US envelope sizes range from small commercial formats like the No. 6¼ to large catalog envelopes like the 10×13, with the No. 10 (9.5 × 4.125 inches) standing as the universal business standard. Announcement envelopes (A1–A10) serve greeting cards and invitations, while catalog envelopes (6×9, 9×12, 10×13) handle unfolded documents — all available in millimeters, centimeters, inches, points, picas, and pixels for design use.

USPS postage rules tie directly to size and shape — standard letters under 1 ounce cost $0.73, large envelopes start at $1.50, and square envelopes always carry a $0.44 surcharge due to automated sorting restrictions. It’s also worth noting that US “A-series” announcement envelopes follow an entirely different system from ISO international C-series envelopes, making size verification essential for cross-border mailing.

Standard US Envelope Sizes and Dimensions

Commercial envelopes are the most common type used in American offices and homes. The No. 10 envelope is the industry standard for business correspondence.

These envelopes have a rectangular shape with aspect ratios between 1:1.6 and 1:2.2. They’re designed to work with automated mail sorting machines.

Commercial Envelope Size Chart

Letter-sized paper (8.5 × 11 inches) fits perfectly in a No. 10 envelope when folded into thirds.

Legal-sized documents (8.5 × 14 inches) can fit when folded into fourths.

Announcement envelopes have a squarer shape with aspect ratios from 1:1.3 to 1:1.6. People use these for greeting cards, invitations, and photos.

Announcement Envelope Dimensions

The A2 envelope is popular for wedding invitations. The A7 size works well for 5 × 7 inch cards.

Business and Commercial Envelope Sizes Explained

The No. 10 envelope measures 9.5 × 4.125 inches and is America’s most-used business envelope. According to USPS data, this size accounts for over 80% of all business mail.

This envelope holds standard 8.5 × 11 inch letter paper folded horizontally into three equal sections. It also accommodates legal-sized paper folded into four sections.

The No. 9 envelope measures 8.875 × 3.875 inches, slightly smaller than its No. 10 counterpart. Companies use this size for reply envelopes and return correspondence.

Both sizes work perfectly with automated mail processing equipment. This compatibility reduces postage costs for bulk mailings.

Large Format Envelopes

The 6×9 envelope falls into the catalog envelope category. Its actual dimensions are 6 × 9 inches (152 × 229 mm).

Businesses use this size for brochures, small catalogs, and documents that shouldn’t be folded. It requires additional postage beyond a standard stamp.

The 9×12 envelope measures exactly 9 × 12 inches (229 × 305 mm). This is the go-to choice for mailing full-size documents without folding.

Window envelopes come in most commercial sizes. They feature a transparent panel that displays the recipient’s address printed on the enclosed letter.

US Envelope Postage Requirements and Weight Limits

USPS First-Class Mail has strict size and weight requirements. Understanding these rules prevents unexpected postage fees.

A standard Forever stamp covers letters up to 1 ounce in weight. The envelope must measure between 5 × 3.5 inches (minimum) and 11.5 × 6.125 inches (maximum).

Weight and Stamp Requirements

How do you know if your letter weighs 1 oz? A single sheet of standard 20-pound copy paper weighs about 0.16 ounces. You can mail up to five sheets with one stamp.

A standard business envelope weighs approximately 0.2 ounces when empty. Add four sheets of paper and you’re at 0.84 ounces—still under the 1-ounce limit.

Letters over 1 ounce need additional postage. Each extra ounce costs $0.28 as of 2024 USPS rates. A 2-ounce letter requires one Forever stamp plus one additional ounce stamp.

Envelopes Requiring Extra Postage

Square envelopes always need extra postage regardless of weight. The USPS charges a $0.44 surcharge because these can’t go through automated sorting machines.

Rigid or lumpy envelopes also require additional fees. Items like greeting cards with buttons, ribbons, or thick embellishments fall into this category.

Does a 6×9 envelope require additional postage? Yes. This size exceeds standard letter dimensions and requires large envelope (flat) postage starting at $1.50.

What size envelope does not require extra postage? Any envelope between 5 × 3.5 inches and 6.125 × 11.5 inches qualifies for standard letter rates if it’s under 1 ounce and not square.

Non-machinable characteristics that trigger surcharges include:

Aspect ratio less than 1.3 or more than 2.5

Polyethylene or plastic construction

Clasps, strings, or buttons

Self-stick attachments

Addresses parallel to the shorter dimension

Weighing your mail at home helps avoid returns. A basic kitchen scale works perfectly. Place your sealed envelope on the scale and check if it’s under 1 ounce.

