USPS Minimum and Maximum Envelope Sizes

Every envelope mailed through USPS falls into one of three categories: letter, flat (also called a large envelope), or parcel. Which category a piece falls into determines what postage it costs, whether it qualifies for automation discounts, and in some cases whether it can be mailed at all.

The boundaries between categories are defined by physical measurements: height, length, thickness, and weight. This page covers the exact USPS minimum and maximum dimensions for each category, the aspect-ratio rule that catches most surprise surcharges, and how to verify your envelope is mailable before committing to a print run.

All rules sourced from USPS Domestic Mail Manual sections 101 and 201.

USPS envelope size categories

Letter-size minimum

Every mailable envelope or postcard must meet these absolute minimums:

• Height: 3.5 inches
• Length: 5 inches
• Thickness: 0.007 inches

Anything smaller than 3.5 by 5 by 0.007 inches is not accepted by USPS. The minimum is the same for envelopes and postcards.

Letter-size maximum

To qualify for letter rates (the lowest postage tier), a piece must stay within all of these:

• Height: 6.125 inches maximum
• Length: 11.5 inches maximum
• Thickness: 0.25 inches maximum
• Weight: 3.5 ounces maximum
• Aspect ratio: length divided by height between 1.3 and 2.5

Cross any one of these and the piece moves to flat rate (currently $1.63 starting versus $0.78 for letter rate).

The aspect ratio rule

Aspect ratio (length divided by height) must fall between 1.3 and 2.5 for a piece to be machinable at letter rate. A square envelope has an aspect ratio of 1.0 — automatically nonmachinable, with a $0.44 surcharge per piece in 2026. A very long, narrow envelope with an aspect ratio above 2.5 is also nonmachinable.

Flat (large envelope) maximums

If a piece exceeds letter-size limits but stays within these dimensions, it's a flat:

• Height: 12 inches maximum
• Length: 15 inches maximum
• Thickness: 0.75 inches maximum
• Weight: 13 ounces maximum at First-Class flat rate

Flat-rate postage starts at $1.63 for the first ounce in 2026, with $0.20 added per additional ounce.

Beyond flat — parcel rates

Pieces over 12 by 15 by 0.75 inches, or over 13 ounces, are rated as parcels and ship under USPS Ground Advantage or Priority Mail rates — significantly higher than flat rates, with separate dimensional rules.

Postcard-specific minimums and maximums

Postcards have their own size rules: minimum 3.5 by 5 inches, maximum at the postcard rate of 4.25 by 6 by 0.016 inches. Pieces between 4.25 by 6 and 6.125 by 11.5 inches are rated at letter rate (oversized postcards). Postcards thicker than 0.016 inches but within letter dimensions also rate at letter rate.

2026 postage rates by size category

Letter rate

• 1 ounce, Forever stamp: $0.78
• 1 ounce, metered: $0.74
• Each additional ounce, up to 3.5 oz: $0.29
• Postcard rate: $0.61

Nonmachinable letter surcharge

If a piece meets letter-size dimensions but fails machinability standards (aspect ratio outside 1.3–2.5, rigid contents, uneven thickness, etc.), the postage is letter rate plus a $0.44 nonmachinable surcharge per piece. For a 1-ounce nonmachinable letter, that's $0.78 + $0.44 = $1.22.

Flat rate (large envelope)

• 1 ounce: $1.63
• Each additional ounce, up to 13 oz: $0.20

Why these limits matter for direct mail

The cost difference between letter rate and flat rate, multiplied across a 5,000-piece campaign, is significant: $0.85 per piece × 5,000 = $4,250. Designing direct mail pieces to stay just inside letter-rate limits is one of the highest-leverage decisions in mail-piece planning. We work through these tradeoffs on every campaign — see our direct mail services.

How to verify your piece

Before printing a run, measure a finished sample piece (envelope plus all contents):

1. Use a ruler, not the spec sheet — measure actual dimensions
2. Weigh on a postal scale
3. Calculate aspect ratio (length ÷ height)
4. Test flexibility by bending around an 11-inch diameter object
5. Test thickness by sliding through a 0.25 inch gauge

If any measurement is at or near a limit, plan for the higher postage rate or redesign the piece.

Last verified: May 2026. Source: USPS Domestic Mail Manual 101 and 201.