First-Class Mail Size and Weight Limits: Complete Requirements

Getting mail size and weight right matters for two reasons: incorrect classification means paying the wrong postage, and certain characteristics trigger surcharges that increase costs. Understanding USPS requirements for First-Class Mail helps you choose the right supplies and avoid unexpected fees.

First-Class Mail Categories#

USPS classifies First-Class Mail into three categories based on size and shape:

Letters: Standard business envelopes and correspondence Flats (Large Envelopes): Larger flexible mail like manila envelopes Postcards: Cards meeting specific dimension requirements

Each category has distinct size limits, weight limits, and pricing.

Letter Requirements#

First-Class letters are the most common business mail. To qualify for letter pricing, mail must meet these specifications:

Size Limits#

Standard #10 business envelopes (4.125" × 9.5") fit comfortably within these limits. So do most common envelope sizes used for business correspondence.

Weight Limit#

Maximum weight for letters: 3.5 ounces

A standard #10 envelope with 4-5 pages of 20-lb paper typically weighs under one ounce. Six or more pages usually pushes over one ounce, requiring additional postage. Once you exceed 3.5 ounces, your mail no longer qualifies as a letter and must be sent as a flat or package at higher rates.

Aspect Ratio Requirements#

This is where specifications get more technical. For a letter to be considered "machinable"—processable by USPS's automated sorting equipment—the aspect ratio must fall between 1.3 and 2.5.

Aspect ratio = Length ÷ Height

For a standard #10 envelope (9.5" × 4.125"): 9.5 ÷ 4.125 = 2.3 — within acceptable range.

For a square 6" × 6" envelope: 6 ÷ 6 = 1.0 — outside acceptable range.

Mail that doesn't meet aspect ratio requirements is "nonmachinable" and incurs a surcharge (currently $0.49) even if it otherwise fits letter dimensions.

Nonmachinable Surcharge Triggers#

Letters that can't be processed by automated equipment cost an extra $0.49 per piece. This surcharge applies to mail that:

Has the wrong aspect ratio: Square envelopes or envelopes with aspect ratios below 1.3 or above 2.5.

Contains rigid items: Mail with items that don't bend—credit cards, keys, USB drives, pens. USPS equipment can't safely process rigid contents.

Is too thick or uneven: Lumpy mail or items with significant thickness variation. A letter containing a small object creates an uneven surface.

Uses certain closures: Envelopes with clasps, strings, or buttons that could catch in processing equipment.

Has address problems: Delivery addresses parallel to the short edge rather than the long edge, or addresses placed in nonstandard locations.

For business mail, the most common surcharge triggers are square envelopes (popular for invitations and special mailings) and rigid inserts. If you're planning a mailing with special characteristics, factor the surcharge into your costs: a one-ounce nonmachinable letter costs $1.27 ($0.78 + $0.49) rather than $0.78.

Postcard Requirements#

Postcards have their own pricing tier—cheaper than letters—but must meet strict specifications to qualify.

Postcard Dimensions#

These dimensions are more restrictive than letters. A 4" × 6" postcard (common for marketing) fits within limits. A 5" × 7" postcard does not—it would be charged letter rates instead.

Postcard Weight#

Postcards don't have a separate weight restriction, but the thickness limits (0.007" to 0.016") effectively constrain weight. Typical cardstock within these thickness ranges weighs well under an ounce.

Postcard Shape#

Postcards must be rectangular, not square. Square postcards incur the nonmachinable surcharge just like square letters.

Price Impact#

A qualifying postcard costs $0.61. A postcard exceeding size limits becomes a letter at $0.78 (or more if it's also nonmachinable). Getting dimensions right saves $0.17 per piece or more.

Large Envelope (Flat) Requirements#

Mail pieces exceeding letter dimensions but remaining flexible are classified as "flats" and have their own specifications.

Flat Dimensions#

Note how these minimums are essentially where letter maximums end. If your mail is too big for a letter, it's probably a flat.

Flat Weight Limit#

Maximum weight for flats: 13 ounces

Flats exceeding 13 ounces can't go First-Class and must use Priority Mail or other services.

Flexibility Requirements#

Flats must be flexible—they need to bend without damage. Rigid items like hardcover books or items with completely stiff contents don't qualify as flats, even if they fit the dimensions.

Uniform Thickness#

Flats should have relatively uniform thickness throughout. Lumpy items with significant thickness variations may not qualify or may require surcharges.

