Envelope sizing is a small secret language — A7, №10, DL, C5 — and like most secret languages, it exists to be simple once someone finally translates it. Buy the wrong size and your card rattles like a seed in a gourd or has to be folded to fit; get it right and card meets envelope like it was born there, which, in a good stationery wardrobe, it was. Here's the translation, both dialects, plus the postal fine print nobody mentions until the surcharge.
The American A-series (announcement envelopes)
The "A" sizes are the social-stationery standard in the US — flap on the long edge, made for cards:
- A2 — 4⅜ × 5¾" · fits a 4¼ × 5½" card (a half-fold of letter paper). The classic small correspondence card and RSVP size.
- A6 — 4¾ × 6½" · fits 4½ × 6¼". The everyday greeting-card size.
- A7 — 5¼ × 7¼" · fits the beloved 5 × 7 card. THE invitation envelope; if you only stock one size beyond A2, it's this.
- A9 — 5¾ × 8¾" · fits half-folded letter sheets with presence; programs, large announcements.
- №10 — 4⅛ × 9½" · the business envelope; a letter sheet folded in thirds. Correspondence can absolutely live here — a №10 in good cotton with a handwritten address is quietly elegant.
The international C-series (and DL)
Europe sizes envelopes to fit ISO paper, and the logic is beautiful: a C-size envelope fits the same-number A-size sheet. C5 (162 × 229mm) takes an A5 sheet flat or A4 folded once; C6 (114 × 162mm) takes A6 cards or A4 folded twice; DL (110 × 220mm) — the "long" envelope — takes A4 folded in thirds and is Europe's №10. Writing to the UK or the Continent? Their letterboxes and rates are built around these; a DL letter is the most at-home format you can send.
The fitting rule
Card-to-envelope clearance: add ⅛ to ¼ inch (3–6mm) to each dimension of the card. Less and insertion becomes surgery (and corners bruise in transit); more and the card travels loose, arriving with its corners softened. Enclosing extras — a photo, pen-pal treasures, a lined liner — sizes you up a notch: an A7 card rides in an A8 or a padded sleeve. Test with the real contents before ordering fifty.
The postal fine print
Three shapes trigger surcharges or machine trouble, and it's better to budget than be surprised:
- Square envelopes — undeniably chic, officially "nonmachinable": they can't ride the automated sorters, so they carry a surcharge and get hand processing. Worth it for the right announcement; just price the stamp accordingly.
- Rigid, lumpy, or thick (over ~¼") — wax seals, pressed flowers, and heavy enclosures can all tip a letter into nonmachinable or parcel territory. The counter clerk is your friend; weigh and measure the real thing.
- Minimums exist too — anything smaller than about 3½ × 5" (roughly A2's floor) may be refused outright. Tiny lovely envelopes are for hand-delivery and gift toppers.
And the perennial: anything precious, hand-lettered, or sealed deserves the two-word request at the counter — "hand cancel, please" — which spares it the machinery entirely. (Full envelope-beauty protocol: the calligraphy guide; who-goes-on-what-line: the etiquette guide.)
What to actually stock
A working drawer needs exactly three: A2 for notes and thank-yous, A7 for cards and invitations, and №10 or DL for letters. Everything else is occasional and can be bought per-project. Match the envelope's paper to the card's gravity — a 300 gsm card in a flimsy envelope is a tuxedo with plastic shoes; the weights guide decodes the numbers, and our cards ship with envelopes that were cut for them, which is the whole point of a matched set.