USPS mailer size rules: postcards, letters & flats

What size a postcard has to be to mail at card prices

At the post office, “postcard” is not a description. It is a size window with a price attached. Miss the window and your card is not a card — it is a letter, and it prices like one.

For retail mail — the kind you hand across the counter with a stamp on it — the Domestic Mail Manual (DMM 101.6.2.2) sets the card-price window at: rectangular, no less than 3-1/2 inches high, 5 inches long, and 0.007 inch thick; no more than 4-1/4 inches high, 6 inches long, or 0.016 inch thick; and no more than 3.5 ounces.

Read it as a box you have to fit inside. Length runs 5 to 6 inches. Height runs 3-1/2 to 4-1/4 inches. Thickness runs 0.007 to 0.016 inch — thin enough to flex, thick enough not to vanish into a machine.

That is why 4 x 6 is the default. Six inches long is exactly the maximum, and four inches high leaves a quarter inch of headroom under the ceiling. It is the biggest common card that still fits on every dimension. A 5 x 7 blows past the six-inch limit — it is a letter, whatever the template calls it. USPS revises the DMM continuously, so confirm the current standard at usps.com before you commit a run. Our postcards get sized to that window from the first sketch.

Why some sources say 6 x 9 and others say 6 x 4-1/4

Because both are right, and they are describing different mail.

USPS keeps two separate physical-standards chapters. DMM 101 covers retail, single-piece mail — walk-in, stamp on it. DMM 201 covers commercial mail: presorted and automation mailings prepared to spec. Postcard maximums are not the same in both, and almost every article that quotes one number without saying which chapter it came from is wrong for half its readers.

Retail card price caps at 6 inches long by 4-1/4 inches high. Commercial card price (DMM 201.1.2.1) goes considerably bigger: up to 9 inches long by 6 inches high, still capped at 0.016 inch thick. USPS opened that door in 2021, and it is the source of every “postcards can now be 6 x 9” headline you have read.

Two catches. First, the 6 x 9 card price only exists inside a presorted or automation mailing — you cannot stamp one and drop it in a blue box at the card price. Second, any commercial card taller than 4-1/4 inches or longer than 6 inches has to be at least 0.009 inch thick, not 0.007. Come in under that and USPS reprices the piece as a nonmachinable letter, which is the opposite of what you were going for. If a 6 x 9 is the plan, it has to be prepared as a commercial mailing — that is what our mailing services side handles.

The minimum mailable size — and the orientation trap

The smallest thing you can mail, letter- or card-shaped, is 5 inches long by 3-1/2 inches high by 0.007 inch thick. DMM 101.2.1 states it plainly for retail mailability: anything 1/4 inch thick or less has to clear that floor. Under it, the piece is not expensive. It is nonmailable. USPS will not take it.

Here is the part that catches people. For mailability, “length” is not the longer side. DMM 601.1.1.2 defines length as the dimension parallel to the delivery address as you read it, and height as the dimension perpendicular to that. Your address block decides which number is which.

DMM 601.1.1.3 spells out the consequence with its own example: a 4 x 6 piece with the address running parallel to the 4-inch dimension is 4 inches long — under the 5-inch minimum — and is therefore nonmailable. DMM 601.1.1.4 repeats it more bluntly: postcard-size pieces with the address parallel to the shorter dimension are nonmailable.

So the same 4 x 6 card, same paper, same ink, mails fine landscape and cannot be mailed at all portrait. Nothing changed except which way the address reads. If you love a vertical layout, keep it on the front and lay the mailing panel out horizontally. Run your dimensions and orientation through our free mailer size checker before the design gets approved.

The letter maximum, and the moment a piece becomes a flat

Once a piece outgrows the card window it becomes a letter, and letters get a much bigger box. DMM 101.1.1 and DMM 201.1.1.1 agree on the numbers — a rare spot where retail and commercial do not split: no more than 11-1/2 inches long, 6-1/8 inches high, or 1/4 inch thick, and no more than 3.5 ounces. The minimums are the same 5 x 3-1/2 x 0.007 floor.

Two thresholds live inside that box. Any letter-size piece more than 6 inches long or more than 4-1/4 inches high needs a minimum thickness of 0.009 inch or it picks up a nonmachinable surcharge. And a letter-size piece over 3.5 ounces pays flat-size prices no matter what its dimensions are.

Exceed any single letter maximum and the piece becomes a flat. DMM 101.2.1 defines flats as more than 11-1/2 inches long, or more than 6-1/8 inches high, or more than 1/4 inch thick, up to a ceiling of 15 x 12 x 3/4. Note the “or.” One dimension does it. A piece 6-1/4 inches high is a flat even if it is only 9 inches long.

That matters because USPS prices by shape, and flats sit above letters. The jump is not proportional to how far you went over — an eighth of an inch and three inches buy you the same reclassification. Working with envelopes? Our envelope size calculator and letterhead & envelopes page cover the standard sizes that stay inside the letter box.

