds liquid and add orientation arrows, use screw-on or otherwise secure closures, and triple-pack any breakable or friction-top container holding more than 4 fluid ounces: absorbent cushioning, a sealed leakproof inner container and a strong outer box.

Our item guides cover the most common cases: shipping AA batteries with USPS, shipping laundry detergent with USPS, shipping matches with USPS, shipping power tools with USPS and shipping insect repellents with USPS.

How can packaging lower your USPS postage?

Packaging choices change the price of every label. For Priority Mail and Ground Advantage packages larger than 1 cubic foot, USPS charges dimensional weight (length × width × height ÷ 139, rounding fractions of an inch up) when it’s higher than the actual weight. USPS moved to the 139 divisor in July 2026, so oversized boxes cost more than they used to.

• Right-size the box. A smaller box can avoid dimensional weight and nonstandard fees for packages longer than 22 inches or larger than 2 cubic feet.
• Use a mailer for soft goods. A poly or padded mailer often keeps light items in a lower weight tier.
• Consider Flat Rate for heavy items. USPS Flat Rate boxes charge one price for up to 70 pounds, and our Priority Mail Flat Rate guide explains when that beats weight-based pricing. The flaps must close within their normal folds.
• Use Media Mail for eligible books. USPS Media Mail is priced by weight only, but it’s limited to qualifying media.

Once the box is packed, compare live USPS and UPS rates in the ParcelPath rate calculator. ParcelPath is free to use and offers up to 89% off USPS retail rates and up to 85% off UPS, with free USPS pickups and USPS Label Broker if you don’t have a printer.

Our USPS frequently asked questions page answers other common mailing questions, and our roundup of the best shipping services and solutions compares carriers.

Frequently Asked Questions

Can I reuse an old box under USPS packaging guidelines?

Yes, as long as the box is still rigid and undamaged. USPS asks you to cover or black out every old logo, barcode, shipping label and address so scanners read only the new label. A box that has lost its strength, with soft corners or tears, should be replaced, because it has to hold its shape alongside packages up to 70 pounds.

What tape does USPS allow for packages?

USPS recommends 2-inch-wide packing tape on every seam. Pressure-sensitive packing tape, reinforced paper tape and water-activated paper tape are all acceptable, but cellophane and masking tape are not allowed under the Domestic Mail Manual. Don’t use string, cord or twine to close a box, because it can catch in sorting equipment.

What happens if my package doesn’t meet USPS packaging guidelines?

USPS can refuse a package that isn’t packed safely or that contains restricted items packed incorrectly. Packages that are too large or oddly shaped for their service can owe nonstandard fees or extra postage, and poorly packed items are more likely to be damaged, which can weaken an insurance claim.

Do USPS packaging guidelines differ for international shipments?

The basic packing rules are the same, but international packages also need a customs form describing each item and its value, and every destination country has its own prohibited and restricted items list. Size and weight limits vary by service and country, so check USPS’s country listing before you pack.

Sources

• USPS Domestic Mail Manual 601: Mailability (packaging, cushioning, closures, box strength, liquids)
• USPS Domestic Mail Manual 101: Physical Standards (letters, flats, parcels)
• USPS: How to Send a Package (boxes, tape, labels, Priority Mail packaging)
• USPS: USPS Ground Advantage (dimensional weight and nonstandard fees)
• USPS: Priority Mail (Flat Rate rules, 108-inch limit)
• USPS Publication 52: Hazardous, Restricted and Perishable Mail