You can mail a small envelope through USPS only if it measures at least 3.5 inches tall, 5 inches long, and 0.007 inches thick. Anything under those minimums on any single dimension is nonmailable and will be returned to you. If your envelope is too small (many coin envelopes and novelty envelopes are), the fix is to tuck it inside a larger envelope that does meet the minimums and pay postage on the outer one.
The Minimum Size USPS Will Accept
Every letter-size piece has to clear three cutoffs:
- Height: at least 3.5 inches
- Length: at least 5 inches
- Thickness: at least 0.007 inches, roughly an index card
These are hard cutoffs, not guidelines. A piece that falls short on any one of them is nonmailable.1Postal Explorer. Sizes for Letters
To keep standard letter pricing, the envelope also has to stay under the maximums: no taller than 6.125 inches, no longer than 11.5 inches, no thicker than 0.25 inches. Anything larger gets bumped into the flats category and costs more.1Postal Explorer. Sizes for Letters
Postcards use the same 3.5 by 5 by 0.007 minimum. A card smaller than that can’t be mailed as a postcard either.2Postal Explorer. Sizes for Postcards
Where Coin and Small Envelopes Actually Fall
Most coin envelopes marketed for coins, seeds, or jewelry are too small to mail on their own. A #1 coin envelope is about 2.25 by 3.5 inches, well under both the height and length minimums. Even a #6 coin envelope, roughly 3.375 by 6 inches, misses the height minimum by an eighth of an inch. The smallest standard coin envelope that clears USPS requirements is the #7 at 3.5 by 6.5 inches, sitting right at the minimum height.
If Your Envelope Is Too Small
USPS rejects mail that falls under the minimums. If the piece has a return address, the carrier sends it back to you with a note that it doesn’t meet mailability requirements.3Postal Explorer. 507 Mailer Services Without a return address, it becomes dead mail, meaning USPS has no way to return it to you or deliver it to the recipient.
The workaround is simple. Put the tiny envelope inside a larger one that meets the minimums. A #6 3/4 envelope (3.625 by 6.5 inches) or an A2 envelope (4.375 by 5.75 inches) works fine as an outer mailer. You pay regular letter postage on the outer envelope, and the small one rides along inside.
The Non-Machinable Surcharge
Meeting the minimums doesn’t mean your envelope will run through USPS sorting machines. Letters that can’t be processed by automated equipment get a non-machinable surcharge of $0.49 on top of standard postage.4United States Postal Service. Notice 123 – January 2026 Price Change
The most common trigger is shape. USPS requires letters to have an aspect ratio (length divided by height) between 1.3 and 2.5. A square envelope has a ratio of 1.0, so every square envelope automatically gets the surcharge.5United States Postal Service. 3-6 Nonmachinable Criteria Square greeting cards and wedding invitations are the usual culprits.
Other features that trigger the same surcharge:
- Rigidity, meaning the envelope doesn’t flex easily when bent
- Closure hardware such as clasps, strings, or buttons
- Lumpy contents like pens, keys, or coins that create uneven thickness
One characteristic is enough.5United States Postal Service. 3-6 Nonmachinable Criteria A thick plastic gift card taped inside an envelope may not pass, though a credit-card-thin flexible item is usually fine.
2026 Postage for a Small Envelope
A one-ounce First-Class Mail letter costs $0.78, which is what a Forever stamp covers. Each additional ounce adds $0.29, up to 3.5 ounces for letter-class mail.6USPS. Types of First-Class Mail
If your envelope is non-machinable (square, rigid, lumpy, or otherwise unable to run through sorting machines), add the $0.49 surcharge. That puts a one-ounce non-machinable letter at $1.27 total.4United States Postal Service. Notice 123 – January 2026 Price Change You can build that from stamps you have on hand, or ask a postal clerk to weigh and rate it.
Underpaying creates a problem for the recipient. USPS marks the letter with the shortfall and delivers only if the recipient pays the balance. If they refuse or aren’t there to pay, the letter goes back to you (assuming a return address). No postage and no return address means dead mail.7Postal Explorer. 604 Postage Payment Methods and Refunds
Addressing a Small Envelope
Address placement matters more when the surface is small. The delivery address goes in the center of the front, the same side as the postage, and needs the recipient’s full name, street address, city, state abbreviation, and ZIP code. Your return address goes in the upper-left corner.8Postal Explorer. 602 Quick Service Guide
On an envelope near the 3.5 by 5 minimum, fitting all of that legibly is tight. Use your smallest clear handwriting or a printed label. USPS prefers uppercase letters for the delivery address, one space between city and state, two spaces between state and ZIP.
Postage goes in the upper-right corner. Seal the envelope fully so the flap doesn’t catch in sorting equipment. If you’re enclosing something with uneven thickness, tape it flat inside, but expect the non-machinable surcharge and price the postage accordingly.
Sending a Small Envelope Internationally
International mail has a stricter length minimum. Domestic letters need to be at least 5 inches long; international First-Class Mail letters must be at least 5.5 inches long. Height and thickness minimums stay at 3.5 inches and 0.007 inches.9USPS. First-Class Mail International A small envelope that just barely clears the domestic minimums may be rejected for international.
A one-ounce international letter is $1.70, covered by a single Global Forever stamp, which stays valid even if the rate rises later.10USPS. First-Class Mail International The $0.49 non-machinable surcharge applies internationally too, bringing a non-machinable international letter to $2.19.4United States Postal Service. Notice 123 – January 2026 Price Change
If the envelope holds anything besides documents or personal correspondence (a gift card, coins, seeds, small merchandise), you need a customs form. Letters carrying only documents don’t.11USPS. U.S. Customs Forms You can complete the form online at USPS.com or at the post office counter.