Updated for 2026 USPS Regulations

Free USPS Postcard Size & Mailability Validator

Enter your postcard dimensions and instantly verify compliance with USPS machinable postcard requirements — aspect ratio, size limits, and barcode clear zone.

Postcard Specifications

Enter dimensions in inches

in

Range: 5.0" – 9.0"

in

Range: 3.5" – 6.0"

in

Range: 0.007" – 0.016"

Length

5.000"

Height

3.500"

Ratio

1.43

Visual Preview & Validation

Scaled representation with barcode clear zone

STAMP
Clear Zone 0.625"
5.00" W
3.50" H

PASS — Machinable Postcard

Your postcard meets all USPS 2026 dimension requirements.

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USPS Postcard Dimensions Guide

The United States Postal Service defines a "postcard" as a rectangular mailpiece that falls within specific dimensional thresholds. For 2026, the minimum postcard size is 3.5 inches high × 5.0 inches long × 0.007 inches thick. The maximum postcard size is 6.0 inches high × 9.0 inches long × 0.016 inches thick. Any mailpiece that falls within these ranges — and also satisfies the aspect ratio rule — qualifies for the lower postcard postage rate rather than the higher First-Class letter rate.

It is critical to understand that the USPS defines "length" as the dimension parallel to the delivery address, and "height" as the dimension perpendicular to it. In practical terms, the longer side is always the length and the shorter side is always the height. If you design a card that is 4" × 6", the 6" side is the length and the 4" side is the height, giving you an aspect ratio of 1.5 — well within the acceptable range.

Cards smaller than the 3.5" × 5" minimum are not mailable at any rate and will be returned to the sender. Cards larger than the 6" × 9" maximum are automatically reclassified as letters or large envelopes and charged accordingly. This is one of the most common reasons direct mail campaigns go over budget — the print shop produces a card that is slightly too large, and the postage cost per piece doubles or triples unexpectedly.

Thickness matters just as much as length and height. A postcard must be at least 0.007" thick to survive automated sorting equipment. Cards thinner than this — typically very lightweight paper stock — get torn or jammed in the machines. On the upper end, anything thicker than 0.016" is classified as a package and charged parcel rates, which can be several dollars per piece compared to the roughly $0.50–$0.70 postcard rate.

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The Aspect Ratio Rule

The aspect ratio is one of the most frequently misunderstood USPS postcard requirements. It is calculated by dividing the length of the card by its height. The result must fall between 1.3 and 2.5 for the card to be considered "machinable" — meaning it can be processed by automated sorting equipment without manual intervention.

Square postcards, which have an aspect ratio of 1.0, always fail this test. A 5" × 5" card has an aspect ratio of 1.0, which is below the 1.3 minimum. Similarly, an extremely elongated card like 10" × 3.5" would have an aspect ratio of 2.86, exceeding the 2.5 maximum. Both scenarios result in a non-machinable surcharge.

The reason for this rule is purely mechanical. USPS sorting machines orient mailpieces by detecting the longer edge and feeding them through the system in a specific direction. If a card is too close to square, the machine cannot reliably determine which way to orient it. If it is too elongated, it may bend or fold during processing. The 1.3–2.5 range represents the sweet spot where automated equipment can handle the card reliably.

When a card fails the aspect ratio test, the USPS applies a non-machinable surcharge (currently $0.40 per piece) on top of the base postage. For a direct mail campaign of 10,000 postcards, that is an unexpected $4,000 in additional postage — a costly mistake that can be avoided by simply checking the aspect ratio before going to print.

The most popular postcard sizes — 4" × 6" (ratio 1.5), 4.25" × 5.5" (ratio 1.29, borderline), and 5" × 7" (ratio 1.4) — all fall within the acceptable range. However, 4.25" × 5.5" is dangerously close to the 1.3 minimum. A slight variation in trimming during print production could push it below the threshold. Always design with a safety margin.

The Barcode Clear Zone

The barcode clear zone is a critical area at the bottom of every postcard's address side that must remain completely free of printed content. The USPS requires a clear zone of 5/8 inch (0.625") measured from the bottom edge of the card upward, extending the full width of the card. This is where the USPS prints its automated barcode for sorting and tracking purposes.

Any text, graphics, dark colors, or patterns in this zone can interfere with the barcode printing process or the ability of sorting machines to read the barcode. If the clear zone is violated, the postcard may be rejected, returned to the sender, or assessed a non-machinable surcharge. In some cases, the USPS will manually apply a barcode sticker over the offending content, but this delays delivery and is not guaranteed.

When designing your postcard artwork, always leave the bottom 0.625" of the address side as a blank, white area. This is a common mistake for designers who want to use the full card surface for marketing content. The visual preview in our validator tool above shows this zone as a hashed red/orange overlay so you can see exactly how much space you need to reserve.

It is also worth noting that the return address should be placed in the upper left corner of the address side, and the delivery address should be centered in the middle area — above the barcode clear zone. Leaving adequate margin around all address elements ensures the automated equipment can read everything correctly.

Frequently Asked Questions

Everything you need to know about USPS postcard mailing requirements

Can I mail a 6x9 postcard with a standard stamp?
Yes, a 6"x9" postcard falls within the USPS maximum postcard dimensions (6" high x 9" long). However, you need to verify the aspect ratio: 9 ÷ 6 = 1.5, which is within the required 1.3–2.5 range. As long as your card is between 0.007" and 0.016" thick, it qualifies for postcard postage rates. A standard First-Class postcard stamp will cover it. If your card exceeds any of these thresholds, it will be processed as a letter and require letter-rate postage.
What happens if my postcard is too thick?
If your postcard exceeds 0.016" (about 1/16 of an inch) in thickness, the USPS classifies it as non-machinable or even as a package, depending on how thick it is. This means it cannot be processed through automated sorting equipment and will incur a non-machinable surcharge — typically an additional $0.40 or more per piece. In extreme cases, very thick mailpieces are routed to package processing and charged parcel rates, which are significantly more expensive than postcard rates.
Why was my direct mail rejected?
Direct mail postcards are most commonly rejected for three reasons: (1) The aspect ratio falls outside the 1.3–2.5 range — square or nearly square cards fail this test. (2) The barcode clear zone at the bottom 0.625" of the card contains text, graphics, or dark colors that interfere with automated sorting. (3) The card is too thin (under 0.007") and gets destroyed or jammed in sorting machines. Always run your design through a validator like this one before sending to print.
What is the USPS postcard aspect ratio rule?
The USPS requires that the length of a postcard divided by its height must yield a ratio between 1.3 and 2.5. This ensures the card can be properly oriented by automated mail sorting machines. A perfectly square card (1:1 ratio) will always fail this test. Cards outside this range are subject to a non-machinable surcharge and may experience delivery delays.
How much is the non-machinable surcharge for postcards?
The non-machinable surcharge for First-Class Mail postcards is currently $0.40 per piece (as of 2025–2026 rate schedules). This surcharge is applied when a postcard fails any of the machinability criteria: aspect ratio outside 1.3–2.5, thickness outside 0.007"–0.016", or dimensions outside the 3.5"x5" minimum and 6"x9" maximum. The surcharge is added on top of the base postcard postage rate.
Does the barcode clear zone apply to all postcards?
Yes. The USPS requires a clear zone of 5/8" (0.625") at the bottom address side of all machinable postcards. This area must be free of any printing, graphics, or dark colors that could interfere with barcode reading and automated sorting. The clear zone extends the full width of the card and is measured from the bottom edge upward. Violating this rule can result in surcharges or returned mail.