Why one stamp is often not the answer
The USPS does not price mail by weight alone. It prices by shape first, weight second, and machinability third, and all three have to be settled before you know what a piece owes. A one-ounce envelope can cost the base letter rate, or that rate plus a surcharge, or the large-envelope rate that is more than double it — with no change in weight at all. The variable is the envelope.
That is why so much invitation mail arrives short-paid. A square envelope is a letter by every measurement a person cares about, but it cannot be turned the right way up by an automated facer-canceller, so it carries a nonmachinable surcharge. A stiff card in a rigid envelope has the same problem. A 6 × 9 announcement that exceeds letter height stops being a letter and becomes a flat, priced from a different column entirely.
This calculator applies the shape tests in the order the Postal Service applies them, then does the part no rate chart does for you: it converts the exact postage into whole stamps. Because postage is priced to the cent and stamps are not, the number you stick on is always a rounding-up, and the difference between what you pay and what you owe is money the mailing costs you for nothing.
The shape tests and the arithmetic behind them
A piece is letter-size when it is rectangular, at least 5 in long and 3½ in high, no more than 11½ in long and 6⅛ in high, between 0.007 in and ¼ in thick, and no heavier than 3.5 oz. Miss any one of those and it is not a letter. Exceed the letter limits but stay within 15 in by 12 in by ¾ in and no more than 13 oz, and it is a flat — what USPS calls a large envelope. Past that it is a package, priced by weight and zone rather than from a stamp sheet.
A letter-size piece over 3.5 oz is an odd case worth knowing: it stays letter-shaped but is charged at large-envelope prices, because the weight rather than the size is what pushes it out of the letter column. The calculator handles that automatically and tells you when it has done so.
Within the letter category, machinability decides whether a surcharge applies. The test that catches most people is the aspect ratio: the length divided by the height must fall between 1.3 and 2.5. A standard #10 business envelope is 9½ × 4⅛, a ratio of 2.30, comfortably inside. A 5½-inch square is a ratio of exactly 1.00 and fails. A piece also fails if it is rigid, if it has a clasp, string or button closure, if its contents make it lumpy or unevenly thick, or if it is too thin for its size — anything more than 4¼ in high or more than 6 in long has to be thicker than 0.009 in to stay machinable.
The postage arithmetic itself is a first-ounce charge plus a per-ounce step on every started ounce after the first: ⌈w − 1⌉ of them. A 1.1 oz letter and a 2.0 oz letter cost the same, because both have started their second ounce. The 3.5 oz letter tier is the one exception to the whole-ounce pattern, and it is priced as first ounce plus three steps.
Then N = ⌈P / v⌉ converts postage P into stamps at denomination v. The ceiling is doing real work here. Postage of $1.07 against a $0.78 stamp needs two stamps worth $1.56 — you pay 46% more than you owe, and no arrangement of that single denomination fixes it.
Worked example: 120 square wedding invitations
You are mailing 120 invitations. Each is a 5½ × 5½ in square envelope holding a card, a vellum overlay, an RSVP card and its own small envelope; the assembled piece weighs 2.1 oz and is 0.05 in thick. The rates entered on this page are $0.78 for the first ounce, $0.29 for each additional ounce, and $0.46 for the nonmachinable surcharge; you are using Forever stamps worth $0.78.
- Classify the shape. 5.5 in by 5.5 in is inside the letter-size envelope of 5–11.5 in by 3.5–6.125 in, thickness is inside 0.007–0.25 in, and 2.1 oz is under 3.5 oz. It is a letter.
- Test machinability. Aspect ratio = 5.5 ÷ 5.5 = 1.00, which is below the 1.3 minimum. The piece is nonmachinable.
- Count additional ounces. ⌈2.1 − 1⌉ = ⌈1.1⌉ = 2 additional ounces.
- Base postage. $0.78 + 2 × $0.29 = $0.78 + $0.58 = $1.36.
- Add the surcharge. $1.36 + $0.46 = $1.82 due per piece.
- Convert to stamps. ⌈$1.82 ÷ $0.78⌉ = ⌈2.33⌉ = 3 stamps, worth $2.34.
- Overpayment. $2.34 − $1.82 = $0.52 per piece.
- Whole mailing. 120 × $2.34 = $280.80 in stamps, of which 120 × $0.52 = $62.40 is overpayment.
That $62.40 is the price of using one denomination. Two Forever stamps plus a $0.29 additional-ounce stamp comes to $1.85 — three cents over instead of fifty-two, saving about $59 across the mailing. Weighing the assembled invitation before you buy anything is the other lever: shedding 0.11 oz drops you to one additional ounce, $1.53 due, and two Forever stamps cover it with $0.03 to spare. If you are budgeting the stationery alongside everything else, the wedding budget allocation calculator puts postage next to the rest of the spend.
How to read the result before you buy stamps
Look at the overpayment first, not the stamp count. Under about $0.10 a piece, one denomination is fine and buying a second stamp type is not worth the trip. Above roughly $0.30 a piece on a mailing of any size, mixing denominations pays for itself immediately — an additional-ounce stamp or a low-value make-up stamp exists precisely for this, and USPS sells both.
