#10 Envelope Size: Dimensions, USPS Rates & When to Use It

The #10 envelope is the most widely used business envelope in the United States. It measures 4.125 inches high by 9.5 inches long — proportions chosen to fit a standard 8.5 by 11 inch sheet of paper folded in thirds, with room left for a reply slip or thin enclosure if needed.

If you've received a piece of business mail in the last week, it almost certainly arrived in a #10. Banks send statements in them. Utility companies bill in them. Nonprofits run donation appeals in them. The size is so common that "business envelope" and "#10 envelope" are used interchangeably in the printing and mailing industry.

This page covers the exact dimensions, what fits inside, USPS postage rules, window and non-window options, and when a #10 is the right choice over a #9, A-series, or larger 9×12 flat envelope.

When to use a #10 envelope

The #10 is the default for almost all standard business mail. The four most common use cases:

Business correspondence

Cover letters, formal letters, contracts mailed as single sheets, and any general one-to-three-page letter mailing. A trifolded 8.5 by 11 inch sheet sits comfortably inside without binding or overstuffing.

Invoices and statements

Window #10 envelopes let you print the recipient address directly on the invoice itself, eliminating a separate addressing step. The address shows through a window typically positioned in the lower-left of the envelope front. This is the format you see from credit card companies, utility providers, and medical billing services.

Donation appeals and direct mail letters

Nonprofits and financial services use #10 envelopes for appeal letters because they read as personal correspondence rather than promotional mail — which improves open rates. A typical appeal mailing pairs a #10 outer envelope with a smaller #9 reply envelope inside.

Bills and account communications

Banks, insurance companies, and government agencies send statements, notices, and account documents in #10 windows or non-window varieties. The format is recognizable, machine-processable, and inexpensive at the volumes these mailings run.

When a #10 is not the right choice

Invitations and announcements should use the A-series (A6, A7, or A9 depending on card size). Unfolded documents like résumés or certificates should go in 9×12 or larger. Pieces with bulky inserts thicker than 0.25 inches will exceed the letter-rate thickness limit and get reclassified as flats.

#10 envelope postage and USPS rules

A #10 envelope at standard weight falls within USPS letter-size limits, which keeps postage at the lowest available First-Class rate.

2026 First-Class postage for a #10 envelope

• 1 ounce, Forever stamp: $0.78
• 1 ounce, metered postage: $0.74
• Each additional ounce, up to 3.5 ounces: $0.29

Most #10 mailings land in the 1-ounce tier. A standard #10 envelope on 24 lb stock with a single trifolded 20 lb letter inside weighs roughly 0.5 ounces. Adding a reply envelope or a second sheet typically still keeps the piece under 1 ounce. Past 3.5 ounces, the piece is rated as a flat and postage jumps to $1.63 starting.

Automation eligibility

A #10 envelope's aspect ratio (length divided by height) is 2.30 — well within the USPS machinable range of 1.3 to 2.5. As long as the envelope is flexible, uniformly thick, and under 0.25 inches, it qualifies for full automation. Bulk mailings can earn presort and automation discounts of $0.10 or more per piece. We cover those discounts on our direct mail services page.

What disqualifies a #10 from letter rate

The most common surcharge triggers: rigid contents (a credit card, a key, a thick brochure), overweight pieces (more than 3.5 ounces), or lumpy contents that fail the uniform-thickness test. If you're mailing anything beyond paper and a thin reply card, weigh and flex-test a sample piece before committing to a print run.

Last verified: May 2026. Source: USPS Domestic Mail Manual 201 and 202.