Envelope Sizes Chart: Every Commercial, Catalog, Coin and Check Size Explained

Ordering the wrong envelope size is one of those mistakes you only make once after the documents go out folded wrong, or arrive unfolded but in the wrong size, or after a full batch of checks misaligns because the envelope was for invoices. This guide is a complete reference for every envelope size we carry, with dimensions in inches and millimeters, maximum enclosure sizes, postage classification, and a plain-language description of what each size is for. Use it as a lookup before every new order.

How Envelope Numbers Work: What the System Actually Means

The commercial envelope numbering system used in the United States was developed by William Irwin Martin in 1876 to standardize what had been an inconsistent naming convention across manufacturers. The numbers in the commercial series (#6 3/4, #9, #10, #11, #12, #14) are legacy catalog identifiers they do not directly correspond to any dimension. A #10 is not ten of anything. It is the name for an envelope that happens to measure 4-1/8 x 9-1/2 inches, because that was its designation in the Martin catalog.

The catalog envelope series (6x9, 9x12, 10x13, 10x15) does use its actual dimensions as the name, in inches, width by height. This is the more intuitive system. A 9x12 is 9 inches wide and 12 inches tall. For catalog envelopes, the name tells you the size. For commercial (numbered) envelopes, you need the reference table.

USPS classifies envelopes as letters or flats based on physical dimensions. Letter rate ($0.78 for the first ounce in 2026) applies to pieces no taller than 6-1/8 inches, no wider than 11-1/2 inches, and no thicker than 1/4 inch. Flat rate ($1.63 for the first ounce in 2026) applies to everything larger. Every 9x12 always mails as a flat, regardless of content weight. Every #10 always mails as a letter on qualifying contents. Several intermediate sizes (6x9) can qualify as either depending on contents.

Commercial Envelope Sizes: #6 3/4 Through #14

Size	Dimensions (inches)	Dimensions (mm)	Max Enclosure (inches)	Postage Class	Primary Use
#6 3/4	3-5/8 x 6-1/2	92 x 165	3-3/8 x 6-1/4	Letter	Reply/remittance envelopes, donation response; fits inside #10 outgoing with clearance
#9	3-7/8 x 8-7/8	98 x 225	3-5/8 x 8-5/8	Letter	Standard reply/return envelope nested inside #10; donation appeals, billing reply
#10	4-1/8 x 9-1/2	105 x 241	3-7/8 x 9-1/4	Letter	US business correspondence standard; fits 8.5 x 11 sheet tri-folded into thirds
#11	4-1/2 x 10-3/8	114 x 264	4-1/4 x 10-1/8	Letter	Slightly larger than #10; fits documents slightly too long for standard tri-fold or with bulkier inserts
#12	4-3/4 x 11	121 x 279	4-1/2 x 10-3/4	Letter	Larger correspondence; specialty applications; fits #10 with more clearance
#14	5 x 11-1/2	127 x 292	4-3/4 x 11-1/4	Letter	Legal-size correspondence and documents; at maximum letter-rate length (11.5")

Browse #10 envelopes, and all commercial sizes at the full catalog.

Catalog and Flat Envelope Sizes: 6x9 Through 10x15

Size	Dimensions (inches)	Dimensions (mm)	Max Enclosure (inches)	Postage Class	Primary Use
6 x 9	6 x 9	152 x 229	5-3/4 x 8-3/4	Letter (light) or Flat	Half-folded Letter sheets, 5x7 photos, brochures, nonprofit correspondence
9 x 12	9 x 12	229 x 305	8-3/4 x 11-3/4	Always Flat	Standard flat-document mailer; unfolded Letter, certificates, reports, brochures
9-1/2 x 12-1/2	9-1/2 x 12-1/2	241 x 318	9-1/4 x 12-1/4	Always Flat	When 9x12 is too tight for thick packets or wide printed borders; bridge size
10 x 13	10 x 13	254 x 330	9-3/4 x 12-3/4	Always Flat	Legal-size (8.5 x 14) documents flat; thick multi-page packets; architectural scale prints
10 x 15	10 x 15	254 x 381	9-3/4 x 14-3/4	Always Flat	Oversized documents, posters, folded blueprints, large certificates, art prints

Browse 6x9 envelopes, 9x12 envelopes, and 10x13 and 10x15 clasp envelopes.

