---
title: "USPS size requirements for postcards and letters"
description: "A USPS postcard runs 3.5 × 5 in to 4.25 × 6 in with a stamp, or up to 6 × 9 in mailed commercially. Letter, flat, thickness, and aspect ratio limits."
canonical: https://trysincerely.com/guides/usps-mail-size-requirements
last_updated: 2026-10-11
---
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# USPS size requirements for postcards and letters

> A USPS postcard runs 3.5 × 5 in to 4.25 × 6 in with a stamp, or up to 6 × 9 in mailed commercially. Letter, flat, thickness, and aspect ratio limits.

Source: https://trysincerely.com/guides/usps-mail-size-requirements

A USPS postcard has to be at least 3-1/2 by 5 inches and 0.007 inch thick. To get the postcard price with a stamp, it can be no bigger than 4-1/4 by 6 inches and 0.016 inch thick. Mailed commercially, through a printer or mail house with a permit, a postcard can reach 6 by 9 inches and still pay the postcard price. A letter can be up to 6-1/8 by 11-1/2 inches and 1/4 inch thick, and its length divided by its height has to land between 1.3 and 2.5. Anything larger is a flat, up to 12 by 15 inches.

![A to-scale drawing of nested mail sizes in inches: a 4.25 × 6 stamped postcard inside a 6 × 9 commercial postcard, inside the 6.125 × 11.5 letter limit, inside the 12 × 15 flat limit.](https://trysincerely.com/images/guides/usps-mail-size-requirements.webp)

*USPS size requirements nest, so a postcard that outgrows its limit pays the letter price, and a letter that outgrows 6-1/8 × 11-1/2 inches becomes a flat.*

The rules come from the USPS Domestic Mail Manual. [Section 101](https://pe.usps.com/text/dmm300/101.htm) covers stamped mail and [section 201](https://pe.usps.com/text/dmm300/201.htm) covers commercial mail. Prices here are from [USPS Notice 123](https://pe.usps.com/text/dmm300/Notice123.htm), effective October 4, 2026. We checked all three on October 11, 2026.

## USPS size limits at a glance

USPS sorts mail by shape before it looks at price. Height is the side perpendicular to the address. Length is the side the address runs along.

| Shape                       | Minimum (height × length) | Maximum (height × length) | Thickness                                         | Weight limit                   |
| --------------------------- | ------------------------- | ------------------------- | ------------------------------------------------- | ------------------------------ |
| Postcard, with a stamp      | 3.5 × 5 in                | 4.25 × 6 in               | 0.007 to 0.016 in                                 | Card price stops at 3.5 oz     |
| Postcard, commercial        | 3.5 × 5 in                | 6 × 9 in                  | 0.007 to 0.016 in (0.009 if larger than 4.25 × 6) | 3.5 oz                         |
| Letter, including envelopes | 3.5 × 5 in                | 6.125 × 11.5 in           | 0.007 to 0.25 in (0.009 if larger than 4.25 × 6)  | 3.5 oz, then flat prices apply |
| Flat (large envelope)       | Larger than a letter      | 12 × 15 in                | Up to 0.75 in                                     | 13 oz for First-Class Mail     |

A piece up to 1/4 inch thick that is smaller than 3-1/2 by 5 inches, or thinner than 0.007 inch, is not mailable at all. A piece over 12 by 15 inches or 3/4 inch thick is a parcel.

## Postcard size requirements

A postcard is one unfolded sheet of card stock. The [section 201 card rules](https://pe.usps.com/text/dmm300/201.htm) set three tests: size, thickness, and shape. A card that is too big pays a letter price. A card that is too thin for its size, or fails the shape test, pays the higher nonmachinable letter price.

The size test depends on how you pay. A card mailed with a stamp from a post office or a mailbox qualifies for the postcard price only up to 4-1/4 by 6 inches. In commercial First-Class Mail, the limit is 6 by 9 inches. USPS raised it in August 2021, as its [PostalPro note on the larger postcard](https://postalpro.usps.com/large-postcard-faqs) explains. A printed campaign sent through a print vendor goes out commercially, so the 6 by 9 inch limit is the one that applies to it.

