Envelope Sizes Chart 2026

Why envelope size matters

Envelope size determines three things on every mailing: which postage class applies, whether the piece qualifies for automation discounts, and whether the contents fit without folding. Get any of them wrong and the cost shows up either at the post office or weeks later — wrinkled inserts, surcharged pieces, or returns.

This page is the comparison reference we use at Cornerstone Services when setting up direct mail and printing jobs. Every standard envelope size is listed below with exact dimensions in inches and millimeters, the postage class it falls under, and what it's typically used for. Each entry links to a deeper page with paper weights, window options, USPS rules, and design specs for that specific size.

If you already know the size you need, jump to it from the comparison table. If you're not sure, the next section covers the three questions that decide it.

How to choose the right envelope size

Three questions decide it:

1. What's going inside?

A single trifolded 8.5×11" letter fits a #10. A flat 8.5×11" page that has to stay unfolded needs a 9×12 or larger. Wedding invitations and announcements use the A-series (A6, A7, A9) because the proportions match standard card sizes. Match the contents to the envelope, not the other way around.

2. Letter rate or flat rate?

USPS classifies any envelope at or under 6.125" × 11.5" and 0.25" thick as a letter. Anything larger is a flat — and flat-rate postage starts at $1.63 versus $0.78 for a letter. For most direct mail, staying inside the letter envelope is the cheaper play. For documents that can't fold, the flat rate is unavoidable.

3. Will the piece qualify for automation discounts?

Bulk and presort discounts require the envelope to be machinable: rectangular, flexible, no clasps or strings, and within the 1.3-to-2.5 aspect ratio (length divided by height). A square envelope has an aspect ratio of 1.0 and is automatically nonmachinable — the surcharge adds up fast on a 5,000-piece campaign. The full rules are in the next section.

Once those three are answered, the size choice is usually narrow. The comparison table above shows every standard option side by side.

USPS rules for letter-size envelopes

Every letter-size envelope has to clear a short list of physical standards before it qualifies for First-Class or Marketing Mail rates. These come from the USPS Domestic Mail Manual (DMM 201) and apply nationwide:

• Minimum dimensions: 3.5" high × 5" long × 0.007" thick. Anything smaller is not accepted for mailing.
• Maximum dimensions: 6.125" high × 11.5" long × 0.25" thick. Beyond any of these, the piece is rated as a flat.
• Aspect ratio: length divided by height must fall between 1.3 and 2.5. A square envelope falls outside this range and is nonmachinable by rule.
• Maximum weight: 3.5 oz for First-Class letters. Heavier pieces are rated as flats.
• Shape: rectangular, with four square corners and parallel opposite sides.
• Construction: flexible, uniformly thick, no clasps, strings, buttons, or rigid contents that create uneven thickness.

Pieces that meet every standard above qualify for machinable letter rates. Pieces that fail one or more criteria are still mailable in most cases but are charged the nonmachinable surcharge.

2026 First-Class postage rates

These rates have been in effect since July 13, 2025. USPS has proposed a rate adjustment effective July 12, 2026 that would raise the First-Class stamp price to $0.82, pending Postal Regulatory Commission approval.

• 1-oz letter, Forever stamp: $0.78
• 1-oz letter, metered postage: $0.74
• Each additional ounce, up to 3.5 oz: $0.29
• Postcard: $0.61
• Large envelope (flat), 1 oz: $1.63
• Each additional flat ounce: $0.20

Metered postage saves $0.04 per piece versus stamps — small per piece, meaningful at volume. Presort and automation discounts take per-piece costs significantly lower than retail rates; we cover those on our direct mail services page.

Last verified: May 2026. Source: USPS Domestic Mail Manual 201; USPS rate announcements.