Then look at what pushed the price up. If the surcharge is the reason, you have a design decision, not a postage decision: a rectangular envelope in the 1.3–2.5 ratio band avoids it entirely, and on 200 pieces that is a meaningful sum. If the additional-ounce count is the reason, the fix is inside the envelope — a lighter card stock, one fewer insert, or a thinner liner.
Weigh a fully assembled piece, stamp included, on a scale that reads to a tenth of an ounce, and weigh three of them rather than one. Assembly varies, and being 0.05 oz over the line on a fifth of your mailing means a fifth of your invitations arrive postage-due. When you are near a boundary, round up. Short-paid mail is either returned to the sender or delivered with postage due collected from the recipient, and on invitations both outcomes are worse than a few cents of overpayment.
One more check before a large mailing: take a single assembled piece to a post office counter and have it weighed and rated. It costs nothing, it settles machinability with the people who actually make the call, and it is the only way to be certain about a piece whose shape is borderline.
Letter postage and Forever stamps by weight
| Weight | Additional ounces | Postage due | Forever stamps | Overpayment |
|---|---|---|---|---|
| 1.0 oz | 0 | $0.78 | 1 | $0.00 |
| 1.5 oz | 1 | $1.07 | 2 | $0.49 |
| 2.0 oz | 1 | $1.07 | 2 | $0.49 |
| 2.5 oz | 2 | $1.36 | 2 | $0.20 |
| 3.0 oz | 2 | $1.36 | 2 | $0.20 |
| 3.5 oz | 3 | $1.65 | 3 | $0.69 |
Add the nonmachinable surcharge to every row if your envelope is square, rigid, clasped or outside the 1.3-2.5 aspect ratio. Above 3.5 oz a letter-size piece moves to large-envelope prices.
The rate fields are placeholders, not a price list
USPS prices change, typically more than once a year, and this page has no way to know what happened after it was written. The authoritative source is Notice 123, the Price List, published on the Postal Explorer site and updated the day each new price schedule takes effect. Open it, copy the four rate figures into the fields above, and the rest of the calculation is exact. The classification rules — the size bands, the 3.5 oz letter ceiling, the 13 oz First-Class Mail ceiling, the 1.3 to 2.5 aspect ratio, the 0.007 in minimum thickness — come from the Domestic Mail Manual and change far less often than the prices do.
Mistakes that cost real money on a mailing
- Weighing an unassembled envelope. The liner, the RSVP envelope, the vellum and the stamp all weigh something. Weigh a finished piece.
- Assuming a Forever stamp covers two ounces. It covers the first ounce at whatever the current letter rate is, and nothing more.
- Forgetting that the surcharge is per piece, not per mailing. On 200 square invitations it is the largest single line in the postage bill.
- Buying stamps before weighing. Denomination choice depends on the postage due, and postage due depends on the assembled weight.
- Treating a 6 × 9 envelope as a letter. It exceeds the 6⅛ in height limit only narrowly, but it is a flat and priced from the other column.
- Mailing a piece with a clasp closure unstamped for the surcharge. Clasps, strings and buttons are all nonmachinable triggers on their own.
- Rounding weight down. A started ounce is a charged ounce; 2.01 oz costs what 3.00 oz costs.
When stamps are the wrong tool
Past 13 oz, First-Class Mail ends. A heavier piece goes as USPS Ground Advantage or Priority Mail, both priced on weight and zone, and both of which can be charged by dimensional weight when the box is large and light — that calculation is a different one entirely and is covered by the dimensional weight shipping calculator. You can still pay with stamps at those prices, but it takes so many that a printed label is the sensible route.
For volume mailings, stamps stop being the cheapest option well before they stop being practical. USPS Marketing Mail and presorted First-Class Mail carry lower per-piece rates in exchange for minimum volumes, address standardisation and barcoding, which in practice means working through a mailing house or online postage software. The break-even sits at a few hundred pieces for most people, and the trade is price against setup effort.
Below that threshold, printing your own postage is usually the better deal for anything non-standard, because it prices to the cent and eliminates the rounding this calculator measures. If you are also printing the invitations or the address labels yourself, the printer ink cost per page calculator will tell you what the printing side actually costs, and the gift wrap paper calculator covers the parcels that go alongside the cards.
Postal terms worth knowing
- Flat
- USPS's name for a large envelope: bigger than letter-size, up to 15 × 12 × ¾ in and 13 oz, flexible and uniformly thick.
- Machinable
- A piece that can be processed on automated mail equipment. Letter-size pieces that are not machinable pay a surcharge.
- Forever stamp
- A non-denominated stamp that is always worth the current one-ounce First-Class Mail letter price, whatever you paid for it.
- Additional-ounce stamp
- A stamp sold at the additional-ounce rate, designed to be used alongside a Forever stamp on a heavier letter.
- Notice 123
- The USPS Price List: the complete, current table of domestic and international prices, republished each time prices change.
- Postage due
- Mail delivered with insufficient postage, where the shortfall is collected from the recipient or the piece is returned to sender.