A #0 (6 x 10 in) with a soft item, 2 oz
Inside letter dimensions if it stays under 0.25 inch thick, flexible, uniform. Rides at First-Class Mail letter rates. The moment the contents push it past a quarter inch, it becomes a First-Class flat.
Envelope or parcel
Is a padded envelope a package?
USPS decides the answer from three things: how big the envelope is, whether it is flexible, and whether it has a uniform thickness. Under the flat maximum of 15 by 12 by 0.75 inches and still bendable, a padded envelope rides at letter or flat rates. Rigid, bulging or over-size and it becomes a parcel and is priced like one. The definitions here are quoted from Domestic Mail Manual 101, read on pe.usps.com on 2026-08-11.
DMM 101, quotedThree testsRate consequence
Bend it, measure it, weigh it
Bendable, uniform and inside the flat maximum: letter or flat. Rigid, uneven or larger: parcel.
The three cut-offs USPS applies
DMM 101 lays out three mailpiece shapes and gives each one a maximum. Anything past the flat maximum is a parcel by definition; anything below the letter maximum and inside the letter shape is a letter. The middle band is a flat.
| Shape | Max length | Max height | Max thickness | Weight cap |
|---|---|---|---|---|
| Letter | 11.5 in | 6.125 in | 0.25 in | 3.5 oz |
| Flat | 15 in | 12 in | 0.75 in | Class dependent |
| Parcel | Anything over the flat maximum, or that fails flexibility or uniform thickness | |||
The two tests that a padded envelope fails most often are the flexibility test and the uniform-thickness test. A padded envelope holding a hard object — a keychain, a book, a small circuit board — is rigid where the object sits, and its thickness varies from lump to seam. DMM 101 treats the envelope as a parcel from that point regardless of the outside dimensions.
Three common padded mailers, three different answers
A #2 (8.5 x 12 in) with a paperback book
Almost always a flat: uniform thickness, flexible, under 15 x 12 x 0.75 inches. Priced as First-Class flat mail or, once the book pushes weight past the flat cap, USPS Ground Advantage.
A #5 (10.5 x 16 in) with a rigid electronics unit
Parcel on two counts: it exceeds the 15-inch length maximum for a flat, and its contents make it rigid. USPS treats it as a parcel and prices it on Ground Advantage or Priority Mail dimensional weight.
For the standard bubble-mailer size chart with the #000 through #7 numbering, see the guide to padded shipping envelope sizes. This page is about the classification rule; that page is about picking the mailer size in the first place.
Why the answer matters at the counter
USPS charges three different rate schedules for the three shapes. A First-Class letter is billed by ounce up to 3.5 ounces at the lowest per-piece rate. A First-Class flat is billed by ounce on a heavier fee schedule. A parcel is billed by weight and dimensional weight on Ground Advantage or Priority Mail. Moving one line up or down between shapes changes the price by dollars, not cents, on the same physical envelope.
The clerk applies the rule when a padded envelope reaches the counter, but a self-service label buyer needs to apply it themselves. Print a letter or flat postage on a padded envelope that fails the flexibility test, and the sorter kicks it out for short-paid postage; the shipment either returns to you for the difference or delivers with postage due.
Two tests you can do on the bench
Bend test
Hold the sealed envelope at both ends and try to arc it into a gentle curve. If it bends without buckling and returns flat, it passes flexibility. If it forms a permanent crease or refuses to bend, it fails and rates as a parcel.
Feel test for thickness
Run a straight edge across the flat face. If the surface stays within roughly a quarter inch of level across its width, it is uniform. A lumpy padded envelope varies by more, and DMM 101 pulls it into parcel classification.
Callipers or a ruler on the seams
Fifteen inches long by twelve inches high by three-quarters of an inch thick is the flat maximum. Over any single dimension, and the envelope is a parcel by size alone, regardless of what is inside it.
The other two carriers do not have this hierarchy
UPS and FedEx do not price letter and flat mail. A padded envelope on a UPS or FedEx label is billed by weight against dimensional weight like any parcel, unless it rides in the carrier's own branded flat-rate envelope. If speed does not require the expedited network and the envelope fits the USPS letter or flat rules, USPS is almost always cheaper on the small end; if the envelope is a parcel by USPS rules, price it side by side on the calculator.
Where these rules come from
- USPS, Domestic Mail Manual 101 — Physical Standards for Letters, Flats and Parcels — the letter, flat and parcel dimension and weight limits, the flexibility test and the uniform-thickness test. Read 2026-08-11.
Common questions
Is a padded envelope a letter, a flat or a package?
Whichever its finished shape fits. Under USPS DMM 101, an envelope up to 11.5 by 6.125 by 0.25 inches is a letter, an envelope up to 15 by 12 by 0.75 inches that stays flexible and uniform is a flat, and anything rigid, bulging or outside those dimensions is a package.
Does adding padding turn a letter into a package?
Not by itself. Padding pushes an envelope past the 0.25-inch thickness or the 3.5-oz weight cap only when the contents themselves do; a thin, light padded mailer under the letter dimensions still qualifies as a letter.
What makes a bubble mailer count as a parcel?
A bubble mailer becomes a parcel when it exceeds the flat maximum of 15 by 12 by 0.75 inches, when it fails the flexibility test because the contents make it rigid, or when its thickness varies by more than 0.25 inch across the surface.
What service class does a padded package usually take?
USPS Ground Advantage or Priority Mail for parcels over the flat dimensions. Below them, First-Class Mail letters or flats. Bubble mailers rigid enough to be parcels are priced by weight and dimension, not by envelope rate.
Do UPS and FedEx treat padded envelopes differently than boxes?
Both bill a padded envelope by billed weight against dimensional weight, same as any package, unless the label uses the carrier's own branded flat-rate envelope. There is no letter or flat rate at UPS or FedEx for a private-purchased bubble mailer.