A6 is one of those sizes that means slightly different things depending on whether you are working in US card terms or ISO paper terms. Here is what actually fits.

A6 envelope size

In the US stationery world, an A6 envelope measures 4.75 x 6.5 inches (about 120 x 162 mm). It is built to hold an A6 card or a 4 x 6 inch photo or flat card with a little room to spare.

Note the potential confusion: in the ISO paper system, A6 paper is 105 x 148 mm. A "6 by 9" or ISO C6 envelope is sized to that ISO sheet. The US "A6 envelope" is the announcement-style envelope above, which is the one most invitation and card sellers mean.

What fits an A6 envelope

A 4 x 6 inch photo or flat card
An A6 folded note card (4.5 x 6.25 inches)
A small invitation or save-the-date

Leave about a quarter inch of clearance so the card slides in cleanly.

Where A6 is used

A6 envelopes are popular for save-the-dates, RSVP cards, thank-you notes, and small invitations. The size feels personal without being fussy, and it posts at standard letter rates in most cases (check thickness and rigidity with your carrier).

Matching cards and envelopes

If you are printing the cards too, size the card to the envelope with that quarter inch of clearance built in. For the full range of card-and-envelope pairings, see our guide to greeting card sizes.

The quick answer: a US A6 envelope is 4.75 x 6.5 inches and fits an A6 card or a 4x6 photo. Just confirm you mean the US announcement A6 and not the ISO C6, since the two are not the same.