How to Choose the Right Envelope Size?

Match your envelope to your document size first. An 8.5 × 11 inch letter needs a No. 10 envelope. A full-size certificate requires a 9×12 flat envelope.

Consider folding when possible. Sending documents flat in large envelopes costs more in postage. If the document can be folded without damage, use a smaller envelope.

Postage Cost Optimization

Standard letter envelopes (No. 6¼ through No. 10) offer the best postage value. They cost only one Forever stamp when under 1 ounce.

Large envelopes start at $1.50 for the first ounce. That’s more than double the cost of standard mail.

Professional correspondence typically uses No. 10 envelopes with printed return addresses. Personal letters can use any size that fits the content.

Automation compatibility matters for bulk mailings. Commercial envelopes process faster through USPS sorting equipment, reducing delivery times.

Does it matter what size envelope you mail? Absolutely. Size directly affects postage costs, delivery speed, and professional appearance.

International vs US Envelope Size Standards

US envelope sizes don’t match the ISO international standard. This creates confusion when mailing documents across borders.

The international system uses C-series envelopes designed to hold A-series paper. A C5 envelope perfectly fits an A5 sheet unfolded.

C5 vs A5 Comparison

Is the C5 envelope the same as A5? No. The C5 envelope measures 162 × 229 mm and is designed to hold A5 paper (148 × 210 mm).

In the US system, announcement envelopes use the “A” designation, but they’re completely different from ISO A-series paper sizes.

US envelopes use a hyphen when referring to announcement sizes (A-2, A-7) to distinguish them from ISO standards. However, this practice isn’t consistent across all manufacturers.

When mailing internationally, check the recipient country’s standard sizes. European businesses expect ISO-sized envelopes and may have trouble processing US formats.

FAQ’s (US Envelope Sizes)

What are standard US envelope sizes?

Standard US envelope sizes include No. 10 (9.5 × 4.125 inches) for business mail, No. 9 (8.875 × 3.875 inches) for replies, and announcement sizes A2 through A10 for greeting cards. Catalog envelopes range from 6×9 to 10×13 inches for larger documents.

How do I know if my letter needs 1 or 2 stamps?

Check the weight using a kitchen scale. Letters under 1 ounce need one Forever stamp. Letters between 1-2 ounces need one Forever stamp plus one additional ounce stamp ($0.28). Square envelopes and rigid items always require extra postage regardless of weight.

What size is a 9×12 envelope?

A 9×12 envelope measures exactly 9 × 12 inches (229 × 305 mm). This catalog-style envelope holds full-size documents without folding and requires large envelope postage starting at $1.50.

What size envelope does not require extra postage?

Standard letter-sized envelopes between 5 × 3.5 inches and 6.125 × 11.5 inches don’t require extra postage when they weigh under 1 ounce and aren’t square.

Does it matter what size envelope you mail?

Yes, Envelope size determines postage costs, processing speed, and professional presentation. Larger envelopes cost more to mail, while non-standard sizes may incur surcharges or processing delays.

Does a 6×9 envelope require additional postage?

Yes, A 6×9 envelope exceeds standard letter dimensions and qualifies as a large envelope or flat. USPS charges $1.50 for the first ounce instead of the $0.73 letter rate.

Is a #9 or #10 envelope bigger?

The No. 10 envelope is bigger. It measures 9.5 × 4.125 inches compared to the No. 9 at 8.875 × 3.875 inches. The No. 10 provides more space and is the standard for business correspondence.

Is the C5 envelope the same as A5?

No. C5 is an ISO international envelope measuring 162 × 229 mm designed to hold A5 paper. In the US, A5 refers to an announcement envelope measuring 159 × 108 mm used for greeting cards. These are completely different sizing systems.

Conclusion

US envelope sizes range from small No. 6¼ commercial envelopes to large 10×13 catalog formats. The No. 10 envelope remains the business standard, offering the best balance of capacity and postage cost.

Remember that envelope size directly affects postage. Standard letters under 1 ounce cost $0.73, while large envelopes start at $1.50. Square envelopes and rigid items always need additional postage.

Choose your envelope size based on document dimensions, folding preferences, and budget. When possible, use standard commercial sizes to minimize costs and maximize delivery speed.