What USPS actually requires for an EDDM-eligible mailpiece
The three thresholds, the minimums, the maximums, and why a few popular postcard sizes don't qualify.
This page is for anyone trying to figure out whether a specific postcard or mailer size will qualify for EDDM Retail. By the end you'll know the exact USPS rules, the math that decides eligibility, and the sizes most printers reject.
The short answer: when a piece qualifies for EDDM
Your mailpiece qualifies for EDDM Retail when USPS classifies it as a flat, it weighs 3.3 oz or less, and it stays inside 15" long by 12" high by 0.75" thick. To be a flat, the piece must exceed at least one of three thresholds: longer than 11.5", taller than 6.125", or thicker than 0.25". One is enough.
If that answered your question, skip to the recommended EDDM postcard sizes. If you want the math, keep reading.
The USPS EDDM flat rule (the three thresholds)
USPS uses a simple test to separate letters from flats. A piece becomes a flat the moment it exceeds any one of these dimensions:
- Length greater than 11.5" — the longest side.
- Height greater than 6.125" — the perpendicular side.
- Thickness greater than 0.25" — for most postcards, this is the paper stock.
Exceed any single one and you have a flat. Stay under all three and USPS calls it a letter, which is not eligible for EDDM Retail.
Rule of thumb: if your piece exceeds ANY ONE of length 11.5", height 6.125", or thickness 0.25", it qualifies as a flat. It does not need to exceed all three.
Minimum size rules
The minimum is set by the flat threshold above. A piece that falls under all three numbers is not a flat, so it cannot run through the EDDM Retail program. In practice that means:
- If your length is your qualifying dimension, the piece must be longer than 11.5". A 11.5" length exactly does not qualify; it must exceed the threshold.
- If your height is your qualifying dimension, the piece must be taller than 6.125". This is why 6.5" is the sweet spot for the short side of most EDDM postcards.
- If your thickness is your qualifying dimension, the piece must be thicker than 0.25". Standard postcard stock is nowhere near that, so thickness almost never carries a flat by itself.
For nearly every EDDM postcard, height is the dimension that crosses the line. That is the entire reason 6.5"x9" exists as the most popular EDDM size: 6.5 is just past 6.125.
Maximum size and weight rules
EDDM Retail also has a ceiling. Once a piece passes these numbers, it stops being eligible:
- Maximum length: 15"
- Maximum height: 12"
- Maximum thickness: 0.75"
- Maximum weight: 3.3 oz
The weight cap matters most on folded newsletters and heavy stocks. A 9"x12" piece on 100 lb gloss cover with a heavy coating can sneak up on 3.3 oz. If you are pushing size or stock weight, weigh a printed proof first.
| Rule | USPS EDDM Retail requirement |
|---|---|
| Mailpiece type | Flat |
| Max weight | 3.3 oz |
| Qualifies by length | > 11.5" |
| Qualifies by height | > 6.125" |
| Qualifies by thickness | > 0.25" |
| Max dimensions | 15" x 12" x 0.75" |
How USPS measures length vs. height on a flat
USPS defines length as the longest side of the piece and height as the perpendicular side, regardless of orientation. That sounds obvious until you look at a 6.5"x9" postcard. Even if your artwork reads vertically, USPS calls 9" the length and 6.5" the height because 9 is the longer number.
This matters for two reasons. First, the rule "length must exceed 11.5"" applies to whichever side is longer, not to whichever side you call the "top." Second, addressing and indicia placement always reference the mailing panel as it would be read by the carrier, not by your design grid. Plan your EDDM artwork layout with the long edge identified before you build the back.
Why 4x6, 5x7, and standard 6x9 fail the test
Here is the math on the three sizes people ask about most often:
- 4" x 6": Length 6" (not > 11.5"), height 4" (not > 6.125"), thickness about 0.014" on 14 pt stock (not > 0.25"). Fails all three thresholds. This is a letter, not a flat. Not EDDM-eligible.
- 5" x 7": Length 7" (not > 11.5"), height 5" (not > 6.125"), thickness about 0.014" (not > 0.25"). Same story. Not EDDM-eligible.
- Standard 6" x 9": Length 9" (not > 11.5"), height 6" (not > 6.125"), thickness on a postcard stock nowhere near 0.25". Fails all three thresholds by tiny margins. USPS will not run it through EDDM Retail.
The fix for the third one is one extra half inch on the short side. A 6.5" x 9" postcard clears the height threshold (6.5 > 6.125) and qualifies as a flat. That single dimension change is why 6.5"x9" is the workhorse size in our catalog.
Watch out for "6x9" specifically. It is one of the most common postcard sizes in commercial print, and it is the size most often confused with the EDDM-eligible 6.5x9. They look almost identical on screen. Confirm the short side is 6.5", not 6.0", before you send art to press.
Common EDDM size mistakes
The same handful of errors come up over and over. Avoiding them saves a reprint:
- Designing a 6x9 instead of a 6.5x9. The most common mistake. Half an inch decides whether USPS accepts the piece.
- Trimming under tolerance. If your finished piece comes off the cutter at 6.10" tall, you've fallen under the 6.125" threshold even though the file was set up correctly. Build to 6.25" or 6.5" so trim variance never pulls you under.
- Forgetting the fold. An 11"x17" sheet folded in half is a 8.5"x11" final piece. EDDM rules apply to the final folded dimensions, not the flat sheet size.
- Pushing thickness with heavy coatings. Layering a UV plus a gloss on a heavy cover stock can creep past the 0.25" threshold on the high side and the 3.3 oz weight cap. Usually fine, but worth checking on large formats.
- Assuming bleed equals trim. A file built at 6.625"x9.125" with bleeds is not a 6.625"x9.125" mailpiece. The trim size is what USPS measures.
- Skipping the printer's EDDM check. Most printers will confirm eligibility before they release the press. If yours doesn't, ask. A five-minute check beats a 5,000-piece reprint.
Frequently asked questions
Does a 6"x9" postcard ever qualify for EDDM?
No. A true 6"x9" fails all three flat thresholds. Some printers will still try to run it and the post office may turn it away at the counter. The safer move is 6.5"x9", which is essentially the same look with a qualifying short edge. See the full breakdown on the EDDM size guidelines pillar page.
Does the size rule change for folded pieces?
The rule itself does not change, but USPS measures the final folded dimensions, not the flat sheet. An 11"x17" piece folded once is 8.5"x11", and the 8.5"x11" measurement is what determines flat eligibility. Always confirm the folded math before ordering.
What happens if my piece is exactly 6.125" tall?
It does not qualify on height. The rule reads "greater than 6.125"," so an exact match is treated as under the threshold. Build to 6.25" or higher to avoid trim variance pulling you under.
Is there a minimum quantity tied to the size rules?
Size eligibility and quantity are separate. The size rule is binary: the piece qualifies or it does not. The minimum quantity for EDDM Retail is 200 pieces per carrier route, and that applies once your size is approved. See the size guidelines pillar for how size, quantity, and route choice fit together.
Can my printer override these rules?
No. The rules are set by USPS, not the printer. A printer can produce any size you want, but the post office decides whether it enters the EDDM stream. If a piece fails the flat test, USPS will reject the bundle at intake.
Pick a size that works the first time
If you want a recommendation rather than a ruleset, the practical follow-up is the EDDM postcard size comparison. Once you've chosen a size, the EDDM design guidelines walk through indicia placement, mailing panel layout, and bleed setup so your artwork passes on the first proof.
Ready to quote a specific size and route? Start on the EDDM quote page or call (713) 300-0687 and we'll confirm your size is eligible before you commit.