Coin Envelope Sizes

Size	Dimensions (inches)	Mailable?	Primary Use
#1 Coin	2-1/4 x 3-1/2	No (too small for USPS)	Single coins, watch parts, small gems, seeds, tiny hardware fasteners
#7 Coin	2-7/8 x 4-7/8	No (too small for USPS)	Coin sets, jewelry, keys, SD cards, USB drives, evidence items
4-5/8 x 6-3/4	4-5/8 x 6-3/4	Yes (letter rate)	Gift cards, 4x6 photos, small booklets; retail and gift packaging

USPS minimum mailable size is 3-1/2 x 5 inches. Coin envelopes are storage and organization tools, not postal mailers (with the exception of the largest 4-5/8 x 6-3/4 format). Browse all sizes at coin envelopes.

Check Envelope Sizes

Format	Dimensions (inches)	Windows	Security Tint	Compatible Check Format
Double Window (check)	8-3/4 x 3-9/16	2	Yes	QuickBooks Desktop voucher, ADP, Sage, Intuit
Double Window (invoice/statement)	8-7/8 x 3-7/8	2	No	Invoice and statement programs, wider format
#10 Double Window	4-1/8 x 9-1/2	2	Yes	Invoice and statement programs in #10 format

Check and invoice double window formats are not interchangeable window positions differ. Always run a 10-sample test with your specific software before ordering. Browse at double window envelopes.

Which Envelope Size for Which Document: Quick Decision Guide

Document or Contents	Fold	Envelope Size	Postage
8.5 x 11 sheet (invoice, letter, statement)	Tri-fold (thirds)	#10	$0.78 (letter rate)
8.5 x 11 sheet	Half-fold	6x9	$0.78 (letter rate on light contents)
8.5 x 11 sheet	No fold	9x12	$1.63 (flat rate, always)
5 x 7 card or photo	No fold	6x9	$0.78 (letter rate)
Printed business check (voucher)	Fold per check format	8-3/4 x 3-9/16 double window	$0.78 (letter rate)
Donation reply device or form	Per form design	#6 3/4 (goes inside #10 outer)	Return envelope no outgoing postage
Reply envelope (donor/customer return)	None	#9 (goes inside #10 outer)	Return envelope no outgoing postage
Legal document 8.5 x 14	No fold	10x13 clasp or Tyvek	$1.63 (flat rate)
Architectural ARCH A drawing (9x12)	No fold	9x12	$1.63 (flat rate)
Certificate, diploma	No fold	9x12	$1.63 (flat rate)
Documents requiring tear/moisture protection	Varies	Tyvek (6x9, 9x12, 10x13, 10x15)	Varies by size

The #6 3/4 and #9: How the Reply Envelope Sizing System Works

These two sizes are almost always used as return/reply envelopes inside an outgoing #10, not as the outgoing envelope themselves. Understanding the nesting math prevents ordering mistakes:

A #9 envelope (3-7/8 x 8-7/8 inches) fits inside a #10 (4-1/8 x 9-1/2 inches) with approximately 1/4-inch clearance on each side. This is the nested configuration used in billing programs, donation appeals, and subscription renewals the outgoing letter and reply form go in the #10 outer, the #9 reply envelope nestles alongside them, and the recipient returns their payment, response card, or donation in the enclosed #9. Both the outgoing #10 and the enclosed #9 can carry window formats, colored stock, or custom printing.

A #6 3/4 envelope (3-5/8 x 6-1/2 inches) is the remittance format slightly smaller than #9, sized for a reply device where the large flap contains a printed donation form or payment slip that folds over the sealed envelope face. It fits inside a #10 with room alongside a letter. For nonprofits and donation programs specifically, the #6 3/4 remittance format is the standard, because the large flap provides space for donation amount boxes, credit card fields, and acknowledgment language without a separate response card insert.

