One standard Forever stamp covers a First-Class letter weighing up to 1 ounce, which works out to about 28 grams.1USPS. Postage Stamps – The Basics Cross that weight by even a gram or two and the letter needs extra postage. At the January 2026 rate, a Forever stamp costs $0.78, and older stamps bought at lower prices still cover the current one-ounce rate.2Postal Explorer (USPS). Notice 123 – January 2026 Price Change
What 28 Grams Looks Like in Real Mail
As a working estimate, four or five sheets of ordinary 20-pound copy paper plus a standard envelope sit close to 1 ounce. A sixth sheet, a couple of photos, a thicker card stock, or an extra return envelope tucked inside can push the piece over. A birthday card in a paper envelope almost always fits under one stamp. A wedding invitation with an insert, an RSVP card, and a rigid backing usually does not.
If you’re close to the limit, weigh before you seal. A basic kitchen scale that reads in grams or ounces is enough. Put the sealed, addressed envelope on the scale with every insert already inside. If it reads 29 grams or more, you need additional postage.
Size Rules Apply Too
Weight is only half of what qualifies a piece for the single-stamp rate. The envelope also has to be rectangular, between 3.5 and 6.125 inches tall, 5 to 11.5 inches long, and no more than a quarter inch thick.3Postal Explorer. Sizes for Letters A standard #10 business envelope or a typical greeting-card envelope fits these limits with room to spare. Anything larger gets reclassified as a large envelope, and the price jumps regardless of weight.
Shape Surcharges That Add to the Price
An envelope can weigh well under 28 grams and still cost more than one stamp. Square envelopes, rigid envelopes, and pieces with clasps or uneven thickness jam automated sorting equipment, so the Postal Service charges a $0.49 non-machinable surcharge on top of the regular stamp price.2Postal Explorer (USPS). Notice 123 – January 2026 Price Change A square greeting card under 1 ounce runs $1.27 to mail, not $0.78. If your envelope is square, rigid, or otherwise irregular, add a second stamp or print exact postage at a kiosk.
When Your Letter Weighs More Than an Ounce
Standard letters can weigh up to 3.5 ounces before they stop qualifying as letters. Each additional ounce past the first costs $0.29 as of January 2026. So a 2-ounce letter runs $1.07, a 3-ounce letter runs $1.36, and the maximum standard letter at 3.5 ounces costs $1.65.2Postal Explorer (USPS). Notice 123 – January 2026 Price Change You can buy additional-ounce stamps to combine with a Forever stamp, or print the exact amount from a self-service kiosk.
Once a piece exceeds standard letter dimensions, it becomes a “large envelope” or flat. Flats can measure up to 12 inches tall, 15 inches long, and 3/4-inch thick, with a 13-ounce ceiling.4Postal Explorer. Sizes for Large Envelopes and Flats The first ounce on a flat costs $1.63, with each extra ounce adding about $0.27, reaching $5.04 at 13 ounces.2Postal Explorer (USPS). Notice 123 – January 2026 Price Change Manila envelopes carrying tax returns, transcripts, or legal documents typically land here.
International Letters Are Different
The 28-gram rule is for domestic mail. A First-Class Mail International letter can weigh up to 3.5 ounces, roughly 99 grams, and must stay within the same basic letter dimensions.5Postal Explorer. First-Class Mail International – Physical Characteristics The first ounce costs $1.70 to any destination as of January 2026,2Postal Explorer (USPS). Notice 123 – January 2026 Price Change and a single Global Forever stamp covers that first ounce to anywhere in the world.6USPS. First-Class Mail International Additional-ounce rates vary by country.
How to Avoid Underpaying
Mail that arrives with too little postage doesn’t just get delivered anyway. A letter with no postage is marked “Returned for Postage” and sent back to the return address with no delivery attempt. Pieces with some but not enough postage are delivered postage-due, meaning the recipient owes the shortfall before they can take the mail. Non-machinable First-Class letters that are shortpaid go back to the sender rather than out for delivery.
The cheapest fix is a scale and honest math. If you don’t have a scale, most post office lobbies have self-service kiosks that weigh mail and print exact postage, and many are available outside counter hours.7USPS. Self-Service Kiosks Overpaying by one stamp costs less than a returned letter or an annoyed recipient.