Envelope size and shape can change postage even when two mailpieces weigh the same.
USPS letter-size limits
A machinable letter must be at least 5 inches long, 3.5 inches high and 0.007 inch thick. The maximum letter dimensions are 11.5 inches long, 6.125 inches high and 0.25 inch thick, with a maximum weight of 3.5 ounces. If a letter exceeds 6 inches long or 4.25 inches high, it must be at least 0.009 inch thick. Measure the finished, sealed piece; a large envelope or flat follows a different set of limits.
Letter basics
A letter must fall within USPS letter dimensions and thickness limits and meet shape requirements. Square, rigid, or uneven pieces may be nonmachinable.
Large envelopes or flats
A large envelope exceeds one or more letter dimensions but must still meet flat standards. The current one-ounce large-envelope price starts at $1.69.
Measure after assembly
Inserts, clasps, padding, and photos can change thickness, flexibility, and weight. Measure the sealed, ready-to-mail item.
Frequently asked questions
Is a 9×12 envelope a letter?
It is normally evaluated as a large envelope or flat, subject to thickness, flexibility, uniformity, and weight.
Do square envelopes cost more?
They may require the nonmachinable surcharge.
Does envelope color change postage?
Not by itself, but address readability and contrast matter.
Large envelope and flat limits
A First-Class Mail large envelope or flat must be more than one letter-size maximum but no more than 15 inches long, 12 inches high and 0.75 inch thick. First-Class Mail flats may weigh up to 13 ounces. Measure and weigh the sealed piece; a rigid, lumpy or nonrectangular item may be priced as a package instead of a flat.
Sources and update note
- USPS Notice 123 — current price list
- USPS DMM 101 — physical standards
- USPS DMM 604 — postage payment methods
Rates and mailing standards can change. This page was reviewed in August 2026; verify the completed mailpiece when its weight, dimensions, shape, destination, or services are uncertain.
After measuring and classifying the mailpiece, continue with the complete USPS envelope mailing steps for addressing, postage and deposit.