Calculate USPS postage and Forever stamps by weight for standard letters or large envelopes, or convert a known postage amount into stamps using an editable stamp value.

Weigh the complete mailpiece. Choose its qualifying mail category.

US domestic stamped First-Class Mail only. Weight does not establish mailpiece eligibility. Rates effective October 4, 2026; checked October 23.

Stamp value

Default: current $0.82 domestic Forever stamp value. Edit for denominated stamps.

Related Calculators

Stamps Per Weight Formula

First find the required postage from the USPS table for the mailpiece's weight and type. Then divide that dollar amount by the dollar value of each stamp and round up:

Stamps = ceil(Required postage / Stamp value)

Stamp value is measured in dollars, not ounces. A Forever stamp pays the current one-ounce domestic First-Class Mail letter rate. Additional ounces do not each require a full Forever stamp.

Current USPS Letter and Flat Rates

The built-in table uses retail First-Class Mail prices in USPS Notice 123 effective October 4, 2026, checked October 23. Standard machinable letters cost $0.82 up to 1 oz, $1.11 up to 2 oz, $1.40 up to 3 oz, and $1.69 up to 3.5 oz. Letters heavier than 3.5 oz must use a different mail category. Large envelopes use the separate flat-price table through 13 oz; the added-ounce price is not constant across every tier.

How to Calculate Stamps Needed

  1. Weigh the complete envelope and contents.
  2. Choose a standard machinable letter or a qualifying large envelope. Weight alone does not determine the mail category.
  3. Calculate the required postage and stamp count. Enter a different stamp value if using denominated stamps.
  4. Review excess postage. Combining denominations can avoid overpaying with whole Forever stamps.

Example: a standard 2.5 oz letter requires $1.40. Using only $0.82 Forever stamps requires ceil(1.40 / 0.82) = 2 stamps, applying $1.64 and overpaying by $0.24. One Forever stamp plus two $0.29 additional-ounce stamps supplies the exact $1.40.

Mailpiece Limits

This tool covers domestic standard machinable letters and qualifying flats only. It does not price parcels, international mail, extra services or the nonmachinable-letter surcharge. A rigid, nonrectangular or unevenly thick large envelope can require parcel pricing even below 13 oz. If you already know the final postage including any surcharge, use the known-postage mode.

USPS currently lists a $0.49 nonmachinable-letter surcharge. Confirm dimensions, flexibility, thickness and service eligibility using the official mailing standards. Prices can change during a year.