One Forever stamp covers a letter weighing up to 1 ounce, which in practice means about four sheets of standard copy paper folded into a regular business envelope. A fifth sheet often pushes the envelope just over the line, so if you’re mailing exactly five, weigh it first.
Four Sheets Is the Safe Number
Most home and office printers use 20 lb bond paper, roughly 75 grams per square meter. A letter-size sheet weighs about 4.5 grams. A standard #10 business envelope adds about a quarter of an ounce on its own, leaving roughly three-quarters of an ounce for whatever you put inside. Four sheets plus that envelope come in comfortably under 1 ounce. USPS gives the same estimate, describing one ounce as “about 4 sheets of regular printer paper in a business-sized envelope.”1USPS. How to Send a Letter or Postcard – International
One Forever stamp currently costs $0.78, a price that held through the January 2026 rate cycle.2USPS. 2026 Postage Price Change
Heavier Paper Cuts the Count
Premium stationery and résumé paper is often 24 lb bond, around 90 grams per square meter. Each sheet weighs roughly 5.4 grams, so three sheets in a standard envelope sit safely under 1 ounce and a fourth brings you to the limit.
Cardstock is heavier still. A 65 lb cardstock sheet weighs nearly twice as much as regular copy paper, so one or two inserts plus the envelope can hit 1 ounce on their own. Weigh anything that involves cardstock, photos, or thick stock before you mail it.
The Envelope Has to Qualify
The 1-ounce, one-stamp price only applies if your envelope meets First-Class letter rules. It has to be rectangular, between 3½ × 5 inches and 6⅛ × 11½ inches, and between 0.007 and ¼ inch thick.3USPS. Types of First-Class Mail Exceed those dimensions and the piece gets reclassified as a large envelope or package, both priced higher regardless of what a single stamp would otherwise buy.
If You Go Over One Ounce
You don’t need a second Forever stamp for a heavier letter. First-Class letters can weigh up to 3.5 ounces, and each additional ounce costs $0.29.4USPS. First-Class Mail That puts a 2-ounce letter at $1.07 and a 3-ounce letter at $1.36.5Postal Explorer. Notice 123 Price List – Effective January 18, 2026
USPS sells Additional Ounce stamps at $0.29 each, meant to sit alongside a Forever stamp on heavier letters.6USPS. Additional Ounces – The Postal Store At the 3.5-ounce ceiling, you can fit roughly 15 to 17 sheets of standard copy paper with a business envelope, which handles most everyday document mailings.
Shape Can Cost You Even With Few Sheets
Weight isn’t the only variable. USPS adds a $0.49 non-machinable surcharge to letters that can’t run through automated sorting equipment.7Postal Explorer. Notice 123 Price List – January 2026 Square envelopes, rigid or lumpy envelopes, and envelopes closed with clasps, string, or buttons all trigger it.4USPS. First-Class Mail
Greeting cards are the common trap. A square card with a single Forever stamp arrives postage-due because the recipient owes the extra $0.49. Slipping small items like coins, keys, or magnets into a letter causes the same problem, even if the envelope holds only a sheet or two.
Check Before You Mail
A kitchen scale accurate to a quarter-ounce is enough. Weigh the sealed, fully stuffed envelope rather than the paper alone. If the scale reads exactly 1.0 ounce, add an Additional Ounce stamp, because USPS rounds any fraction up to the next whole ounce.8Postal Explorer. 133 Prices and Eligibility
If you underpay and included a return address, USPS will usually send the letter back stamped “Returned for Postage” so you can add postage and re-mail it.9Postal Explorer. 604 Postage Payment Methods and Refunds Sometimes the letter is delivered with the shortage collected from the recipient, who can refuse and send it bouncing back to you. A letter with no return address and short postage becomes dead mail, so always put a return address on anything you’re not sure about. For borderline pieces, the online postage calculator or a trip to the counter settles it in under a minute.10USPS. Retail Postage Price Calculator