#9 Envelopes

The #9 envelope has one primary purpose that no other size fills as cleanly: it fits inside a standard #10 envelope. That nested fit is the entire reason this size exists and why billing departments, nonprofits, law firms, and membership organizations order it consistently alongside their outgoing #10 stock. You send the #10 with your letter or invoice, you include an unsealed #9 inside, and the recipient drops their payment or reply in and mails it back. No addressing. No label. Just a pre-fitted return envelope that arrives with the outgoing piece and goes right back. At Business Envelopes, we carry #9 envelopes in plain white, tinted security, window, window tinted, wove colored, and starburst formats, all available blank or with custom printing. Free shipping on every order.

Number 9 Envelope Size and Dimensions

The #9 envelope measures 3 7/8 inches tall by 8 7/8 inches wide (3.875 x 8.875 inches). That's a quarter inch shorter and a quarter inch narrower than a #10 on each dimension, which is precisely the clearance needed to nest cleanly inside a #10 without wrinkling or binding. In metric, the #9 envelope measures approximately 98 x 225 mm.

It holds a standard 8.5 x 11 sheet folded into thirds, same as a #10, and mails at USPS First-Class letter postage when the contents are under 3.5 oz. For outgoing mail programs that include a pre-addressed return envelope, the #9 is the standard spec that all major billing and direct mail operations use. The #9 also fits inside a #10 window envelope, so when the outgoing piece is a window mailer, the pre-printed return address on the #9 can be positioned to show through the outer window, eliminating the need for any addressing on the outer piece at all.

No 9 Envelope Dimensions: What the Size Number Means

Like the #10, the #9 designation comes from the commercial envelope numbering system standardized in the 1870s. The number doesn't correspond directly to any physical measurement. What it tells you is where the envelope sits in the commercial size hierarchy: one step below the #10 in both height and width, with exactly the clearance needed to be a standard return-mail companion for the most widely used business envelope in the country. When buyers search "no 9 envelope dimensions" or "number 9 envelope size," they're typically confirming this nesting relationship before placing an order. The answer is 3 7/8 x 8 7/8 inches, and it fits inside a #10 with a quarter inch to spare on each side.

#9 Envelope Size: All Styles We Carry

#9 White Envelopes, Gum Flap

Plain white 24 lb wove with a moistenable gum flap. The baseline format for return mail enclosures, reply envelopes, and standard outgoing correspondence at the #9 size. Available blank or with custom logo and return address printing.

#9 White Tinted, Gum Flap

Adds a security tint interior that blocks contents from showing through the paper when held to light. Used when the return mail piece carries a check, payment details, or any sensitive personal information that shouldn't be readable in transit.

#9 White Tinted, Peel and Seal

Security tinted interior with peel-and-seal closure. For return mail programs where recipients are expected to seal the envelope before mailing it back. No moisture needed. The adhesive bonds on contact, which is more reliable than a gum flap for a return piece that gets handled by someone unfamiliar with your mailing process.

#9 Window Envelopes, Gum Flap

Clear poly-film window on a #9 body. When the pre-printed return address on the enclosed document aligns with the window position, the return address shows through without any label or printing on the envelope face. Used in sophisticated outgoing mail setups where even the return envelope carries no loose elements.

#9 Window Tinted, Gum Flap

Window plus security tint. The window shows the address, the tinted body conceals the contents. Right choice for financial, medical, and legal return mail where privacy of the response is as important as privacy of the outgoing piece.

Available in a range of wove colors. Used when the return envelope needs to stand out or when organizational branding extends to the return piece. Custom printing available on all wove colors.

#9 Starburst, Peel and Seal

Bold starburst color pattern with peel-and-seal closure. Used in direct mail and fundraiser programs where the return envelope needs to prompt action visually. The color pattern makes the return piece distinctive in a pile of mail and signals a purpose-built response piece rather than a generic envelope.

Why #9 Envelopes Matter for Return Mail Programs

Enclosing a return envelope with an outgoing billing or donation piece measurably improves response rates. The recipient doesn't need to find their own envelope, address it, or look up the mailing address. Every barrier removed between receiving a piece and responding to it makes response more likely. That's been the operating principle of direct response mailing for decades, and the #9 enclosed in a #10 is the physical implementation of it.

For nonprofits running annual appeals, including a pre-addressed #9 in every outgoing piece is standard practice. For billing operations where payment by check is still common, a return envelope speeds the payment cycle by reducing friction on the payer's end. For law firms sending executed documents that require signatures and return, the enclosed envelope signals professionalism and makes compliance easier for the client. The #9 is a small cost per piece that earns its place through what it makes possible on the response side.

For the complete window envelope range including #9 and #10 window formats across all closure types, browse our window envelopes category. For double window check envelopes that show both sender and recipient address, the double window category covers those formats. Browse the full size catalog for every envelope size we carry, or request a bulk quote for volume orders.

Frequently Asked Questions About #9 Envelopes