Envelope Sizes Explained: #9, #10, 9x12 and Window Options

Envelope sizes look like a secret code, #9, #10, 9x12, A7, until you know the system. Pick the wrong one and your letter won’t fit, your reply card gets lost, or your address misses the window. Here is the plain-English guide to the sizes Anchorage businesses actually use.

The everyday business sizes

The ”#” numbers refer to standard commercial envelope sizes. These are the workhorses of business mail.

Size	Dimensions	Best for
#10	4-1/8 x 9-1/2 in	The standard business letter and invoice envelope
#9	3-7/8 x 8-7/8 in	Reply envelopes that slip inside a #10
#6-3/4	3-5/8 x 6-1/2 in	Statements, remittances, small notices
9 x 12	9 x 12 in	Flat documents and reports, unfolded
10 x 13	10 x 13 in	Catalogs, large reports, presentation packets
A7	5-1/4 x 7-1/4 in	Invitations, thank-you cards, announcements

The #10 plus #9 pairing is the classic combo: you mail a letter and invoice in the #10, and the customer mails their payment back in the #9 reply envelope.

Window vs. no window

A window envelope has a clear panel so the recipient’s address on your document shows through, no separate addressing needed.

Window envelopes shine for invoicing and statements because the address is printed once (on the document) and shows through the window. Double-window envelopes show both the return and recipient addresses, common for checks and payroll.

Security tints and paper

For anything confidential, checks, statements, payroll, a security tint (the patterned blue or gray printing inside the envelope) hides the contents from being read through the paper.

Match your envelopes to your mailings

The right envelope is the one that fits your document, protects what is inside, and lines up with your address block. We print custom envelopes in every standard size with window and security-tint options, branded to match your letterhead. Request a free quote and tell us what you are mailing.