How to Ship Envelopes | USPS Letter Rates & Rules

Mailing a letter sounds simple, but the difference between letter pricing and flat pricing comes down to a few precise measurements. Whether you’re sending a bill, a greeting card, or a contract, knowing the current USPS letter rules keeps your envelope in the cheap tier and out of the “why did this cost $1.69?” surprise. The standard No. 10 envelope most households keep in a drawer fits comfortably within letter limits, so this is mostly about knowing the boundaries.

What Are the Current USPS Letter Size and Weight Limits?

USPS letter rates apply only to paper envelopes that are rectangular and flat. The maximum dimensions are 11-1/2 inches long and 6-1/8 inches high, with a maximum thickness of 1/4 inch. Minimums are 5 inches long, 3-1/2 inches high, and 0.007 inch thick. Weight tops out at 3.5 ounces for letter-size mail.

A standard No. 10 envelope measures 9-1/2 by 4-1/8 inches, which sits comfortably inside those limits. If your envelope exceeds the 11-1/2 by 6-1/8 by 1/4 inch boundaries, it moves to flat pricing, which starts at $1.69. Flats can go up to 15 inches long, 12 inches high, and 3/4 inch thick.

Folded contents must stay flat. Stuffing a letter with a bulky item that pushes the envelope past 1/4 inch thick triggers flat pricing even if the length and height are fine.

How Much Postage Does an Envelope Need in 2025?

How to Address an Envelope the Right Way

Getting the addresses in the correct positions matters more than most people realize. The return address goes in the top-left corner, and the recipient address goes in the bottom center of the envelope. USPS’s addressing guidance specifies that the top of the delivery address should be no more than 2-3/4 inches from the bottom edge, with the bottom no more than 5/8 inch from the bottom.

Keep the delivery address at least 1 inch from the sides. The lower-right corner must stay empty because that’s where USPS prints the barcode. Any text, logos, or decorations in that zone can delay processing or trigger a surcharge for manual handling.

Write or print clearly. USPS scanning equipment reads typed addresses better than handwriting, so if you have access to a printer, use it. For handwritten envelopes, use a dark pen with clear block letters rather than cursive.

Where Can You Drop Off a Letter?

You have four practical options for sending a letter:

• Your home mailbox with the flag raised for the carrier to pick up
• A cluster mailbox outgoing slot if your neighborhood uses those
• A blue USPS collection box on the street
• The lobby drop at your local Post Office

Collection boxes get emptied on published schedules, so check the pickup time on the box label if your letter is time-sensitive. Home mailbox pickup happens during regular carrier delivery, which means a letter dropped after pickup waits until the next day.

Some mailpieces that look like envelopes don’t qualify as letter mail. Bubble mailers, poly mailers, and other plastic packaging are not accepted as First-Class envelopes, and neither are envelopes with clasps or strings. Those ship as packages or flats at higher rates. When in doubt, USPS’s letter-mail page lists the full qualification rules.

What Can’t Go in the Mail?

USPS Publication 52 lists hazardous, restricted, and perishable items that cannot ship through standard mail. Ammunition, explosives, gasoline, liquid mercury, and marijuana are nonmailable in the U.S. mail. Check that list before sending anything unusual, especially if you’re shipping to a collector or buyer who might expect something offbeat.

If your contents are restricted but mailable, pack the envelope securely and mark it as USPS rules require. When you’re unsure, bring the item to a Post Office window rather than guessing. Postal clerks can confirm whether something qualifies and what packaging it needs. For trading cards or other flat collectibles that need extra protection, envelopes built for shipping trading cards stay within letter thickness while adding rigidity. That keeps fragile contents safe without pushing you into package pricing.