Envelope & Mail Piece Sizes
Standard envelope sizes for business mail, direct mail campaigns, and USPS mailing — with dimensions in inches and millimeters, postal class, and common uses for each size.
Standard Envelope Sizes (Inches & Millimeters)
| Envelope | Size (inches) | Size (mm) | USPS Mail Class |
|---|---|---|---|
| #6¼ | 3½ × 6 | 89 × 152 | Letter |
| #6¾ | 3⅝ × 6½ | 92 × 165 | Letter |
| Monarch | 3⅞ × 7½ | 98 × 191 | Letter |
| #9 | 3⅞ × 8⅞ | 98 × 225 | Letter |
| #10 | 4⅛ × 9½ | 105 × 241 | Letter |
| #11 | 4½ × 10⅜ | 114 × 264 | Letter |
| #12 | 4¾ × 11 | 121 × 279 | Letter / Flat |
| #14 | 5 × 11½ | 127 × 292 | Flat |
| A2 | 4⅜ × 5¾ | 111 × 146 | Letter |
| A6 | 4¾ × 6½ | 121 × 165 | Letter |
| A7 | 5¼ × 7¼ | 133 × 184 | Letter |
| A8 | 5½ × 8⅛ | 140 × 206 | Letter |
| A9 | 5¾ × 8¾ | 146 × 222 | Letter |
| A10 | 6 × 9½ | 152 × 241 | Letter |
| 6×9 | 6 × 9 | 152 × 229 | Letter / Flat |
| 9×12 | 9 × 12 | 229 × 305 | Flat |
| 10×13 | 10 × 13 | 254 × 330 | Flat |
| 12×15½ | 12 × 15½ | 305 × 394 | Flat / Parcel |
The #10 Envelope
The #10 envelope measures 4⅛ × 9½ inches (105 × 241 mm). It's the standard business envelope used for letters, invoices, statements, and most commercial mail in the United States. It fits a standard 8½ × 11 letter folded in thirds and qualifies for USPS letter-rate postage.
USPS Size Requirements: Letter vs. Flat
Envelopes that exceed letter dimensions get classified as flats or parcels, which cost significantly more to mail. Choosing the right envelope size directly affects your postage costs.
✉ USPS Letter Size
- Minimum: 3½ × 5 inches
- Maximum: 6⅛ × 11½ inches
- Max thickness: ¼ inch
- Aspect ratio: Length ÷ height between 1.3 and 2.5
- Max weight: 3.5 oz (First-Class)
📦 USPS Flat Size
- Minimum: Exceeds letter dimensions (6⅛ × 11½ in.)
- Maximum: 12 × 15 inches
- Max thickness: ¾ inch
- Shape: Must be rectangular and flexible
- Cost: Roughly double letter-rate postage
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. Our commercial printing services ensure every piece is cut, folded, and finished to exact USPS dimensions, and our full-service mailing handles presort, postage, and USPS entry so your campaign qualifies for maximum 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.
Get a Free EstimateFrequently Asked Questions
What size is a #10 envelope?
A #10 envelope measures 4⅛ × 9½ inches (105 × 241 mm). It is the standard business envelope used for letters, invoices, and direct mail. It qualifies for USPS First-Class letter-rate postage when properly addressed and within weight limits (under 3.5 oz).
What is the largest envelope I can send at USPS letter rate?
The maximum size for USPS letter rate is 6⅛ × 11½ inches, with a maximum thickness of ¼ inch. Envelopes exceeding these dimensions are classified as flats or parcels and cost more to mail.
What envelope size is best for direct mail campaigns?
For direct mail letters, the #10 envelope is the most common choice. For oversized direct mail that stands out in the mailbox, 6×9 or 9×12 envelopes qualify as flats but offer more visual impact. Cornerstone can advise on the best format for your campaign goals and budget.
What's the difference between a booklet envelope and a catalog envelope?
A booklet envelope (like 6×9) opens on the long side and is used for booklets, brochures, and thick mailings. A catalog envelope (like 9×12 or 10×13) opens on the short side and is used for magazines, catalogs, and flat documents. Both are classified as USPS flats.
What is the maximum postcard size for USPS letter-rate postage?
The maximum postcard size that qualifies for USPS letter-rate postage is 6⅛ × 11½ inches with a maximum thickness of 0.016 inches. Anything larger is classified as a flat and carries a higher postage rate. The 6×11 postcard (popular for EDDM campaigns) is the largest commonly used size that stays within letter-rate pricing.
What is the difference between a letter and a flat in USPS mail?
A USPS letter is a mail piece between 3½ × 5 inches and 6⅛ × 11½ inches, no more than ¼ inch thick and 3.5 oz. A flat (large envelope) exceeds those dimensions — typically 9×12 or 10×13 envelopes — and carries higher postage rates. The practical cost difference can be significant at scale: flats cost roughly double letter-rate postage per piece.
What causes a non-machinable surcharge on a letter?
USPS adds a $0.22 non-machinable surcharge to letters that are square, too rigid, poly-wrapped, have clasps or buttons, or fall outside the 1.3:1 to 2.5:1 aspect ratio requirement. The most common mistake we see is square invitation-style envelopes — they look professional but automatically trigger the surcharge on every piece.
What size envelopes qualify for bulk mail automation rates?
To qualify for USPS bulk mail automation discounts, letters must be between 3½ × 5 inches and 6⅛ × 11½ inches, no more than ¼ inch thick, flexible, and rectangular with an aspect ratio between 1.3:1 and 2.5:1. The address must be machine-readable with a valid barcode. Meeting these specs can reduce postage by $0.10 or more per piece compared to non-automation rates.
What is the minimum size for a USPS postcard?
The USPS minimum postcard size is 3½ × 5 inches with a minimum thickness of 0.007 inches (7pt cardstock). Pieces smaller than this are not accepted for mailing. The most commonly mailed postcard size is 4¼ × 6 inches (standard 4×6), which qualifies for letter-rate postage.
What are the USPS size requirements for self-mailers?
USPS self-mailers must be tabbed or sealed per DMM 201.3 standards. Letter-size self-mailers require at least 1 tab (2 recommended), flat-size self-mailers require 2 tabs minimum. Tabs must be at least 1 inch long placed on the top edge. The address panel must face outward and the open edge must be at the top after folding. Common sizes include bi-fold 8.5×11 (folds to 5.5×8.5) and tri-fold 8.5×11 (folds to 3.67×8.5).
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