Envelope sizes: commercial, announcement, booklet, square and ISO C
Four naming systems in one place — numbered commercial, lettered announcement, booklet and ISO C — plus the quarter-inch fit rule and what costs extra to mail.
Envelopes are described by four naming systems that don't reference each other: numbered commercial sizes, lettered announcement sizes, plain-inch booklet sizes, and the ISO C series. Here they are together, with the fit rule that saves most of the guesswork.
Fig. 6 — #10 and DL do the same job on different continents; A7 and C6 are the invitation workhorses.
Commercial (numbered)
Commercial numbered envelope sizes in inches and millimetres
Number
Inches
Millimetres
Holds
#6¾
3⅝ × 6½
92 × 165
Letter folded in 4 · statements
#7
3¾ × 6¾
95 × 171
—
#7¾ (Monarch)
3⅞ × 7½
98 × 191
Monarch letterhead folded in 3
#9
3⅞ × 8⅞
98 × 225
Reply envelope · fits inside a #10
#10
4⅛ × 9½
105 × 241
Letter folded in 3 · the business default
#11
4½ × 10⅜
114 × 264
Legal folded in 3
#12
4¾ × 11
121 × 279
—
#14
5 × 11½
127 × 292
Legal · bulky enclosures
Announcement (lettered) and baronial
Announcement and baronial envelope sizes in inches and millimetres
Name
Inches
Millimetres
Card it takes
A1 / 4-bar
3⅝ × 5⅛
92 × 130
3.5 × 4.875
A2
4⅜ × 5¾
111 × 146
4.25 × 5.5
A6
4¾ × 6½
121 × 165
4.5 × 6.25
A7
5¼ × 7¼
133 × 184
5 × 7 — wedding standard
A8
5½ × 8⅛
140 × 206
5.25 × 7.875
A9
5¾ × 8¾
146 × 222
5.5 × 8.5
A10
6 × 9½
152 × 241
5.75 × 9.25
Baronial 4-bar
3⅝ × 5⅛
92 × 130
Deep pointed flap · formal
Baronial #5½
4⅜ × 5¾
111 × 146
Deep pointed flap
Baronial #6
4¾ × 6½
121 × 165
Deep pointed flap
Flap shape matters for liners and seals. Announcement envelopes have a shallow square or slightly pointed flap; baronial have a deep triangular one. The baronial flap is what you want under a wax seal — there's more surface to press into.
Booklet, catalog and square
Booklet, catalog and square envelope sizes in inches and millimetres
Name
Inches
Millimetres
Notes
Booklet 6 × 9
6 × 9
152 × 229
Flat Letter-half, open side
Booklet 9 × 12
9 × 12
229 × 305
Flat Letter unfolded
Catalog 10 × 13
10 × 13
254 × 330
Open end, centre seam
Square 5½
5½ × 5½
140 × 140
Non-machinable surcharge
Square 6½
6½ × 6½
165 × 165
Non-machinable surcharge
Square 7½
7½ × 7½
191 × 191
Non-machinable surcharge
ISO C series
ISO C series envelope sizes in millimetres and inches
Envelope
Millimetres
Inches
Takes
C4
229 × 324
9.0 × 12.8
A4 flat
C5
162 × 229
6.4 × 9.0
A5 flat · A4 folded once
C6
114 × 162
4.5 × 6.4
A6 flat · A5 folded once
C6/C5
114 × 229
4.5 × 9.0
A4 folded in three
C7
81 × 114
3.2 × 4.5
A7 flat
C7/C6
81 × 162
3.2 × 6.4
A6 folded once
DL
110 × 220
4.3 × 8.7
A4 folded in three — the European business default
What costs extra to mail
In the US, an envelope mails at the standard letter rate only if it's rectangular, between 3.5 × 5 and 6.125 × 11.5 inches, no more than ¼ inch thick, and flexible. Break any of those and you're paying a non-machinable surcharge — which is per piece, so on 200 invitations it's real money.
The usual culprits at a wedding: square envelopes (always surcharged), wax seals and ribbon (rigid and lumpy), thick suites over ¼ inch, and vellum wraps that stiffen the whole packet. None of them are mistakes — just budget for hand-cancelling, and take one assembled sample to the counter before you buy 200 stamps.