How Many Stamps Do I Need?

One Forever stamp mails a standard letter up to one ounce. Past that, the answer depends on weight and on what shape the thing is.

The tables below give exact counts at current rates. Rates changed on 12 July 2026, so anything you read from before then is out of date.

Standard Letters

A letter is anything rectangular up to 11.5 by 6.125 inches and no thicker than a quarter inch. First-Class letters stop at 3.5 ounces.

Weight	Postage due	Simplest way to pay it
Up to 1 oz	$0.82	1 Forever stamp
Up to 2 oz	$1.11	1 two-ounce stamp
Up to 3 oz	$1.40	1 three-ounce stamp
Up to 3.5 oz	$1.69	2 Forever + 1 additional-ounce

Each ounce past the first adds $0.29. That rate did not change in July 2026 — only the base letter rate did.

Note the trap in row two. Two Forever stamps come to $1.64 for a two-ounce letter that only owes $1.11, so you overpay 53¢ and clutter the envelope. A single two-ounce stamp covers it exactly.

Large Envelopes

Anything bigger than letter size, or thicker than a quarter inch, is a flat. Flats are a separate, higher rate tier that starts at $1.69 for the first ounce and adds $0.29 per ounce after, up to 13 ounces.

Paying flat rates with 82¢ Forever stamps means overpaying on almost every tier. For anything heavier than a letter, weigh it and buy the matching denomination instead of stacking stamps.

Postcards

A domestic postcard is $0.65 — less than a letter. Using a Forever stamp works but wastes 17¢ per card, which adds up across a mailing.

Postcard stamps are priced for exactly this. The card has to be rectangular and between 3.5 by 5 and 6 by 4.25 inches, or it gets charged as a letter.

International Letters

A Global Forever stamp covers a one-ounce letter to any country at a single price, which saves working out per-country rates.

Heavier international mail is priced by country and weight, so check at the counter. See Global Forever stamps.

Things That Change the Answer

• Square envelopes. Invitations in square envelopes are nonmachinable and carry a $0.49 surcharge, whatever they weigh.
• Rigid or lumpy contents. Wax seals, ribbon, anything that will not bend adds the same surcharge.
• Clasps and string ties. Also nonmachinable.
• Guessing. Weigh one finished piece on a kitchen scale before buying postage for the whole batch.

Common Questions

How many stamps for a 2 oz letter? $1.11 of postage. One two-ounce stamp, not two Forever stamps.

Can I just add extra stamps to be safe? Yes, USPS keeps the overpayment. It is only wasteful, not a problem.

What happens if I underpay? The letter comes back marked "Return to Sender", or the recipient is asked for the difference.

Do Forever stamps still work after a rate rise? Yes. A Forever stamp always covers a one-ounce letter, whatever the current rate. See our Forever stamp price guide.

Need the postage itself? Browse first class stamps for everyday letters, or a book of 20 if you mail regularly.