About DirectMailValidator

DirectMailValidator is a free tool that checks whether a postcard or direct-mail piece meets USPS size and mailability rules before it goes to print. Enter a card's width, height, and thickness and you instantly see which mail class it falls into — the cheaper retail postcard rate, the letter rate, or an oversized flat/parcel — plus aspect-ratio and barcode-clear-zone checks.

Why it exists

The most common and expensive direct-mail mistake is assuming any rectangular card mails at the cheap "postcard rate." It doesn't: the USPS reserves that price for cards up to 4.25" × 6". A slightly larger card jumps to letter pricing, and an out-of-range aspect ratio adds a per-piece non-machinable surcharge. On a campaign of thousands of pieces, those surprises add up to real money. This tool is built to catch those errors in seconds.

Who it's for

  • Graphic designers preparing client postcards and mailers.
  • Print shops checking customer files before production.
  • Marketers, realtors, and small businesses planning a mailing and budgeting postage.

How we try to stay accurate

The rules encoded here are based on published USPS standards (the Domestic Mail Manual and the Notice 123 price list) for 2026, including the retail postcard size box (3.5"×5" to 4.25"×6"), the 0.007"–0.016" thickness range, the 1.3–2.5 machinable aspect ratio, and the barcode clear zone. USPS updates prices and rules periodically, so for a large or high-stakes run we always recommend confirming against the official USPS sources.

What's next

We're expanding the tool from a size checker into a direct-mail preflight workspace — upload a postcard PDF and get automated checks for trim size, bleed, safe margins, image resolution, and the barcode clear zone, with a clear pass / needs-review report. The goal is a tool designers and print shops can rely on for every job.

A note on independence

DirectMailValidator is an independent tool. It is not affiliated with, endorsed by, or sponsored by the United States Postal Service. USPS® is a registered trademark of the United States Postal Service. All postal information here is referenced for convenience — the official USPS Domestic Mail Manual is always the authoritative source.

Have feedback or a feature request? Get in touch.