Postcard sizes: every standard size and what it costs to mail
The size of a postcard decides its postage. USPS sorts every piece into one of three shapes, postcard, letter or flat, and each one has its own price. Here are the standard sizes, the exact limits, and the size we recommend for each kind of mailing.
The short answer
The standard postcard size is 4 × 6 inches. Anything up to 4.25 × 6 inches mails for $0.65 with a postcard stamp. Bigger cards, such as 5 × 7, 6 × 9 and 6 × 11, are mailed as letters, and cards bigger than 6.125 × 11.5 inches, such as 8.5 × 11, are mailed as flats.
- Postcard price: up to 4.25 × 6 in
- Letter price: up to 6.125 × 11.5 in
- Flat price: up to 12 × 15 in
Standard postcard sizes at a glance
| Size (inches) | USPS shape | One card with a stamp | Best for |
|---|---|---|---|
| 4 × 6 | Postcard | $0.65 | Reminders, thank-you notes, save the dates |
| 4.25 × 6 | Postcard (the largest) | $0.65 | Getting the most room at the postcard price |
| 5 × 7 | Letter | $0.82 | Invitations, announcements |
| 5.5 × 8.5 | Letter | $0.82 | A half sheet with room for a short offer |
| 6 × 9 | Letter | $0.82 | Marketing with one photo and one offer |
| 6 × 11 | Letter (the largest) | $0.82 | Marketing mail to a list (our pick) |
| 6.25 × 9 and 6.25 × 11 | Flat | Flat price | Every Door Direct Mail® |
| 8.5 × 11 | Flat | Flat price | Every Door Direct Mail® (our pick) |
Stamp prices are USPS retail prices since July 12, 2026. A business mailing hundreds or thousands of cards pays much less per card with bulk USPS Marketing Mail or EDDM®. See how much it costs to mail a postcard.
The USPS size rules
Postcard
To get the postcard price, a card must be rectangular and between 3.5 × 5 inches and 4.25 × 6 inches, and between 0.007 and 0.016 inch thick. Most postcard stock is about 0.014 inch, so thickness is rarely a problem.
Letter
A card bigger than 4.25 × 6 is mailed as a letter, up to 6.125 × 11.5 inches and 0.25 inch thick. It must be at least 0.009 inch thick. A single card needs a $0.82 Forever stamp, the same as a letter in an envelope.
Flat
A card longer than 11.5 inches, taller than 6.125 inches or thicker than 0.25 inch is a flat, the same shape as a large envelope, up to 12 × 15 inches. Flats cost the most to mail one at a time, but they are the shape Every Door Direct Mail® uses, where postage is about $0.26 a piece for any eligible size.
Shape
Length is the side parallel to the address, and height is the side across it. For postcards and letters, the length should be 1.3 to 2.5 times the height. Square cards and cards that are too long and thin can't go through the sorting machines, so they pay a surcharge and start at $1.31 each.
Source: USPS Postal Explorer, Business Mail 101 sizes for postcards and letters, and the USPS First-Class Mail price page, current as of October 2026.
Which size is best for marketing?
A marketing postcard has to be noticed in a pile of mail, so bigger usually wins. The trick is to go as big as possible without moving into a more expensive shape.
- Mailing to a list of addresses: 6 × 11. It's the largest card USPS still prices as a letter, so you get about 20% more space than a 6 × 9 for the same postage. It fits a photo, a clear offer and a phone number in large type. See our direct mail postcards service.
- Mailing every home on a route: 8.5 × 11. Every Door Direct Mail® charges the same postage for every eligible size, so the biggest card costs only a little more to print. See which sizes qualify for EDDM®.
- Reminders to your own customers: 4 × 6 or 4.25 × 6. Appointment reminders and thank-you cards don't need to compete for attention, so the postcard price makes sense.
Not sure whether to mail a list or every home? Compare EDDM® vs addressed direct mail by cost per likely customer.
Size mistakes we see
- A 6 × 9 card sent to EDDM®. It looks big, but it's a letter, not a flat, so USPS won't take it for EDDM®. A 6.25 × 9 card fixes it.
- A 5 × 7 card with a postcard stamp. It's over 4.25 × 6, so it needs letter postage and may be returned for more postage.
- Square cards. They look fresh, but every card pays the surcharge.
- Text too close to the edge. Keep words and logos at least 0.125 inch inside the trim, and leave the address area on the back clear.
Find a design in your size
Our postcard design library has designs for dozens of industries, and we size any of them for your mailing, whether it's a 6 × 11 card to a targeted mailing list or an 8.5 × 11 EDDM® postcard.
Frequently asked questions
What is the standard postcard size?
4 × 6 inches is the most common postcard size. It fits the USPS postcard rules, so one card mails for $0.65 with a postcard stamp in 2026. The largest size that still gets the postcard price is 4.25 × 6 inches.
What is the maximum size for a postcard stamp?
4.25 inches high, 6 inches long and 0.016 inch thick. The smallest is 3.5 × 5 inches and 0.007 inch thick. A card bigger than 4.25 × 6 needs a letter stamp, and a card bigger than 6.125 × 11.5 is mailed as a flat.
Can I mail a 5 × 7 or 6 × 9 postcard with a postcard stamp?
No. Both are bigger than 4.25 × 6, so USPS treats them as letters and a single card needs a $0.82 Forever stamp. Mailed in bulk as USPS Marketing Mail, they cost far less per card.
What size postcard is best for marketing?
For mail addressed to a list, we recommend 6 × 11 inches: it's the biggest card USPS still prices as a letter. For Every Door Direct Mail®, we recommend 8.5 × 11 inches, because EDDM® postage is the same for every eligible size.
Do square postcards cost more to mail?
Yes. Square cards can't run through the Post Office sorting machines, so they pay a nonmachinable surcharge. A single square card starts at $1.31 instead of $0.65 or $0.82.