USPS charges large envelopes — which it also calls flats — on a simple formula: find the piece’s weight in ounces, round up to the next whole ounce, and read the price off the First-Class Mail flats table. There is no distance component for First-Class Mail, so a 9 x 12 envelope costs the same across the state or across the country. Here is the full current table, the classification rules that decide whether your piece is really a flat, and three worked examples. Prices are from USPS Notice 123, effective October 4, 2026.
Key Takeaways
- The flat formula has three steps: confirm the piece is a flat, weigh it and round up to the next whole ounce, then read the price — First-Class prices have no distance component.
- First-Class flats run from $1.69 at 1 ounce to $5.24 at 13 ounces; over 13 ounces, the piece is a parcel.
- A letter-size envelope over 3.5 ounces is priced from this table, not at letter prices — weight alone can make a flat.
- Two Forever stamps are worth $1.64, which does not cover even a 1-ounce flat ($1.69); combine denominations or pay at the counter.
- There is no nonmachinable surcharge on flats — but a rigid, non-rectangular, or uneven flat is repriced as a parcel instead.
Step 1: Confirm the Piece Is Actually a Flat
A piece is priced as a large envelope when it exceeds at least one letter maximum — more than 11-1/2 inches long, more than 6-1/8 inches high, or more than 1/4 inch thick — while staying within the flat maximums:
- Length: up to 15 inches
- Height: up to 12 inches
- Thickness: up to 3/4 inch
- Weight: up to 13 ounces at First-Class Mail prices
Two traps. First, a letter-size envelope that weighs more than 3.5 ounces is also charged flat prices — weight alone can move a piece into this table. Second, the piece must behave like a flat: if it is rigid, non-rectangular, or not uniformly thick, USPS charges parcel prices instead, even if the dimensions fit. That is why a stiff document mailer with a hard photo inside sometimes costs package rates. If your piece exceeds even the flat maximums, it is a parcel, period. The full classification system is explained in USPS letter size and weight requirements.
Step 2: Weigh It and Round Up
Weigh the finished piece — envelope, contents, and anything you will seal inside — and round any fraction up to the next whole ounce. A 2.1-ounce flat pays the 3-ounce price. For most senders a kitchen scale is precise enough; if a piece sits within a tenth of an ounce of a boundary, reweigh it or have a Post Office counter confirm.
Step 3: Read the Price
First-Class Mail large-envelope (flat) retail prices, Notice 123, effective October 4, 2026:
| Weight up to | Price | Weight up to | Price |
|---|---|---|---|
| 1 oz | $1.69 | 8 oz | $3.72 |
| 2 oz | $1.98 | 9 oz | $4.02 |
| 3 oz | $2.27 | 10 oz | $4.32 |
| 4 oz | $2.56 | 11 oz | $4.62 |
| 5 oz | $2.85 | 12 oz | $4.93 |
| 6 oz | $3.14 | 13 oz | $5.24 |
| 7 oz | $3.43 |
For stamps, translate the price into pieces you have: two Forever stamps are worth $1.64 at the current $0.82 rate — five cents short of the 1-ounce flat price — so most senders either add a small-denomination stamp or pay the exact amount at a Post Office counter or with a meter. There is no nonmachinable surcharge for flats; that $0.49 charge applies to letters only (see do greeting cards need extra postage).
Paying With Stamps: Exact Combinations for Common Flat Prices
If you pay with stamps rather than a meter or counter label, the useful building blocks are the Forever stamp ($0.82) and the additional-ounce stamp, sold at the $0.29 additional-ounce rate. Because the flat table rises in $0.29 steps through the middle bands, several common prices can be hit exactly:
| Flat price | Combination | Math |
|---|---|---|
| $1.69 (1 oz) | 1 Forever + 3 additional-ounce | $0.82 + (3 x $0.29) = $1.69 |
| $1.98 (2 oz) | 1 Forever + 4 additional-ounce | $0.82 + (4 x $0.29) = $1.98 |
| $2.27 (3 oz) | 1 Forever + 5 additional-ounce | $0.82 + (5 x $0.29) = $2.27 |
| $2.56 (4 oz) | 1 Forever + 6 additional-ounce | $0.82 + (6 x $0.29) = $2.56 |
Beyond four or five ounces the stamp stack gets unwieldy and the price steps stop being uniform near the top of the table, which is why regular flat mailers use a meter or counter labels. Remember the trap from the table above: two Forever stamps alone ($1.64) do not cover even the 1-ounce flat.
