To qualify for the standard First-Class letter rate, a USPS letter must measure between 3.5 and 6.125 inches tall, between 5 and 11.5 inches long, and between 0.007 and 0.25 inches thick. It also has to be rectangular, weigh no more than 3.5 ounces, and have a length-to-height ratio between 1.3 and 2.5. Miss any one of those USPS letter size requirements in inches and the piece gets bumped to a higher price tier or hit with a surcharge.
The Four Measurements That Define a Letter
The Postal Service sets both a floor and a ceiling on every dimension:1Postal Explorer. Domestic Mail Manual 101 – Physical Standards
- Height: 3.5 inches minimum, 6.125 inches maximum
- Length: 5 inches minimum, 11.5 inches maximum
- Thickness: 0.007 inches minimum, 0.25 inches maximum
- Weight: 3.5 ounces maximum
The minimum thickness exists so pieces don’t disappear inside sorting machines. A single sheet of printer paper is roughly 0.004 inches thick, so a folded letter in a typical envelope clears the 0.007-inch floor with no trouble.
There is one wrinkle worth knowing. If your envelope runs longer than 6 inches or taller than 4.25 inches, the minimum thickness rises to 0.009 inches.2United States Postal Service. DMM 101 Physical Standards for Retail Letters, Flats, and Parcels That covers most business envelopes, including the standard #10 (9.5 inches long by 4.125 inches high). A near-empty #10 that falls under 0.009 inches gets flagged as non-machinable and picks up a surcharge even though it fits the size box.
The #10 is the reference envelope USPS itself cites as an example of a letter that fits comfortably.3United States Postal Service. How to Send a Letter or Postcard: Domestic Most A-series greeting card envelopes (A2, A6, A7) also qualify on dimensions, though square styles do not, for reasons explained below.
Shape and the Aspect Ratio Trap
Letters must be rectangular with square corners. Automated sorting relies on belts and rollers that grip flat, uniform edges, and anything round or oddly shaped can’t move through the equipment reliably.4Postal Explorer. Business Mail 101 – Mailpiece Shape
Rectangular alone isn’t enough, though. The USPS calculates an aspect ratio by dividing the length by the height, and that ratio has to fall between 1.3 and 2.5.1Postal Explorer. Domestic Mail Manual 101 – Physical Standards A square envelope has a ratio of 1.0. It falls under the minimum and is automatically non-machinable. A very long, narrow envelope can exceed 2.5 and land in the same spot.
To check your own: divide the longer side by the shorter side. A #10 at 9.5 by 4.125 inches works out to about 2.3, safely inside the window. An A2 invitation envelope at 5.5 by 4.25 inches comes in at roughly 1.29, just below the 1.3 floor. That’s the ratio problem that catches people sending invitations and greeting cards.
Weight and When a Letter Becomes a Flat
Once a piece crosses 3.5 ounces, USPS charges large envelope (flat) rates even if the envelope itself is letter-sized.1Postal Explorer. Domestic Mail Manual 101 – Physical Standards A standard sheet of 20-pound paper weighs about 0.16 ounces, so a five-page letter in a #10 comes in around an ounce with room to spare.
Any dimension over the letter maximum also pushes the piece into flat pricing. Flats can run up to 12 inches tall, 15 inches long, and 0.75 inches thick.5Postal Explorer. Sizes for Large Envelopes and Flats The cost jump is steep. As of January 2026, a one-ounce letter with a stamp runs $0.78, while a one-ounce large envelope runs $1.63.6United States Postal Service. Notice 123 – January 2026 Price Change More than double, for a piece that’s a fraction of an inch too tall.
Stamped letter rates by weight are:6United States Postal Service. Notice 123 – January 2026 Price Change
- 1 ounce: $0.78 stamped, $0.74 metered
- 2 ounces: $1.07 stamped, $1.03 metered
- 3 ounces: $1.36 stamped, $1.32 metered
- 3.5 ounces: $1.65 stamped, $1.61 metered
Each additional ounce above the first adds $0.29 on stamped mail. Anything past 12, 15, or 0.75 inches (or over 130 combined inches of length and girth) leaves the flat category entirely and gets priced as a parcel.7United States Postal Service. Parcel Size, Weight and Fee Standards
Other Traits That Trigger the Non-Machinable Surcharge
A letter can hit every size and weight limit and still cost extra if something about it prevents automated processing. The non-machinable surcharge is $0.49 per piece as of January 2026, added on top of regular postage.6United States Postal Service. Notice 123 – January 2026 Price Change So a one-ounce square envelope actually costs $1.27 to send: $0.78 in postage plus the $0.49 add-on.
The traits that trigger it:2United States Postal Service. DMM 101 Physical Standards for Retail Letters, Flats, and Parcels
- Aspect ratio below 1.3 or above 2.5. Square envelopes are the common example.
- A non-paper exterior, such as plastic wrap or polybagging. Clear film windows on paper envelopes are fine.
- Clasps, strings, buttons, or metal brads that jam equipment.
- Uneven contents like pens, keys, or loose coins that create bumps. Coins are only allowed if firmly attached to the insert.
- Rigidity. If the piece won’t bend around an 11-inch-diameter curve, it can’t clear the rollers.
- Thickness under 0.009 inches when the piece is longer than 6 inches or taller than 4.25 inches.
- An address that isn’t parallel to the longest side of the envelope.
Rigid inserts are the trap most senders don’t see coming. A wooden postcard, a thick plastic card, or a piece of chipboard that won’t flex easily forces manual handling. If the envelope fights back when you bend it gently, expect the surcharge.
Quick Check Before You Mail
Measure height, length, and thickness. Weigh the piece. Confirm it’s rectangular, that the long side divided by the short side lands between 1.3 and 2.5, and that nothing inside creates a bump or a stiff spot. If any single check fails, the price changes. Post office counters have scales and clerks who will quote the exact postage before you commit to a stamp.