Introduction
Envelope sizes aren’t random — they cluster into a handful of families, each built around a specific kind of insert. Knowing the families makes the whole subject make sense: instead of memorizing dozens of dimensions, you learn five categories and match your contents to one of them.
This guide explains the different envelope sizes by family — commercial, A-series, catalog, square, and coin — what each one holds, and how to choose. For a pure dimension-by-dimension lookup table, see our list of envelope sizes page; this one explains the why behind the families.
The Five Size Families
Envelope sizes organize into five practical families.
| Family | Sizes | Primary use |
|---|---|---|
| Commercial | #10, #9, #6¾, #7 | Business letters |
| A-Series | A2–A10 | Invitations, cards |
| Catalog | 6×9, 9×12, 10×13 | Flat documents, catalogs |
| Square | 5×5, 6×6 | Invitations, cards |
| Coin | #1–#7 | Small items |
Each family exists because of a specific insert type:
| Family | Built around |
|---|---|
| Commercial | Letter paper, folded |
| A-Series | Card and photo sizes |
| Catalog | Flat, unfolded content |
| Square | Square cards, design |
| Coin | Small objects |
Commercial Family
Commercial envelopes are sized to letter paper folded to fit.
| Size | Dimensions | Holds |
|---|---|---|
| #10 | 4⅛” × 9½” | Letter in thirds |
| #9 | 3⅞” × 8⅞” | Reply inside #10 |
| #6¾ | 3⅝” × 6½” | Check, short form |
| #7 | 3¾” × 6¾” | Small form |
#10 is the standard business envelope; #9 is its reply companion. For #10 specs, see our number 10 envelope standards guide.
A-Series Family
The A-series is the invitation and stationery family, keyed to card sizes.
| Size | Dimensions | Holds |
|---|---|---|
| A2 | 4⅜” × 5¾” | 4×5½ card |
| A6 | 4¾” × 6½” | 4×6 photo/card |
| A7 | 5¼” × 7¼” | 5×7 card |
| A8 | 5½” × 8⅛” | Larger invitation |
| A10 | 6″ × 9½” | Folded letter |
A7 is the most common invitation size. For invitation sizes, see our invitation envelope sizes guide.
Catalog Family
Catalog envelopes hold flat, unfolded content — catalogs, reports, booklets.
| Size | Dimensions | Holds |
|---|---|---|
| 6×9 | 6″ × 9″ | Booklet, brochure |
| 9×12 | 9″ × 12″ | Flat 8½×11 |
| 10×13 | 10″ × 13″ | Thick folder, report |
The catalog family is distinguished by its open-end design and flat-mail postage class.
| Trait | Catalog family |
|---|---|
| Design | Open-end (flap on long side) |
| Contents | Unfolded |
| Postage | Flat rate |
| Thickness | Extra room for stack |
Square Family
Square envelopes are the design-driven family for invitations and cards.
| Size | Dimensions | Use |
|---|---|---|
| 5×5 | 5″ × 5″ | Small card, RSVP |
| 5½×5½ | 5.5″ × 5.5″ | Invitation |
| 6×6 | 6″ × 6″ | Invitation, announcement |
Square mail carries a non-machinable postal surcharge — a key consideration before choosing. For details, see our square envelope sizes guide.
| Trait | Square family |
|---|---|
| Look | Modern, distinctive |
| Postage | Surcharge applies |
| Best for | Invitations, cards |
Coin Family
Coin envelopes are the small-item family, from #1 to #7.
| Size | Dimensions | Holds |
|---|---|---|
| #1 | 2¼” × 3½” | Coins, small parts |
| #3 | 2½” × 4¼” | Seeds, jewelry |
| #5 | 3⅛” × 5½” | Parts, small items |
| #7 | 3½” × 6½” | Components |
Coin envelopes serve small, non-paper contents:
| Use | Coin envelope |
|---|---|
| Coins | #1 |
| Seeds | #3 |
| Jewelry | #3 |
| Electronic parts | #5 |
| Screws/fasteners | #7 |
How to Choose by Contents
Match your contents to the right family.
| Your contents | Family | Size |
|---|---|---|
| Letter, folded | Commercial | #10 |
| Card or photo | A-Series | A6/A7 |
| Flat document | Catalog | 9×12 |
| Square card | Square | 6×6 |
| Small object | Coin | #1–#7 |
The decision flow is simple — identify the insert type, then pick the family, then the exact size.
Family Summary at a Glance
| Family | Key sizes | Distinguishing feature |
|---|---|---|
| Commercial | #10, #9 | Letter-paper fit |
| A-Series | A2–A10 | Card-size keyed |
| Catalog | 9×12, 10×13 | Open-end, flat |
| Square | 5×5, 6×6 | Square, surcharge |
| Coin | #1–#7 | Small-item sized |
For the complete dimension-by-dimension table, see our list of envelope sizes page and the envelope size chart.
Conclusion
The different envelope sizes make sense once you see the five families — commercial for folded letters, A-series for cards and invitations, catalog for flat documents, square for design-driven invites, and coin for small objects. Match your contents to the family first, then the exact size within it. For a full lookup table, see our list of envelope sizes guide.
Need any of these sizes at production volume? Request a quote.