Aspect ratio, squares, and what else triggers a surcharge

Aspect ratio is length divided by height, and DMM 201.3.7 requires machinable letters to land between 1.3 and 2.5, inclusive. Fall outside and the piece is a nonmachinable letter. Be clear on what that is: a price penalty, not a rejection. DMM 601.1.1.3 works the example itself — a 5 x 8 piece addressed parallel to the 5-inch side is 5 long by 8 high, a ratio of 0.625, and the DMM calls it mailable as a nonmachinable letter. It goes. It just costs more, on every piece in the run.

Square envelopes are the famous casualty. A square has an aspect ratio of 1.0, below the 1.3 floor, so it is nonmachinable — USPS says so in as many words in its own Quick Service Guide. Wedding invitations pay for that geometry every year.

DMM 101.1.2 lists the full set of triggers:

• Aspect ratio

  Length divided by height below 1.3 or above 2.5. Squares fail at 1.0.

• Non-paper exterior

  Polybagged, polywrapped, or any exterior surface that is not paper. Paper envelope windows are fine.

• Closures

  Clasps, strings, buttons, or similar closure devices.

• Uneven or loose contents

  Pens, pencils, keys, or coins that make thickness uneven, or loose objects rattling around inside.

• Too rigid

  USPS quantifies this one: the piece must bend easily under 40 pounds of transport-belt tension around an 11-inch diameter turn.

• Too thin for its size

  Under 0.009 inch when the piece is more than 6 inches long or 4-1/4 inches high.

• Address orientation

  Delivery address parallel to the shorter dimension.

One honest caveat: that 1.3 to 2.5 rule is written into the commercial chapter. The retail card section only requires a card to be rectangular and states no ratio. If you are mailing single-piece retail cards below 1.3, ask your post office how they will price it rather than assuming.

The sizes actually worth designing to

• 4 x 6

  The safe default. Retail card price, landscape, with a quarter inch of headroom on height. Ratio 1.5 — dead center of the machinable band.

• 4-1/4 x 6

  The absolute retail card maximum. Ratio 1.41. Anything past it is a letter.

• 6 x 9

  The largest card price there is, and commercial only. Ratio 1.5. Needs 0.009 inch minimum thickness and a presorted or automation mailing.

• 6-1/4 x 9

  A quarter inch taller than 6 x 9 and a completely different animal — 6-1/4 clears the 6-1/8 letter ceiling, so it is a flat. Wrong for a card mailing, right for EDDM.

• 5-1/2 x 4-1/4

  Four-up on an 11 x 8.5 sheet. Ratio 1.294 — technically under the 1.3 floor, and permitted anyway.

That last one is worth knowing because it looks like a violation and is not. DMM 201.1.2.1 carries a written exception for exactly this layout: an 11 by 8.5 sheet cut into four identical postcards, whose 1.294 ratio USPS names in the text and allows. The four-up layout print shops have run forever is specifically blessed, ratio math notwithstanding.

Now look again at the 6 x 9 and 6-1/4 x 9 pair. A quarter of an inch moves a piece from the top of the card window into a different pricing shape entirely. That is not something you catch in proofing. It is a decision you make before you open the file. Price it both ways in our mailing cost calculator and let the postage pick the size.

EDDM size limits: the piece has to be big enough

Every Door Direct Mail inverts the usual logic. Most postal rules tell you how big you may go. EDDM Retail tells you how big you must go.

The USPS Quick Service Guide for EDDM Retail sets a minimum of 5 inches long, 3-1/2 inches high, and 0.007 inch thick; a maximum of 15 inches long, 12 inches high, and 0.75 inch thick; and a maximum weight of 3.3 ounces. So far, ordinary.

Then the rule that catches designers. An EDDM Retail piece has to be a flat, which means it must affirmatively exceed one of three thresholds: greater than 10-1/2 inches long, greater than 6-1/8 inches high, or greater than 1/4 inch thick. Being under the maximum is not enough.

Note the length number. Ordinary flats start above 11-1/2 inches long, but DMM 101.2.1 writes EDDM Retail its own exception at 10-1/2 — so a piece can sit inside letter dimensions and still qualify as an EDDM Retail flat. Confirm the current EDDM standards in USPS Quick Service Guide 140 before you commit a run.

Work that through and the whole EDDM size menu explains itself. A 9 x 6 card is comfortably letter-size and cannot go EDDM Retail at all. Add a quarter inch of height and 6-1/4 x 9 clears the 6-1/8 bar. So does 6-1/2 x 9. A 6 x 12 clears it on length, and 8-1/2 x 11 clears it on both. Those are not industry conventions — they are the smallest useful shapes that pass.