Worked Examples
| Piece | Classification | Price |
|---|---|---|
| 9 x 12 catalog envelope with a 12-page booklet, 3.4 oz | Flat, 4 oz band | $2.56 |
| Standard #10 business envelope, thick contract inside, 4.2 oz | Letter-size but over 3.5 oz → flat prices, 5 oz band | $2.85 |
| 10 x 13 envelope with a calendar, 8.05 oz | Flat, 9 oz band (fractions round up) | $4.02 |
| 9 x 12 envelope weighing 14 oz | Over the 13 oz First-Class limit → parcel service (e.g., USPS Ground Advantage or Priority Mail), priced by weight and distance | Varies |
One Piece, Start to Finish
Walk a single piece through all three steps. You are mailing a 24-page report in a 9 x 12 envelope.
- Classify. The envelope is 12 inches long and 9 inches high. The height exceeds the 6-1/8 inch letter maximum, so this is a flat regardless of weight — Step 1 done.
- Weigh. Sealed, with the report inside, it reads 6.3 ounces. Fractions round up, so it bills at the 7-ounce band.
- Price. The 7-ounce row of the table gives $3.43. Pay it at the counter, meter it exactly, or apply stamps: one Forever plus nine additional-ounce stamps is $0.82 + (9 x $0.29) = $3.43 exactly — correct, if inelegant.
Notice what did not enter the calculation: the destination. The same envelope to a neighboring town or across the country pays the same $3.43, because First-Class Mail prices do not vary by distance.
Three Atlanta Flats, Priced
The same three steps, run on pieces mailed from Atlanta this season:
| Piece | Classification and weight | Price |
|---|---|---|
| A Midtown architecture firm’s plan sets in a 10 x 13 envelope | Flat — height passes the letter maximum; 11.6 oz bills at the 12 oz band | $4.93 |
| A Decatur bakery’s holiday catering menus in a 9 x 12 envelope | Flat — 2.6 oz rounds up to the 3 oz band | $2.27 |
| An Atlanta nonprofit’s annual reports to donors in Savannah, in a 9 x 12 envelope | Flat — 5.4 oz bills at the 6 oz band | $3.14 |
The Savannah row is the one worth staring at. From an Atlanta desk, Savannah feels like a trip — it is most of the way across Georgia. The price table does not care: the same report mailed to a donor in Decatur, twenty minutes away, costs the same $3.14, because First-Class Mail prices are built on weight and shape only. In my years in commercial mail, this was the fact new business mailers found hardest to believe and most useful once they did: with flats, your only levers are what the piece weighs and how it is built, so the time spent lightening an insert is worth more than any time spent thinking about destinations.
Paying for Flats in Practice
Because flat prices rarely match a single stamp denomination, senders usually pay one of three ways. At a Post Office counter, the clerk weighs the piece and prints an exact-value label — the cleanest option for a handful of pieces. A postage meter or online postage account prints exact amounts at home or in the office and suits regular mailers. With stamps, combine denominations to meet or exceed the price; small overpayment is normal and harmless, underpayment is the only real error. Two mistakes account for most flat overcharges and postage-due surprises: pricing a letter-size envelope over 3.5 ounces at letter rates (it belongs in the flat table above), and forgetting that fractions of an ounce always round up to the next price band.
Cross-Check With USPS Tools
Before a large or expensive mailing, confirm your number with the USPS Price Calculator (linked from the USPS First-Class Mail page): enter the weight, shape, and dimensions and it returns the current price for every available service. Postal prices change periodically — recent years have seen changes take effect in January and during the holiday season — so a calculator check is the right habit if you are reading this months after publication. Save the result with your mailing records; if a piece is ever questioned, the dated calculator output shows the price you relied on. For how flats fit into the broader service, see USPS First-Class Mail: a practical guide, and for more postage questions, the USPS Mailing Rules hub.
Frequently Asked Questions
My flat weighs exactly 13 ounces. First-Class or parcel?
First-Class — 13 ounces is the top band of the First-Class flats table, priced at $5.24. Only a piece that goes over 13 ounces leaves First-Class flats pricing and moves to a parcel service, where price depends on weight and distance.
Is there a nonmachinable surcharge on large envelopes?
No. The $0.49 nonmachinable surcharge applies to letters only; flats are not run through the same letter-sorting process. Do not relax completely, though: a flat that is rigid, non-rectangular, or unevenly thick is not charged a surcharge — it is repriced as a parcel, which is a larger jump.
Why was my letter-size envelope charged flat prices?
Almost certainly weight. A letter-size piece over 3.5 ounces is charged large-envelope prices at its full weight even though its dimensions are letter-perfect — the worked examples in USPS letter size and weight requirements show the boundary cases. Weighing the finished piece is the only reliable way to know which table you are in.
Sources
The rates, dates, and rules in this guide were checked against these official USPS pages:
Related Guides
- USPS Mailing Rules hub — the classification system, addressing, and surcharge rules in one place.
- USPS letter size and weight requirements — the letter window and the boundaries that push a piece into this price table.
- Do greeting cards need extra postage? — the letter-side surcharge and weight rules for the cards in your stack.
- USPS First-Class Mail: a practical guide — how flats fit into the wider First-Class service.
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.