A10 Envelope Size: Dimensions and Mailability Rules

The A10 envelope measures 6 inches high by 9.5 inches long and represents the upper boundary of letter-rate envelope mailing. At 6 inches high, it sits just below the USPS maximum letter height of 6.125 inches.

That tight margin matters. Manufacturing variance can push some A10 envelopes a hair over the 6.125 inch limit, which reclassifies them as flats and triples the postage. If you're running a large A10 mailing, measure the actual envelope dimensions — not just the spec sheet — before committing to letter-rate postage.

This page covers dimensions, the letter-rate boundary issue, common use cases, and how the A10 compares to the smaller A9 and the larger 9×12 flat.

When to use an A10 envelope

Premium and oversized announcements

Corporate announcements, milestone celebrations, and high-end social invitations sometimes use A10 envelopes for the visual presence of a larger format while staying within letter rates. The size feels substantial in the recipient's hand without the cost of flat-rate postage.

Photography deliveries

Photographers shipping prints up to 6 by 9 inches use A10 envelopes when the order doesn't need a rigid backer. For prints requiring a flat shipping surface, a rigid mailer in 9×12 is typically a better choice.

Substantial direct mail

Real estate agents, financial advisors, and luxury brands sometimes mail in A10 envelopes to differentiate from the standard #10 business envelope flow. The format reads as more deliberate and premium without the cost of a 9×12 flat.

Trifolded 8.5×11 pieces with bulk

An 8.5 by 11 inch sheet folded in thirds (to roughly 3.67 by 8.5) fits inside an A10 with substantial room. This works for marketing pieces with multiple inserts that would feel cramped in a #10.

The boundary problem

If you're considering an A10, also consider whether a 9×12 flat is a better fit. The A10 saves on postage but sacrifices the unfolded 8.5×11 layout. Going to 9×12 means flat-rate postage but lets you mail an unfolded letter or document. Choose based on what the recipient experience needs to be.

A10 envelope postage and USPS rules

The A10 sits at the very top of the letter-rate range — close enough to the boundary that you should verify dimensions on actual envelope stock before committing to a mailing.

2026 First-Class postage for an A10 envelope

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

The A10's aspect ratio is 1.58, comfortably inside the machinable range of 1.3 to 2.5.

The 6.125 inch boundary

USPS letter-rate maximum height is 6.125 inches. The A10 spec is 6.0 inches. That's a margin of 0.125 inches — narrow enough that any manufacturing inconsistency, slight cutting variance, or paper expansion can push the actual envelope over the limit. If a postal acceptance clerk measures a piece at 6.13 inches or more, it's reclassified as a flat — postage jumps from $0.78 to $1.63.

Before mailing a large A10 run: measure 5 to 10 random envelopes from your stock with a ruler. If any of them measure 6.10 inches or higher, plan for flat-rate postage or use a different envelope.

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