Greeting cards come in a handful of standard sizes, named with an "A" series that, confusingly, is not the same as ISO A-paper. Here is what the common ones actually measure.

The common card sizes

These are folded card dimensions (the finished card):

A2: 4.25 x 5.5 inches, for thank-you notes and small cards
A6: 4.5 x 6.25 inches, a common all-purpose card
A7: 5 x 7 inches, the most popular size for invitations and standard greeting cards
A9: 5.5 x 8.5 inches, a larger statement card
Square: 5.25 x 5.25 inches and 6 x 6 inches are popular for a modern look

The "A" here is a US card convention, not the ISO paper series.

Matching cards to envelopes

Each card size has a matching envelope sized with a little clearance:

An A7 card pairs with an A7 envelope (5.25 x 7.25 inches)
An A6 card pairs with an A6 envelope (see our A6 envelope guide)
An A2 card pairs with an A2 envelope (4.375 x 5.75 inches)

If you print the card, size it so it slides into the envelope without forcing, usually about a quarter inch smaller on each dimension.

Choosing a size

A7 is the safe default for invitations and most greeting cards: large enough to feel substantial, standard enough that envelopes are easy to find. A2 suits short notes; square sizes stand out but sometimes cost more to mail. For a birthday card specifically, see our note on birthday card size.

In short: the common greeting card sizes are A2, A6, and A7, with A7 (5x7) being the everyday standard.