Postcard sizes and USPS rules: what counts as a postcard

To the Postal Service, a postcard is a rectangular card that is at least 3.5 inches high, 5 inches long and 0.007 inch thick. To get the First-Class Mail postcard price on a single card you stamp yourself, it can be no more than 4.25 inches high, 6 inches long and 0.016 inch thick. Those limits are in the Domestic Mail Manual, section 101.6.2. A bigger card is still mailable. It is just priced as a letter, or as a large envelope (a "flat") if it is bigger still.

There is one important exception. Commercial First-Class Mail, which means presorted mailings of 500 or more pieces, allows the card price on postcards up to 6 inches high and 9 inches long. That rule is in DMM section 201.1.2. So the answer to "is a 6 x 9 a postcard?" depends on how it is mailed.

What size is a standard postcard?

The Domestic Mail Manual (DMM) is the USPS rulebook for mail inside the United States. It sets a minimum and a maximum for cards that claim a card price. USPS measures a card this way: length is the side parallel to the address, and height is the side perpendicular to it.

A card that does not meet the card standards is charged the letter-size price. The DMM also says a card that misses the standards must not carry the words "Postcard" or "Double Postcard".

Is a 4 x 6, 6 x 9 or 6 x 11 card a postcard?

These are the three sizes most small businesses choose between. Here is how each one is treated in First-Class Mail.

A 6 x 11 card is too long for any card price, but it fits inside the letter limits. Letter-size mail can be up to 11.5 inches long, 6.125 inches high and 0.25 inch thick. Anything longer, taller or thicker than that is a flat, which can be up to 15 inches long, 12 inches high and 0.75 inch thick.

How thick does a postcard have to be?

The minimum is 0.007 inch. Once a card is more than 4.25 inches high or more than 6 inches long, the minimum rises to 0.009 inch. The maximum for a card price is 0.016 inch. All three numbers are in DMM 201.1.2.

The paper matters too. A card must be of uniform thickness and made of unfolded, uncreased paper or cardstock of about the quality and weight of a USPS stamped card, according to DMM 101.6.2.4. Ask your printer for the thickness of the stock in inches or "points" (one point is 0.001 inch), so a 9-point stock is 0.009 inch.

What is aspect ratio, and why does it matter?

Aspect ratio is the length of the card divided by its height. USPS sorting machines are built for mail that is wider than it is tall. For commercial cards the ratio must be between 1.3 and 2.5, inclusive. A letter-size piece outside that range is classed as nonmachinable, which means it cannot run through the standard sorting machines and can cost more.

The three sizes above are all inside the range: 6 ÷ 4 = 1.5, 9 ÷ 6 = 1.5 and 11 ÷ 6 = 1.83. A square card would have a ratio of 1.0 and fall outside it. Cards and letters must also be rectangular. Cardstock pieces may have rounded corners with a radius of no more than 1/8 inch.

What has to go on the address side?

The address side is the side with the delivery address and the postage. It may also carry a message area, so you can print on both sides (DMM 101.6.2.8). Three placement rules from DMM section 202 are worth knowing:

• Delivery address. It must be visible and legible, on the same side as the postage. USPS recommends placing it inside the area its machines read: at least 1/2 inch in from the left and right edges, and between 5/8 inch and 2-3/4 inches up from the bottom edge.
• Return address. A domestic return address goes in the upper left corner of the address side.
• Barcode clear zone. This is a strip in the lower right corner of the address side, 4-3/4 inches wide and 5/8 inch high, measured from the right and bottom edges. USPS reserves it for a barcode. The simplest approach is to leave it blank.

The address must also run parallel to the long side of the card. A delivery address printed parallel to the shorter side is one of the things that makes a letter-size piece nonmachinable.

First-Class Mail or Marketing Mail?

The postcard price described in this article is a First-Class Mail price. First-Class Mail has no minimum quantity for single pieces, so you can mail one card. Commercial First-Class Mail prices need presorted mailings of 500 or more pieces and a yearly presort mailing fee at the office where you mail.

USPS Marketing Mail is a separate bulk class for advertising. It is meant for mailings of 200 or more pieces, or 50 or more pounds, and it also has an annual mailing fee. If you mail a few dozen cards at a time, you are almost always using First-Class Mail.

How do I choose a size?

1. Count your words. A 4 x 6 card holds one offer, one photo and one way to respond. If you need more than that, go up a size.
2. Decide how you will mail. If you stamp cards yourself, only cards up to 4.25 x 6 inches get the postcard price.
3. Ask your printer or mail service for the stock thickness, and check that it is at least 0.009 inch for anything larger than 4.25 x 6.
4. Leave the address area and the lower right strip of the address side free of artwork.
5. Look up the current price for your size and class before you approve the order.

If you send through a mail service, the service normally handles the address block, barcode and postage for you. PostalNest prints 4 x 6, 6 x 9 and 6 x 11 inch cards and mails them by First-Class Mail at $1.10 per card, with printing and postage included.