A6 Envelope Size: Dimensions and Use Cases

The A6 envelope measures 4.75 inches high by 6.5 inches long. It's the smallest of the A-series envelopes commonly used for social mailings — invitations, announcements, photo cards, and greeting cards.

The A-series is a sizing convention developed for the stationery industry, where each size is built to fit a specific card insert with a small margin around it. The A6 fits an A6 card, sized at 4.5 by 6.25 inches.

This page covers the exact dimensions, what cards fit inside, USPS rules for the size, and how the A6 compares to the larger A7 and A9.

When to use an A6 envelope

The A6 is the go-to envelope for smaller social and announcement mailings.

Save-the-dates

The size has become a default for save-the-date cards because it pairs naturally with the 4.5 by 6.25 inch card stock that printers stock as A6 size. Send-outs typically arrive 6 to 12 months before a wedding and need to feel substantial without competing visually with the formal invitation that follows.

Shower and party invitations

Bridal showers, baby showers, birthday parties, and casual gatherings all use A6 envelopes routinely. The size signals "social" rather than "business," and the proportions read as personal correspondence.

Photo announcement cards

4 by 6 inch and 5 by 5 inch photo cards mail comfortably in an A6 with room for a return-address printed flap. Birth announcements, graduation announcements, and holiday photo cards are common formats.

Greeting cards

Many commercial greeting card lines are sized to fit A6 envelopes for shipping and retail packaging consistency.

When the A6 isn't the right size

Standard 5 by 7 invitation cards are slightly too large for an A6 — use an A7 envelope instead. For larger formal invitations, programs, or trifold materials, step up to A9 or A10.

A6 envelope postage and USPS rules

The A6 falls within USPS letter-size limits and qualifies for the lowest First-Class postage rate.

2026 First-Class postage for an A6 envelope

• 1 ounce, Forever stamp: $0.78
• 1 ounce, metered postage: $0.74
• Each additional ounce, up to 3.5 ounces: $0.29

The A6's aspect ratio (length divided by height) is 1.37, which sits inside the USPS machinable range of 1.3 to 2.5. The piece is automation-eligible as long as it's flexible, uniformly thick, and under 0.25 inches.

Watch the thickness

Heavy cardstock, embellishments, vellum overlays, and ribbon ties can push an A6 piece beyond the 0.25 inch letter-rate thickness limit, which triggers the nonmachinable surcharge or moves the piece to flat rate. Test a fully assembled sample by sliding it between two flat surfaces 0.25 inches apart — if it doesn't slide freely, plan for the surcharge.

Last verified: May 2026. Source: USPS Domestic Mail Manual 201.