USPS prices letters by shape first and weight second. If your envelope fits the letter-size window, it travels at letter prices; exceed any single maximum — length, height, thickness, or weight — and the piece is repriced as a large envelope or a parcel. Here are the current official standards, verified against USPS Postal Explorer and the USPS price list (Notice 123, effective October 4, 2026).
Key Takeaways
- Letters must be 5–11.5 inches long, 3.5–6.125 inches high, 0.007–0.25 inch thick, and no more than 3.5 ounces — exceed any single limit and the piece is priced as a large envelope.
- Letter prices run from $0.82 at 1 ounce, plus $0.29 per additional ounce, to $1.69 at 3.5 ounces (metered mail starts at $0.78).
- Square, rigid, clasped, or lumpy letters add the $0.49 nonmachinable surcharge even when they fit the size window — a 1-ounce square card costs $1.31.
- Large envelopes (flats) can measure up to 15 x 12 x 0.75 inches and weigh up to 13 ounces, with prices starting at $1.69.
- Anything smaller than 3.5 x 5 inches is nonmailable — tiny enclosures must ride inside a standard envelope.
The Letter-Size Window
To qualify for letter prices, a piece must be rectangular and fall inside all of these limits at once:
| Dimension | Minimum | Maximum |
|---|---|---|
| Length (the side parallel to the address) | 5 inches | 11-1/2 inches |
| Height | 3-1/2 inches | 6-1/8 inches |
| Thickness | 0.007 inch | 1/4 inch |
| Weight (for letter prices) | — | 3.5 ounces |
Notes that catch people out:
- Below the minimum is nonmailable. A piece smaller than 3-1/2 x 5 inches cannot be mailed at all — USPS equipment simply cannot handle it. Tiny gift enclosures must ride inside a standard envelope.
- The 0.007-inch minimum thickness is roughly the thickness of an index card. Pieces more than 6 inches long or 4-1/4 inches high must be at least 0.009 inch thick, or they pick up the nonmachinable surcharge described below.
- Weight is a size rule too. A letter-size envelope over 3.5 ounces is charged large-envelope (flat) prices even if its dimensions are perfectly standard — no number of stamps turns it back into a letter.
Current First-Class Letter Prices
Retail prices for stamped letters (Notice 123, effective October 4, 2026):
| Weight up to | Stamped | Metered |
|---|---|---|
| 1 oz | $0.82 | $0.78 |
| 2 oz | $1.11 | $1.07 |
| 3 oz | $1.40 | $1.36 |
| 3.5 oz | $1.69 | $1.65 |
Each additional ounce above the first costs $0.29, and partial ounces round up — a 1.2-ounce letter pays the 2-ounce price. One ounce is about four sheets of standard copy paper plus an envelope; a typical greeting card with nothing else inside usually fits comfortably. For stamp math, see how many stamps do I need for a letter.
The Nonmachinable Surcharge: Within Size, Still Extra
A letter can fit the size window and still cost an extra $0.49 if it cannot run through automated sorting equipment. Under USPS standards (DMM 101 / Quick Service Guide 201), a letter is nonmachinable if it:
- Has an aspect ratio (length divided by height) below 1.3 or above 2.5 — a square envelope has a ratio of 1.0, so all square cards qualify;
- Is too rigid to bend;
- Has clasps, strings, buttons, or similar closure devices;
- Contains items like pens, pencils, or keys that make the thickness uneven;
- Has its delivery address parallel to the shorter side of the envelope (a “vertical” orientation);
- Is more than 4-1/4 inches high or 6 inches long but thinner than 0.009 inch;
- Has a nonpaper surface (beyond allowed windows or attachments);
- Is a folded self-mailer that is not sealed or tabbed.
The math for the classic case: a 1-ounce square greeting card costs $0.82 + $0.49 = $1.31. This is the single most common surprise in holiday mail — see do greeting cards need extra postage for the card-specific walkthrough.