The thickness test has a floor and a ceiling. Every card needs at least 0.007 inch. A card larger than 4-1/4 by 6 inches needs 0.009 inch, and no card may be thicker than 0.016 inch. Paper thickness is often quoted in points, where one point is a thousandth of an inch. A 14 pt card is 0.014 inch, inside the limit.

The shape test is the aspect ratio, covered below. Corners can be square or rounded to a radius of no more than 1/8 inch.

## Letter size requirements

A letter is anything up to 6-1/8 inches high, 11-1/2 inches long, and 1/4 inch thick. That includes envelopes, folded self-mailers, and postcards too big for the card price. A letter heavier than 3.5 ounces pays the flat price even when it fits the letter dimensions.

| Envelope or piece       | Size (height × length) | Aspect ratio | USPS shape           |
| ----------------------- | ---------------------- | ------------ | -------------------- |
| #10 business envelope   | 4.125 × 9.5 in         | 2.30         | Letter               |
| 6 × 9 booklet envelope  | 6 × 9 in               | 1.50         | Letter               |
| 6 × 11 postcard         | 6 × 11 in              | 1.83         | Letter               |
| 6 × 6 square envelope   | 6 × 6 in               | 1.00         | Nonmachinable letter |
| 9 × 12 catalog envelope | 9 × 12 in              | Not tested   | Flat                 |

Mailed one at a time, a 1-ounce letter costs $0.82 with a stamp and $0.78 metered. Each extra ounce adds $0.29 at the stamp price, up to 3.5 ounces. Weigh a finished sample before the full print run.

For folded self-mailers, section 201 adds its own limits: 3-1/2 to 6 inches high, 5 to 10-1/2 inches long, no more than 3 ounces, and tabs or glue in set places. The [self-mailer entry](https://trysincerely.com/glossary/self-mailer) covers them.

## The aspect ratio rule

USPS sorting machines need a piece that is longer than it is tall. The rule is length divided by height, and the result has to fall between 1.3 and 2.5. Length is measured along the address, so a piece addressed parallel to its short side fails even when its outline looks fine.

Some quick checks:

- A 4 × 6 card is 6 ÷ 4 = 1.5. It passes.
- A #10 envelope is 9.5 ÷ 4.125 = 2.3. It passes.
- A 6 × 6 square envelope is 1.0. It fails, and pays the nonmachinable price.
- A 3.5 × 11 slim card is 3.14. It fails on the long side.

There is one named exception. Four postcards cut from an 8-1/2 by 11 inch sheet measure 4-1/4 by 5-1/2 inches, a ratio of 1.294, and USPS accepts them at the card price anyway.

## What makes a piece nonmachinable

[Section 101](https://pe.usps.com/text/dmm300/101.htm) lists what keeps a letter off the sorting machines. A letter is nonmachinable when it:

- has an aspect ratio under 1.3 or over 2.5
- is wrapped in plastic or has an outside surface that is not paper, though a paper envelope with a window is fine
- closes with a clasp, string, or button
- holds a pen, key, coin, or other lump that makes it uneven
- is too stiff to bend around an 11-inch turn on a sorting belt
- is thinner than 0.009 inch while larger than 4-1/4 by 6 inches
- has its address running parallel to the short side
- is a self-mailer or booklet that misses its folding and tabbing rules

The penalty depends on how the letter is paid for. As of October 2026, a stamped or metered letter adds a $0.49 nonmachinable surcharge to the normal price. A commercial letter moves to a different price row instead. At the 3-digit presort level, a nonmachinable First-Class letter costs $0.979, against $0.672 for a letter the machines can sort. In Marketing Mail the same move is $1.090 against $0.435. On 2,000 square envelopes, that gap comes to $614 in First-Class postage. The [nonmachinable surcharge entry](https://trysincerely.com/glossary/nonmachinable-surcharge) has more.

