What “1 oz” really means on the scale
Kitchen scales and postage scales may show decimals. USPS pricing for letters uses whole ounces for the rate ladder in common retail scenarios: weights round up. Anything from just over 0 oz up to 1 oz is treated as one ounce for that step; once you pass 1 oz, you move into additional-ounce pricing. If you are at 1.0 oz on the nose, you stay in the one-ounce bucket. If you are at 1.2 oz, you are in the two-ounce bucket—see our 2 oz stamps page for that scenario.
Envelope shape matters as much as weight
People search “how many stamps for 1 oz” because weight feels like the whole story. In practice, dimensions and flexibility decide whether you are even mailing a “letter” for pricing purposes. A one-ounce invitation in a square envelope can cost more than a one-ounce letter in a #10 envelope because of the non-machinable surcharge. That is not a trick—it is how the Postal Service prices pieces that cannot ride automated letter sorting the same way.
Letter limits in plain language
If your piece is too thick, too big, or too heavy for letter treatment, you may need large-envelope (flat) pricing instead. This tool compares your numbers to letter limits and tells you when you are out of scope. Do not force letter stamps onto a flat: underpaid mail gets delayed or returned.
Quick checklist before you seal the envelope
Weigh the sealed piece. Measure thickness at the thickest point. Decide honestly whether the envelope is square or stiff. Those three steps, plus the fields in our calculator, give you a trustworthy answer for how many stamps you need at one ounce.
Seasonal mail—holiday cards, tax paperwork, school forms—often lands right at one ounce because senders use a single folded sheet or a thin card. If you add a printed photo, a second sheet, or a stiff liner, you may cross into two ounces without noticing. When in doubt, weigh again after you assemble everything. The calculator’s note about rounded weight exists so you can reconcile the scale readout with the ounce step USPS uses for retail letter pricing.
Finally, remember that stamps are denominated in whole units at retail, while your total might include cents that do not match a single stamp exactly. In practice you combine Forever stamps and additional ounce stamps until you meet or slightly exceed the required postage. The calculator’s dollar total is the target; the stamp sentence describes a common retail combination that matches the rate structure we model.