#9 Envelope Size: Dimensions and Reply Envelope Uses

The #9 envelope measures 3.875 inches high by 8.875 inches long — about a quarter inch smaller in each dimension than a #10. That sizing is deliberate: a #9 is designed to slip inside an unsealed #10 without folding.

If you've ever received a donation appeal, an invoice, or a charity solicitation, you've seen this pairing. The outer #10 carries the letter; the inner #9 carries the response. It's the default architecture for any mailing that asks for something back.

This page covers the exact dimensions, the #9's role as a reply envelope, Business Reply Mail (BRM) considerations, and when to choose a #9 over a postcard or a smaller envelope.

When to use a #9 envelope

Almost every #9 mailing falls into one of three patterns:

Reply envelope inside a #10 outer

This is the dominant use. A nonprofit sends an appeal letter in a #10. Inside, along with the letter and reply card, sits a pre-addressed #9 reply envelope. The donor fills out the card, places it in the #9, adds postage (or uses BRM if the organization pays), and drops it in a mailbox. The same pattern shows up in invoice mailings, pledge drives, and survey campaigns.

Business Reply Mail (BRM)

BRM lets the recipient mail back a reply at no cost to themselves — your organization pays postage only on pieces actually returned. The #9 is the most common BRM envelope size because it pairs cleanly with a #10 outer. BRM envelopes have specific design requirements: a permit number, FIM (facing identification mark), Intelligent Mail barcode, and the FACING IDENTIFICATION block on the front. Cornerstone designs and produces BRM envelopes to USPS specification — see our direct mail services for details.

Stand-alone use

Less common, but a #9 can be used as a primary mail envelope for short notes, single-sheet correspondence, or remittance slips. The size is somewhat undersized for a folded 8.5 by 11 letter — a #10 is better suited. The #9 is most at home as the reply piece in a paired mailing.

What a #9 isn't designed for

It's too small to comfortably hold a trifolded 8.5 by 11 letter without binding. Use a #10 for outgoing letter mail and reserve the #9 for replies.

#9 envelope postage and USPS rules

A #9 envelope falls comfortably within USPS letter-size limits and qualifies for First-Class letter rates.

2026 First-Class postage for a #9 envelope

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

The #9's aspect ratio (length divided by height) is 2.29 — squarely inside the USPS machinable range of 1.3 to 2.5. Empty, a #9 weighs around 0.15 ounces, so even with a reply card and check enclosed, a #9 reply piece typically stays well under the 1-ounce tier.

Business Reply Mail considerations

BRM #9 envelopes require a USPS permit and have strict design rules: the words "BUSINESS REPLY MAIL," a permit number, the FIM-B mark in the upper-right corner, and the Intelligent Mail barcode in the address area. Pieces that don't meet design specs are returned by USPS or charged at full retail rates instead of the discounted BRM per-piece price.

BRM is paid on a return-only basis. You don't pay postage on the envelopes that don't come back, which is why it works well for response-rate-driven mailings like donation appeals and surveys.

Last verified: May 2026. Source: USPS Domestic Mail Manual 201 and 505.