## Common postcard sizes and their price category

Three printed postcard sizes are common, and they do not all price the same way. Prices are from Notice 123, effective October 4, 2026, for a piece under one ounce. The commercial and Marketing Mail columns show the 3-digit automation price, the middle of three presort tiers.

| Postcard size | Aspect ratio | Mailed with a stamp   | Commercial First-Class | USPS Marketing Mail  |
| ------------- | ------------ | --------------------- | ---------------------- | -------------------- |
| 4 × 6 in      | 1.50         | Postcard price, $0.65 | Postcard price, $0.478 | Letter price, $0.435 |
| 6 × 9 in      | 1.50         | Letter price, $0.82   | Postcard price, $0.478 | Letter price, $0.435 |
| 6 × 11 in     | 1.83         | Letter price, $0.82   | Letter price, $0.672   | Letter price, $0.435 |

In commercial First-Class Mail, a 6 × 9 card costs the same as a 4 × 6, $0.478 at the 3-digit automation level. A 6 × 11 card pays the letter price in every column, because it is longer than 9 inches.

USPS Marketing Mail has no postcard price, so every postcard pays the letter price there. Moving a postcard from commercial First-Class to Marketing Mail saves $0.043 a piece. Moving a letter saves $0.237.

A 6 × 11 card has 66 square inches against 54 for a 6 × 9, about 22% more room. That room costs $0.194 more per piece in commercial First-Class postage. Marketing Mail needs at least 200 pieces in one mailing, and presorted First-Class prices need 500. The [direct mail cost calculator](https://trysincerely.com/tools/direct-mail-cost-calculator) runs these prices at your volume, and [postcard vs letter](https://trysincerely.com/guides/postcard-vs-letter) covers which format suits which message.

## Where flats begin

USPS calls a piece a flat, or a large envelope, once any one dimension goes past the letter limit. That means more than 6-1/8 inches high, more than 11-1/2 inches long, or more than 1/4 inch thick. Flats run up to 12 by 15 inches and 3/4 inch thick. They also have to bend and be uniformly thick, with no bump more than 1/4 inch higher than the rest. A box is never a flat, even a thin one.

Flats cost about twice as much to mail. A 1-ounce First-Class flat with a stamp is $1.69 as of October 2026, against $0.82 for a 1-ounce letter. A 9 × 12 envelope that carries one flat sheet pays the flat price. The same sheet folded into a #10 envelope pays the letter price.

## How Sincerely sizes its mail

Sincerely prints two postcard sizes, 4 × 6 and 6 × 9, and one-page letters in a #10 double-window envelope. Handwritten pieces come as 4 × 6 postcards and letters in a hand-addressed envelope, and each one is genuinely written by hand. Sincerely does not print 6 × 11 postcards, self-mailers, or flats.

Sincerely mails US campaigns First-Class by default. A campaign can choose USPS Marketing Mail, but only for printed 6 × 9 postcards and letters sent to US addresses. A 4 × 6 postcard always goes First-Class. The same sizes mail inside Canada under different limits, covered in [Canada Post size requirements](https://trysincerely.com/guides/canada-post-size-requirements). Delivery times are in [how long direct mail takes](https://trysincerely.com/guides/how-long-does-direct-mail-take).

## Related questions

- [Postcard or letter, which should you send?](https://trysincerely.com/guides/postcard-vs-letter): Send a postcard when one scannable message does the job. Send a letter when you need room, formality, or privacy. Decision rules with current format costs.
- [Direct mail cost calculator: postage, print, and data](https://trysincerely.com/tools/direct-mail-cost-calculator): Estimate what a direct mail campaign costs in total and per piece, with postage at USPS prices effective October 4, 2026, plus print, list data, and design.
- [Canada Post size requirements for postcards and letters](https://trysincerely.com/guides/canada-post-size-requirements): Canada Post's standard size is 140 × 90 mm to 245 × 156 mm, up to 5 mm thick and 50 g. The limits for postcards, letters, and oversize mail.
- [What determines direct mail postage rates?](https://trysincerely.com/glossary/postage-rates): Direct mail postage rates depend on mail class, shape, weight, presort depth, and where the mailing enters the USPS network.
- [What is a self-mailer?](https://trysincerely.com/glossary/self-mailer): A self-mailer is a folded printed piece that mails without an envelope. USPS rules, costs, and use cases compared to postcards and letters.
- [What is first-class mail vs marketing mail?](https://trysincerely.com/glossary/first-class-vs-marketing-mail): The difference between USPS First-Class Mail and Marketing Mail, and why B2B senders usually pay for First-Class.

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Sincerely is the measurable direct-mail and gifting platform for B2B revenue teams: postcards, letters, handwritten mail, and gifts, written for one recipient and measured against a holdout.

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