Four ounces usually leaves letter stamps
This site caps First-Class letters below 4 oz actual weight. Searchers land here after a scale readout or a thick packet—expect flat or parcel retail, not “add another Forever.”
When a 4 oz readout matters
Use this page when your sealed piece weighs about four ounces or when you are budgeting postage for thick document stacks, small booklets, or dense paper—then verify whether the calculator still calls it a letter or moves you to large-envelope class.
Example scenarios
Four ounces often appears when someone stacks thick documents, adds booklets to a packet, or mails dense cardstock. Small product samples, thick annual reports, and padded paper stacks can reach four ounces quickly. Parents mailing school projects and offices sending contract packets sometimes see this weight on a kitchen scale. If you are stepping up from a standard 1 oz letter, remember letter class here tops out below 4 oz. The important part for postage is not the story behind the weight but whether the piece still fits letter rules. In this calculator, four ounces of actual weight usually puts you past the letter weight cap, which moves the conversation from letter stamps to large-envelope or other retail pricing.
Common mistakes
A frequent mistake is treating four ounces like a heavier letter and adding four Forever stamps. Letter class here tops out at 3.5 ounces of actual weight, so four ounces is past that line for letters. Another mistake is ignoring rounding and limits together. You might have 3.8 ounces on the scale and think you are close to a letter, but billable ounces and physical limits both matter. People also assume every flat envelope mails like a letter if it looks thin. Length, height, and thickness can disqualify letter treatment even when weight alone seems fine.
How USPS calculates postage
Letter rates use whole ounces for the ladder in this model, rounded up from actual weight. Letter size uses maximum length, height, thickness, and weight together. If any limit fails, the mail may be a large envelope or another class. Machinable pieces move through automation more easily; non-machinable pieces can carry surcharges when they still qualify as letters. None of that changes the letter weight ceiling. If actual weight is above 3.5 ounces, this tool treats the piece as outside letter classification for the weight rule, which is why letter stamp math stops applying.
Why four ounces becomes a large envelope
In the simplified rules used here, a First-Class letter cannot weigh more than 3.5 ounces. Four ounces crosses that line. A large envelope, or flat, uses different retail prices and different retail products than letter stamps. That is why the calculator does not show a letter total for a typical four-ounce piece that otherwise fits the flat model. Your options are to buy the correct flat postage from USPS, use metered or online postage with the right price, or ask a clerk. If you can trim weight or move content into a smaller packet under the letter cap, letter stamps may apply again. International mail, parcels, and odd shapes follow other rules entirely.
Using this page with the calculator
The preset weight is four ounces so you can see the large-envelope path clearly. Change dimensions to match your envelope. If the tool still shows a flat, compare your measurements to a ruler at the longest and thickest points. When in doubt, retail confirmation beats guessing.
Before you mail
If you must hit a deadline, buy postage at the counter or online with the class the clerk or the official tool assigns. Guessing with letter stamps on a four-ounce piece risks underpayment or returned mail.