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USPS Postcard Sizes & Mailing Requirements

Designing a postcard for direct mail? Size, thickness, and layout all affect whether your piece qualifies for postcard postage rates and processes smoothly through the mail. This guide covers the practical requirements so your postcards print right and mail right.

Postcard vs Letter vs Flat — Why Size Matters for Mailing

USPS classifies mail pieces into categories based on size. The classification determines your postage rate and how the piece is processed. A piece that qualifies as a "postcard" gets the lowest First-Class rate. Slightly larger pieces are classified as "letters" and cost more. Even larger pieces become "flats" (large envelopes).

The practical takeaway: if you want the cheapest postage, your postcard must fit within USPS postcard dimensions. If you want a bigger piece for visual impact, that's fine — just know it'll mail at a higher rate.

USPS Size Classifications for Postcards

Classification	Size Range	Typical Postage
Postcard rate	Min 3.5″ × 5″, Max 4.25″ × 6″	Lowest First-Class rate
Letter rate	Larger than 4.25″ × 6″, up to 6.125″ × 11.5″	Standard First-Class letter rate
EDDM	Up to 6.125″ × 11.5″	Flat-rate per piece (no address list needed)

Most printed postcards fall into one of these categories. The 4″ × 6″ postcard is the most common size — it qualifies for the postcard rate and is large enough for effective marketing.

Common Postcard Sizes and How They Mail

Paper Thickness Requirements

USPS requires postcards to be at least 0.007 inches thick (roughly 7 pt). Anything thinner may be rejected or damaged during automated processing. In practice, most printed postcards use 14–16 pt cardstock, which is well above the minimum. This weight ensures the piece survives sorting machines, stacking, and handling without bending or tearing.

For help choosing the right stock, see our Paper Weight Guide and Postcard Paper & Finish Guide.

Mailing Side Layout

The back of a mailed postcard has specific layout considerations. The right half is generally reserved for the delivery address, return address, postage, and any postal barcodes. The left half is available for your message or additional design.

Practical guidelines for the mailing side:

• Address block: Right half of the back, positioned at least ⅝″ from the bottom edge
• Return address: Top-left corner of the back
• Postage area: Top-right corner
• Barcode clear zone: A clear strip along the bottom edge (approximately ⅝″ tall) — no dark backgrounds or text in this area
• Left half: Available for your message, secondary offer, or call-to-action

EDDM (Every Door Direct Mail)

EDDM lets you mail to every address on selected postal routes without a mailing list. You choose ZIP codes and carrier routes, and USPS delivers to every door on those routes. It's one of the most cost-effective direct mail methods for local businesses.

EDDM size requirements: the piece must be larger than standard letter size — generally at least 6.125″ long × 11″ wide or meet other USPS dimensional minimums for EDDM flats. Maximum size is typically 6.125″ × 11.5″ for flat pieces. Check current USPS EDDM guidelines for the latest requirements.

For a full breakdown of EDDM including route selection and preparation, see our Direct Mail & EDDM Guide.

Designing for Mailability — Common Mistakes

Making a postcard just slightly too large for the postcard rate. A piece that's 4.3″ × 6.1″ won't qualify for the postcard rate because it exceeds the 4.25″ × 6″ maximum. Even a quarter inch matters for postal classification.

Putting text or dark graphics in the barcode clear zone. The bottom ⅝ inch of the mailing side needs to be clear for the USPS delivery barcode. Dark backgrounds in this area can cause scanning failures and mail delays.

Using paper that's too thin. A postcard on lightweight paper bends, jams in sorting machines, or gets classified differently. Stick with 14 pt cardstock or heavier.

Not leaving enough space for the address block. The postal service needs room for the delivery address, barcode, and routing marks. A design that crowds or overlaps the address area can cause delivery problems.

Frequently Asked Questions