The 60-Second Nonmachinable Self-Check
Run one finished piece through these five tests before you stamp a batch. Any single failure adds the $0.49 surcharge:
| Test | How to check | Pass looks like |
|---|---|---|
| Ratio test | Divide the length by the height | Between 1.3 and 2.5 — a 5 x 7 envelope: 7 ÷ 5 = 1.4, pass. A 6 x 6 square: 1.0, fail |
| Bend test | Flex the sealed piece gently between your hands | It bends like ordinary mail, without forcing |
| Closure test | Look at how the envelope closes | A gummed or self-seal flap — no clasps, strings, or buttons |
| Lump test | Run your fingers across both sides | Uniform thickness — no pen, key, ornament, or seal mound standing out |
| Thin-piece test | For pieces over 6 in long or 4-1/4 in high, check the stock | At least 0.009 in thick — flimsy, paper-thin large pieces fail |
The ratio test is the one worth doing with a ruler in hand, because envelope names mislead: many “square-ish” card sizes are rectangular enough to pass, and some tall formats fail in the other direction. When a test is borderline, the piece is a candidate for a Post Office counter check before the batch is stamped.
When a Letter Becomes a Large Envelope (Flat)
Exceed any one letter maximum and the piece is a large envelope, which USPS also calls a flat. Flats may be up to 15 inches long, 12 inches high, and 3/4 inch thick, and First-Class Mail flats can weigh up to 13 ounces. Prices start at $1.69 for 1 ounce — the full table is in how to calculate postage for large envelopes. One more boundary: a flat that is rigid, non-rectangular, or not uniformly thick is charged parcel prices instead, so a stiff photo mailer may cost parcel rates even when its dimensions fit the flat window.
How to Measure Your Own Mailpiece
- Length is the side parallel to the address as it will be read; height is perpendicular to it. USPS defines the dimensions by the address, not by how the envelope came out of the box.
- Measure thickness at the thickest point of the sealed, finished piece — inserts included.
- Weigh the finished piece, sealed, with everything inside. Kitchen scales are accurate enough to place a piece in the right ounce band for most mail.
- If a piece sits within a hair of any limit, take it to a Post Office counter. Clerks have templates and gauges, and asking before a mailing is free; paying surcharges or postage-due after is not.
Worked Examples: Which Price Applies?
| Piece | Verdict | Price |
|---|---|---|
| 5 x 7 greeting card envelope, flexible, 0.9 oz | Inside every letter limit | Letter, $0.82 |
| 6 x 6 square card envelope, 1 oz | Letter size, but ratio 1.0 → nonmachinable | $0.82 + $0.49 = $1.31 |
| 9 x 12 envelope with a few sheets, 0.8 oz | Exceeds the 6-1/8 in letter height → flat, 1 oz band | $1.69 |
| Letter-size envelope, 0.3 in thick with a bulky insert | Exceeds the 1/4 in thickness maximum → flat prices | Priced as a large envelope at its weight |
| #10 business envelope with a thick report, 3.8 oz | Dimensions fine, but over the 3.5 oz letter weight limit → flat prices | Priced as a large envelope at its weight |
The last two rows are the ones that generate postage-due surprises, because the envelope looks like a letter. Weight and thickness reclassify a piece just as surely as length and height do. For flat pricing by weight, see how to calculate postage for large envelopes.