Volume rules apply too: at least 200 pieces (or 50 pounds) per mailing, no more than 5,000 per ZIP code per day, bundled in stacks of 50 to 100. Our EDDM explained guide covers routes, indicia, and paperwork, and EDDM vs. targeted mailing lists helps you choose between them.

Self-mailers and booklets lose an inch

If your piece folds and mails without an envelope, a different set of numbers applies — and they are tighter than most people expect.

Folded self-mailers (DMM 201.3.14.2) run 3-1/2 to 6 inches high and 5 to 10-1/2 inches long, 0.007 to 1/4 inch thick, no more than 3 ounces, aspect ratio inside the same 1.3 to 2.5 band, and no more than 12 panels. The 0.009 thickness floor kicks in once height passes 4-1/4 inches or length passes 6. Booklets (DMM 201.3.16.3) are similar: 3-1/2 to 6 inches high, 5 to 10-1/2 inches long, 0.009 to 1/4 inch thick, 3 ounces maximum, same ratio band.

Catch the difference. An enveloped letter can be 11-1/2 inches long. A self-mailer or booklet stops at 10-1/2. Take the envelope away and you lose an inch of length and half an ounce of weight. Designers who assume “it is a letter, so 11-1/2 is fine” find out at the counter.

Both formats also carry preparation standards — where the folded edge sits, how many tabs, where they go. DMM 101.1.2 is explicit: a self-mailer or booklet not prepared to spec is nonmachinable. That is a tabbing detail, not a design one, but it prices exactly like a design mistake. Rack cards and folded mailers both live in this territory, so settle the folds and tabs before the layout is final.

Design to the window before you design anything else

Here is the whole guide in one line: pick the size before you design, not after.

Postage is usually the largest number on a direct mail invoice — often more than the paper and the printing combined. Which makes the size decision a postage decision, and the highest-leverage choice on the entire project. It is also the one most people make last, by eyeballing a template.

Better order of operations. Decide the mail class and price category first: retail card, commercial card, letter, flat, EDDM. That hands you a size window. Design inside it. Check the finished dimensions, the orientation of the address block, and the aspect ratio. Then print.

Do it backward and the fixes get expensive. A quarter inch of extra height reclassifies a card as a flat. An address block rotated 90 degrees makes a mailable card nonmailable. A 5 x 8 with a portrait mailing panel earns a surcharge on every piece — not once, on all of them. None of that shows up in a PDF proof. All of it shows up on the postage bill.

Two last honest notes. USPS revises the Domestic Mail Manual continuously and adjusts prices on its own schedule. The dimensional standards above have been stable for years, but confirm the current numbers at usps.com or in the DMM before you commit a run, and see our note on the 2026 USPS rate change for what moves. And once the size is settled, the real question is response, not postage — our direct mail ROI guide covers the break-even math. Send us your dimensions and we will tell you which category you are in before you spend a dollar designing into the wrong one.

Frequently asked questions

• What size is a postcard for mailing?

  For retail mail with a stamp on it, USPS card pricing requires a rectangular piece between 5 and 6 inches long, 3-1/2 to 4-1/4 inches high, and 0.007 to 0.016 inch thick. That is why 4 x 6 is the standard — it sits at the top of the window with room to spare on height. Confirm current standards at usps.com, since USPS updates the Domestic Mail Manual regularly.

• Can I mail a 6 x 9 postcard at postcard prices?

  Only as part of a commercial mailing. USPS keeps separate standards for retail and commercial mail. Retail card pricing caps at 6 inches long by 4-1/4 inches high, while commercial presort and automation mailings allow cards up to 9 inches long by 6 inches high. A 6 x 9 also has to be at least 0.009 inch thick. You cannot stamp one and get the card price.

• What is the smallest piece of mail USPS will accept?

  For anything letter- or card-shaped, the floor is 5 inches long, 3-1/2 inches high, and 0.007 inch thick. Below that the piece is nonmailable, not just expensive. Watch orientation too: USPS measures length parallel to the address, so a 4 x 6 card addressed portrait is only 4 inches long and cannot be mailed. Turn the mailing panel landscape and the same card is fine.

• Why do square envelopes cost more to mail?

  Because of the aspect ratio rule. USPS requires machinable letters to have a length divided by height between 1.3 and 2.5. A square comes out at 1.0, below the floor, so it is nonmachinable and picks up a surcharge. It still mails — it just costs more on every piece. The same applies to any letter proportioned outside that range.

• What size does an EDDM postcard have to be?

  EDDM Retail pieces must qualify as flats, meaning at least one dimension has to exceed 10-1/2 inches long, 6-1/8 inches high, or 1/4 inch thick, while staying under 15 x 12 x 0.75 inches and 3.3 ounces. Being small enough is not enough — the piece must be affirmatively big on one dimension. That is why 6-1/2 x 9, 6 x 12, and 8-1/2 x 11 are the usual EDDM sizes. Confirm current standards in USPS Quick Service Guide 140.