The number of stamps you need depends primarily on the weight, size, and destination of your mail. For standard letters within the U.S.:

• A single First-Class Forever Stamp (worth $0.73 as of 2025) covers postage for a letter weighing up to 1 ounce.
• For each additional ounce over 1 ounce, you need to add extra postage. The additional ounce typically costs about $0.24 more. For example:
  • 1 ounce = 1 stamp
  • 2 ounces = 1 stamp + additional postage (about 24 cents)
  • 3 ounces = 1 stamp + about 48 cents extra, and so on.
• Letters over 3.5 ounces are considered large envelopes (flats) and require different postage rates starting at about $1.35 for the first ounce, plus additional charges for extra weight

For non-standard or large envelopes:

• Larger envelopes (like 9x12 or manila) start at two stamps for the first ounce, with additional postage for each extra ounce

For international mail:

• A Global Forever Stamp or multiple domestic Forever Stamps are needed. For a letter up to 1 ounce, one Global Forever Stamp or three domestic Forever Stamps are typically required

How to calculate:

1. Weigh your mail accurately.
2. Check the current USPS postage rates for your mail type and weight.
3. Use the value of your stamps to cover the total postage. If your letter weighs more than 1 ounce, add stamps to cover the extra cost.
4. Round up to the nearest ounce when calculating postage

In summary, for a standard U.S. letter weighing up to 1 ounce, one Forever Stamp is sufficient. For heavier letters, add additional postage stamps accordingly. For large envelopes or international mail, postage requirements increase and may require multiple stamps or special stamps like Global Forever Stamps