Three Atlanta Pieces, Measured
Standards become easier to use when they are attached to real envelopes. These three came across Atlanta desks this season; every measurement below is of the sealed, finished piece:
| Piece | Measurements | Verdict and price |
|---|---|---|
| A Buckhead event planner’s square invitation, with reply card and envelope inside | 6.5 x 6.5 in, 0.2 in thick, 1.3 oz | Inside the letter window, but the aspect ratio is 1.0, so it is nonmachinable. Weight bills at 2 oz: $1.11 + $0.49 = $1.60 |
| A Decatur bakery’s holiday catering menu in a 9 x 12 envelope | 12 in long, 9 in high, 2.6 oz | Height exceeds the 6-1/8 in letter maximum, so it is a flat. Weight rounds to the 3 oz band: $2.27 |
| A Midtown firm’s #10 envelope carrying a thick signed agreement | 9.5 x 4.125 in, 0.3 in thick at the fold, 4.1 oz | Both thickness and weight pass the letter limits — the piece is priced as a flat at the 5 oz band: $2.85 |
Each piece fails a different test, which is the point of measuring all four dimensions instead of eyeballing the envelope. The invitation looks like a letter and is priced like one — plus a surcharge. The menu looks like a letter and is not one at all. The agreement is the quietest case: an ordinary business envelope that crossed two limits nobody checked because it “looked standard.”
In my years in commercial mail, the advice that saved senders the most money was unglamorous: measure the piece you made, not the piece you ordered. Card stock swells with inserts, folds add thickness, and the sealed envelope in your hand is the only version USPS ever prices.
Quick Reference
| Shape | Key limits | Price starts at |
|---|---|---|
| Postcard (retail) | 3-1/2 x 5 in min; 4-1/4 x 6 in max; 0.007–0.016 in thick | $0.65 |
| Letter | Up to 11-1/2 x 6-1/8 in; up to 1/4 in thick; up to 3.5 oz | $0.82 (1 oz) |
| Letter, nonmachinable | Square, rigid, clasps, uneven, etc. | Letter price + $0.49 |
| Large envelope (flat) | Up to 15 x 12 in; up to 3/4 in thick; up to 13 oz | $1.69 (1 oz) |
| Parcel | Exceeds flat limits, or rigid/non-uniform flat | Package prices |
Frequently Asked Questions
My letter weighs exactly 3.5 ounces. Letter prices or flat prices?
Letter prices. The letter weight limit is 3.5 ounces, so a piece at exactly 3.5 ounces still qualifies and pays the top letter price — $1.69 stamped. The moment it goes over 3.5 ounces, the whole piece is priced as a large envelope at its full weight; there is no partial credit for the letter-sized dimensions.
Is a 6 x 9 envelope a letter?
Yes, if the rest of the piece cooperates. At 9 inches long and 6 inches high it sits inside the 11-1/2 x 6-1/8 inch window, so it mails at letter prices as long as it is no more than 1/4 inch thick, weighs no more than 3.5 ounces, and passes the nonmachinable checks above. Booklet envelopes in this size are a common, economical choice for cards and documents.
My envelope is exactly 1/4 inch thick. Does it still qualify?
At exactly the maximum, yes — 1/4 inch is the limit, not a target to stay under. The catch is measurement: thickness is judged at the thickest point of the sealed piece, so an envelope that is 1/4 inch at the flap but bulges past it over an insert is a flat, not a letter. Measure the bulge, not the edge.
Standards are drawn from the USPS Domestic Mail Manual via Postal Explorer and prices from USPS Notice 123 (effective October 4, 2026). Postal prices change periodically — if you are reading this well after publication, confirm current figures in our USPS Mailing Rules hub before a large mailing.
Sources
The rates, dates, and rules in this guide were checked against these official USPS pages:
Related Guides
- USPS Mailing Rules hub — addressing, extra postage, and large-envelope pricing collected in one place.
- Do greeting cards need extra postage? — these size and shape rules applied to the cards in your December stack.
- How to calculate postage for large envelopes — the full flats price table and worked examples once a piece leaves letter prices.
- How many stamps do I need for a letter? — the letter prices on this page translated into stamps.
About the author — James Miller is a retired U.S. Postal Service employee whose career was in commercial mail, where he helped businesses prepare and send large-volume mailings. He edits Buy Holiday Stamps from Atlanta, Georgia. More about James and this site.
Last fact-checked against official USPS sources: November 2, 2026.