Standard Postcard Size in the USA: The Complete Guide

The standard postcard size in the USA is 4" x 6", but the actual USPS-approved range runs from 3.5" x 5" up to 4.25" x 6" if you want the lowest First-Class postcard rate, which is $0.65 as of the July 2026 rate change. Go bigger than that, even by an eighth of an inch, and your card gets reclassified and charged as a letter. That single rule is the reason 4x6 dominates print shops, but it's not the whole story.

I've printed and mailed postcards for small business clients for years, and the size question always comes down to one thing people don't ask: are you designing for the mailbox or for the coffee table? The two questions have different answers.

What USPS Actually Says About Postcard Dimensions

Most articles quote "4x6" and stop there. USPS is more specific, and the specifics matter if you're mailing at scale.

To qualify for the First-Class Mail postcard price, your card must be:

• Rectangular, with square corners (cardstock pieces can have rounded corners up to a 1/8 inch radius)
• At least 3.5 inches high, 5 inches long, and 0.007 inch thick
• No more than 4.25 inches high, 6 inches long, and 0.016 inch thick

Miss any of those numbers and USPS doesn't call it a postcard anymore. It becomes a letter, and letter postage applies instead, which is $0.82 for a one-ounce piece under the current rate schedule.

Here's the detail almost nobody mentions: length and height aren't interchangeable. USPS defines length as the side that runs parallel to the address, and height as the side perpendicular to it. Print your address across the short side by mistake, and you've technically flipped your own dimensions, which can trip up automated sorting even if the card is otherwise the right size.

Print Size vs. Mail Size: The Gap That Trips People Up

Here's where a lot of competitor guides get sloppy. They treat "postcard size" as one number. In practice, there's a print-shop default and a postage ceiling, and they're not the same thing.

A 5x7 card looks and feels premium, but the moment it leaves the 4.25x6 window, you're paying letter postage on it whether you like it or not. I've had clients design a beautiful 5x7 campaign, order 10,000 pieces, then get hit with a postage bill nearly 25% higher than they budgeted because nobody checked the size against the rate chart first. Check postage before you finalize art, not after.

The 6x9 Loophole Nobody Talks About

This is the piece almost every competitor article misses entirely, and it's genuinely useful if you mail in volume.

The 4.25x6 ceiling applies to single-piece, stamped postcards, the kind an individual customer buys a stamp for and drops in a mailbox. But USPS also has a separate category called "card prices" for commercial, presorted, and automation mail under DMM section 201. Under that category, a machinable card can go up to 6 inches high by 9 inches long and still qualify for card-level pricing, which is cheaper than letter pricing at the same dimensions.

That's how big real estate firms and political campaigns mail large, eye-catching 6x9 postcards without paying full letter or flat rates. It requires a USPS business mailing permit, bulk quantities, and CASS-certified address lists, so it's not something you can access by buying a single Forever stamp. But if you're setting up a recurring direct mail program, this is the size-to-cost lever that actually moves the needle, and it's worth a conversation with a Mailpiece Design Analyst before you commit to a print run.

Why Aspect Ratio Quietly Decides Your Postage

Dimensions alone don't guarantee the rate you expect. USPS also checks aspect ratio, which is length divided by height. For a mailpiece to be machinable, that ratio has to fall between 1.3 and 2.5.

A standard 4x6 card has a ratio of 1.5, comfortably inside the range. A 6x9 card comes in at 1.5 too. But a square-ish 5x5.5 card sits right around 1.1, which fails the test. A perfectly square card has a ratio of exactly 1.0, and USPS flags it as nonmachinable regardless of how small it is.

Why does this matter? Nonmachinable pieces get pulled for manual handling, and manual handling means surcharges. I've seen mailers design a trendy near-square card for aesthetics, only to find out at the post office counter that every single piece triggers an extra fee. If your design leans square for visual reasons, run the ratio math before you print, not after 5,000 cards are sitting in boxes.

Thickness Is the Spec Everyone Forgets to Check

Size gets all the attention. Thickness quietly disqualifies more postcards than people realize.

USPS requires a minimum of 0.007 inch, roughly the thickness of a standard index card. Their own guidance literally tells clerks to use an index card as a field test if they're unsure. Most print shops describe stock in "point" weight instead of inches: 100lb cover stock generally lands around 10pt (0.010"), while a heavier 16pt card is about 0.016", which is the absolute maximum before it stops qualifying for the postcard rate at all.

