Envelope & Mail Piece Sizes
Exact USPS dimensions for every mail format — postcards, #10 envelopes, 6×9, flats, and self-mailers. Avoid surcharges, qualify for automation rates, and mail with confidence.
Why Mail Piece Size Matters
The USPS classifies every mail piece by size, shape, and weight. This classification directly determines your postage rate. A postcard that's slightly too large becomes a "flat" — and your postage rate can double. A letter that's too square gets a non-machinable surcharge. Understanding these boundaries saves real money on every mailing.
At Cornerstone, we design and produce every mail piece to meet USPS automation specifications. That means maximum postage savings with zero risk of surcharges, returns, or delays. For a complete breakdown of postcard-specific requirements, see our guide to USPS Postcard Printing Requirements.
Written by Sean Griffen, Owner — Cornerstone Services, Inc.
Sean has managed direct mail production for Hudson Valley businesses since 1998. Over 2.3 million pieces mailed. This guide reflects the USPS size and postage rules we apply to every job we produce.
Last reviewed: January 2025 · Source: USPS Domestic Mail Manual (DMM)
Postcards
The most cost-effective mail format. No envelope needed — your message is immediately visible.
| Size | Dimensions | Notes |
|---|---|---|
| Minimum | 3.5" × 5" | Smallest USPS-accepted postcard size |
| Standard (4×6) | 4.25" × 6" | Most common — qualifies for letter-rate postage |
| Jumbo (6×9) | 6" × 9" | Higher visibility; still qualifies as letter if ≤ 0.016" thick |
| Jumbo (6×11) | 6" × 11" | Maximum postcard size before flat rate applies. Popular for EDDM. |
| Maximum | 6.125" × 11.5" | Beyond this requires flat-size postage |
USPS Rules for Postcards
Letters
Standard business mail in envelopes. Letters qualify for the lowest automated rates when properly formatted.
| Size | Dimensions | Notes |
|---|---|---|
| #10 Envelope | 4.125" × 9.5" | Standard business envelope — most common for letters |
| #9 Envelope | 3.875" × 8.875" | Reply envelope that fits inside a #10 |
| 6×9 Envelope | 6" × 9" | Booklet style — fits 8.5×11 folded in half |
| Minimum | 3.5" × 5" | Smallest USPS letter size |
| Maximum | 6.125" × 11.5" | Beyond this = flat-size postage |
USPS Rules for Letters
Flats (Large Envelopes)
Larger mail pieces like catalogs, magazines, and oversized envelopes. Higher postage but more real estate for your message.
| Size | Dimensions | Notes |
|---|---|---|
| 9×12 Envelope | 9" × 12" | Fits 8.5×11 unfolded — very common for flats |
| 10×13 Envelope | 10" × 13" | Common catalog envelope with extra room |
| Minimum | 6.125" × 11.5" (or taller than 6.125") | Anything exceeding letter dimensions |
| Maximum | 12" × 15" | Largest flat before parcel pricing |
USPS Rules for Flats (Large Envelopes)
Self-Mailers
Folded pieces that mail without an envelope. Great for newsletters, menus, and promotional pieces.
| Size | Dimensions | Notes |
|---|---|---|
| Bi-fold (8.5×11) | 5.5" × 8.5" (folded) | Standard letter sheet folded in half |
| Tri-fold (8.5×11) | 3.67" × 8.5" (folded) | Fits in standard letter trays |
| Bi-fold (11×17) | 8.5" × 11" (folded) | Large format — counts as a flat |
| Z-fold / Accordion | Varies | Multiple panels — check thickness limits |
USPS Rules for Self-Mailers
Common Surcharges & Penalties
Mail pieces that don't meet USPS standards get hit with surcharges — or rejected entirely. Starting from a correctly sized template is the easiest way to avoid these penalties. Our USPS direct mailer templates are pre-sized to spec for every format below. Here's what to watch for:
| Condition | Surcharge | Why |
|---|---|---|
| Non-machinable letter | $0.22 | Square, too rigid, poly-wrapped, or has clasps/buttons |
| Non-standard aspect ratio | $0.22 | Length ÷ width is not between 1.3 and 2.5 |
| Barcode not readable | Loss of automation discount | Up to $0.10+ per piece vs. automation rate |
| Underweight postcard | Rejected | Below 0.007" thickness or smaller than 3.5×5 |
| Oversize letter | Flat-rate pricing | Exceeds 6.125×11.5 or 0.25" thick — jumps to flat rate |
Not Sure About Your Mail Piece Size?
Send us your design or dimensions and we'll confirm USPS compatibility, suggest the best format for your budget, and handle the production. If you're mailing under a nonprofit or bulk permit, see how our mailing permit services can reduce your per-piece cost further.
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