What Happens at Each USPS Size Boundary

Knowing exactly where the size thresholds fall tells you how to keep your mailing costs optimized.

The letter-rate maximum height is 6-1/8 inches. The 6x9 envelope at exactly 6 inches tall clears this threshold qualifying for letter rate on contents that are under 1/4 inch thick and 3.5 oz. The 9x12 at 12 inches tall is double the maximum always a flat, no exceptions. The #14 at 5 x 11-1/2 inches is at the maximum letter-rate length (11.5 inches) qualifying for letter rate.

The nonmachinable surcharge of $0.49 triggers on any letter-rate piece that cannot run through automated sorting: rigid contents, clasps or buttons, square orientation, or an aspect ratio outside 1.3:1 to 2.5:1. A standard #10 has an aspect ratio of approximately 2.3:1 safely within the machinable range. A 6x9 has an aspect ratio of exactly 1.5:1 safely machinable. A square envelope (equal height and length) has an aspect ratio of 1.0:1 always nonmachinable, always triggers the surcharge.

Why Choose Business Envelopes for Every Size in Your Program

Business Envelopes has carried every size in this chart since 1997: commercial #6 3/4 through #14, catalog 6x9 through 10x15, coin #1 through 4-5/8 x 6-3/4, and check envelope formats for every major accounting software. Every size is available in white wove, brown kraft, Tyvek, or colored stock where applicable. Every format is available blank or custom printed with your logo and return address. Self-seal and gummed formats available on all commercial and catalog sizes. Window formats in single and double window across the #9, #10, #11, #12, and #14 commercial range. Free ground shipping on every order, no minimums, same or next business day on blank stock.

Frequently Asked Questions

What is the most common envelope size for business mail?

The #10 envelope (4-1/8 x 9-1/2 inches) is the most commonly ordered commercial envelope in the United States. It holds a standard 8.5 x 11 sheet folded into thirds and mails at First-Class letter rate. It is the default for invoices, statements, business letters, billing correspondence, and most direct mail programs.

What envelope size holds an 8.5 x 11 sheet without folding?

A 9x12 envelope (9 x 12 inches, interior approximately 8-3/4 x 11-3/4 inches) holds a standard 8.5 x 11 sheet flat with about 1/4-inch clearance on each side. No folding. Note that 9x12 always mails as a USPS flat at $1.63 minimum significantly more expensive than the #10 letter rate of $0.78 for the same contents folded.

What is the size difference between a #9 and #10 envelope?

A #9 envelope is 3-7/8 x 8-7/8 inches. A #10 envelope is 4-1/8 x 9-1/2 inches. The #9 is slightly smaller in both dimensions, which allows it to fit neatly inside a #10 the standard configuration for reply envelopes enclosed in outgoing billing and appeal mailings.

Does a 6x9 envelope qualify for letter-rate postage?

Yes, on qualifying contents. At 6 inches tall, it clears the USPS letter-rate maximum height of 6-1/8 inches. Contents must be under 3.5 oz and under 1/4 inch thick for letter rate to apply. Thicker or heavier contents reclassify the piece as a flat at $1.63 minimum. Always measure a fully assembled piece before a large mailing.

What is the difference between a booklet and catalog envelope?

Booklet envelopes open on the long edge (the flap runs across the long dimension). Catalog envelopes open on the short edge (the flap runs across the width). Both 9x12 booklet and 9x12 catalog are the same outer dimensions the difference is opening orientation and seam construction, which affects machine insertability and structural strength.

What is the #6 3/4 envelope used for?

The #6 3/4 (3-5/8 x 6-1/2 inches) is the standard remittance envelope for nonprofit fundraising, donation programs, and payment reply. Its large flap provides surface area for printed donation fields, suggested amounts, and credit card capture. It nests inside a #10 outgoing envelope alongside the appeal letter.

When do I need a 10x13 instead of a 9x12?

Use 10x13 when your document is legal-size (8.5 x 14 inches, which does not fit in a 9x12), when the contents are thick enough that sealing a 9x12 requires forcing the flap, when printed borders compress against the 9x12 interior walls, or when inserting multiple documents simultaneously creates seam stress on the 9x12.