The part that catches people off guard: coatings add thickness. A gloss UV coating or a thick laminate can push a card that measured fine as raw stock over the 0.016" ceiling once it's finished. If you're ordering a heavier premium card with spot UV or foil, ask your printer to confirm the finished thickness, not just the base stock weight. I've had a client's holiday card get bounced back for exceeding thickness purely because of a laminate layer nobody accounted for.

Matching Size to the Job, Not the Trend

Bigger isn't automatically better, and smaller isn't automatically cheaper once you factor in response rates. Here's how I actually steer clients:

• 4x6 for repeat mailings: appointment reminders, coupon drops, anything you're sending monthly where postage adds up fast.
• 5x7 for one-off promotions and event invites where you want a size bump without going oversized.
• 5.5x8.5 for menus, service lists, or anything with enough content that 4x6 feels cramped.
• 6x9 for real estate listings, high-value offers, or campaigns where standing out in the mailbox matters more than shaving pennies off postage.
• 6x11 for jumbo, poster-style mailers meant to be genuinely hard to ignore, usually for high-ticket offers where a few extra cents of postage is irrelevant next to the deal size.

The honest rule of thumb: if your card is disposable and frequent, stay small. If it's a rare, high-value touchpoint, size up and eat the postage difference.

Design Margins for Standard Postcard Sizes

Getting the paper size right doesn't help if your text gets trimmed off during cutting. For any standard postcard size:

• Build in a 0.125 inch bleed on all four sides so background colors and images extend past the trim line with no white edges.
• Keep all text and important elements at least 0.25 inch inside the trim edge, and a bit more on the address side.
• Leave the bottom right corner of the address side clear, roughly 4.75" x 2" as USPS convention, for the postage indicia and barcode area. Printing artwork under a barcode zone can cause automated readers to misfire.
• Remember length runs parallel to the address line. Design your address block on the correct axis or your "6x9" card can accidentally read as a 9x6 to USPS scanners.

When "Standard" Doesn't Apply

Not every postcard needs to fit USPS rules at all. If you're handing cards out at a trade show, tucking them into product packaging, or using them purely as a leave-behind, none of the mailing dimensions matter, and you can print at any size that suits the design, including oversized jumbo formats well beyond 6x11.

The USPS rules only bite when the card actually travels through the mail stream. EDDM (Every Door Direct Mail) campaigns follow a separate size and weight framework from single-piece First-Class mail, so if you're planning a saturation mailing to every address on a route, check EDDM specs separately rather than assuming the same 4.25x6 ceiling applies.

Frequently Asked Question

What is the most common postcard size printed in the US?

4" x 6" is the default at nearly every print shop, mainly because it's the largest size that comfortably clears the USPS postcard rate window without cutting it close.

Can I mail a 5x7 postcard for the postcard rate?

No. A 5x7 card exceeds the 4.25" x 6" maximum, so USPS charges it as a letter, not a postcard, even though it's still a flat piece of cardstock.

Does a square postcard cost more to mail?

Often yes. A square card has an aspect ratio of 1.0, which falls outside the 1.3 to 2.5 machinable range, so it typically gets hit with a nonmachinable surcharge.

How thick can a postcard be and still qualify for postcard postage?

Between 0.007 inch and 0.016 inch. Thicker cardstock, especially with a heavy laminate or UV coating, can push you past the limit without you realizing it until it's rejected.

Is there a smaller minimum size for postcards?

Yes, 3.5" x 5" is the legal floor. Anything smaller isn't mailable at all as a standard postcard.

Can businesses mail larger postcards for the cheaper card rate?

Yes, but only through commercial presorted or automation mail with a USPS permit, where machinable cards up to 6" x 9" can still qualify for card-level pricing instead of letter rates.

What size postcard is best for real estate mailers?

5.5x8.5 or 6x9 tend to perform well for real estate because they carry more photos and detail without tipping into flat-rate postage, assuming you're mailing in bulk.

Do postcard size rules apply to postcards I'm not mailing?

No. USPS dimension and thickness rules only matter once a card enters the mail stream. Cards handed out in person or included in packaging can be any size you want.