Practical Sizing Guidance#

For most business mail, staying within letter specifications keeps costs lowest. Here's how to approach common scenarios:

Standard Business Letters#

Use #10 envelopes (4.125" × 9.5") for most correspondence. These fit standard letter-sized paper folded in thirds and stay well within letter specifications. Keep content to 4-5 pages to stay under one ounce.

Multi-Page Documents#

For documents requiring more pages, you have choices:

Stay within letter specs: Use lighter paper (24-lb vs. 28-lb) to fit more pages while staying under weight limits. Or accept the additional ounce charge ($0.29 per additional ounce).

Switch to flats: For truly thick documents, 9×12 manila envelopes (flats) eliminate folding and accommodate substantial page counts. But flat postage starts at $1.63—significantly more than letter rates.

The crossover point varies by content, but often a heavier letter (2-3 oz at $1.07-$1.36) costs less than a flat ($1.63+). Weigh your options literally.

Special Mailings (Invitations, Cards)#

For invitations and special cards, dimensions matter:

Rectangular cards: Keep within letter dimensions and avoid the nonmachinable surcharge when possible. Consider standard A7 envelopes (5.25" × 7.25")—they fit within letter limits.

Square cards: Accept the $0.49 nonmachinable surcharge as a cost of the design choice. Budget accordingly.

Thick or embellished cards: Cards with ribbons, wax seals, or dimensional decorations will trigger nonmachinable surcharges regardless of dimensions.

Postcards#

For marketing postcards, standard 4" × 6" size fits postcard specifications perfectly. Larger sizes (5" × 7", 6" × 9") move into letter pricing and may not be worth the extra cost for the additional size.

Weighing Your Mail#

Accurate weight determination prevents postage problems:

Underweight postage: If you don't use enough postage, USPS either returns the mail to you (adding delay) or delivers it postage-due to the recipient (professional embarrassment).

Overweight postage: Using more postage than required wastes money—especially at volume.

For regular business mailings, weigh a few representative pieces to establish baseline weights. A standard letter with 3 pages might consistently weigh 0.7 oz; you know one stamp covers it. A letter with 6 pages might hit 1.1 oz; you know to add additional postage.

Postal scales are inexpensive (under $20) and useful for any business sending regular mail. Avoid guessing on borderline weights.

Common Envelope Sizes and Classifications#

Here's how common envelope sizes classify:

Frequently Asked Questions#

What is the maximum weight for a First-Class letter?#

First-Class letters can weigh up to 3.5 ounces. Items heavier than 3.5 ounces must be sent as flats (up to 13 ounces) or packages.

Why do square envelopes cost more?#

Square envelopes don't meet USPS aspect ratio requirements (1.3-2.5) for automated processing. They're classified as "nonmachinable" and incur a $0.49 surcharge because they require manual handling.

What triggers the nonmachinable surcharge?#

Several factors trigger the $0.49 surcharge: square aspect ratios, rigid contents (keys, USB drives, pens), lumpy or uneven thickness, clasps or buttons, unusual address placement, and certain paper types (very thin or very thick).

How many pages can I mail with one stamp?#

A single stamp ($0.78) covers mail up to one ounce. Typically, that's about 4-5 pages of standard 20-lb paper in a #10 envelope. Heavier paper, inserts, or additional pages may require additional postage.

What's the difference between a letter and a flat?#

Letters are smaller (maximum 6.125" × 11.5" × 0.25", up to 3.5 oz). Flats (large envelopes) are larger (up to 12" × 15" × 0.75", up to 13 oz). Flats cost more—starting at $1.63 versus $0.78 for letters. Stay within letter specifications when possible to minimize costs.

Keeping Costs Down#

The most cost-effective approach to First-Class Mail:

Use standard envelope sizes. #10 envelopes fit within letter specs, work with standard paper, and avoid nonmachinable issues.

Stay within one ounce when possible. The jump from $0.78 to $1.07 for the second ounce adds up at volume. Edit for conciseness, use lighter paper, or consider whether all enclosures are necessary.

Avoid nonmachinable triggers. Unless you have a specific reason for square envelopes or rigid inserts, stick with rectangular, flexible mail.

Choose letter over flat when feasible. A heavy letter often costs less than a flat. A 3.5-ounce letter costs $1.65; a 1-ounce flat costs $1.63. Calculate before choosing.

Getting size and weight right isn't just about compliance—it's about controlling costs for your regular business mailings.

Related reading: First-Class Mail Delivery Times: How Long Does It Take?

Related reading: First-Class Mail for Business: The Complete Small Business Guide