USPS letter vs flat vs parcel: mail classification guide 2026
The exact USPS size and weight requirements for letters, flats, and parcels. Nonmachinable surcharge triggers, First-Class Mail rates by weight, Canada Post Lettermail specs, and the thresholds that can save $3–$5 per mailpiece.
One ounce can cost you $3.50
USPS mail is classified as a letter, flat, or parcel based entirely on size and weight — not on content. Each class has its own postage structure. The gap between classes is not gradual; it's a step function. Crossing a threshold by one ounce can multiply your postage cost.
| Class | Starting rate | At the max | Over the limit |
|---|---|---|---|
| Letter | $0.82 (1 oz stamp, Jul 2026) | $1.54 (3.5 oz) | Becomes a flat → $1.63+ |
| Flat (large envelope) | $1.63 (1 oz) | ~$4.03 (13 oz) | Becomes a parcel → $7+ |
| Parcel | ~$7+ (Ground Advantage) | Scales by zone/weight | Oversized surcharge at 130" girth |
The 13 oz threshold: A padded envelope at 13 oz ships as a flat for ~$4.03. The same envelope at 14 oz must ship as a parcel (USPS Ground Advantage minimum ~$7+) — a $3+ jump for a single ounce. Staying under 13 oz and 0.75 inch thick is one of the most reliably impactful cost decisions in direct mail and e-commerce packaging.
Once a piece crosses into parcel territory, Covantra compares carrier rates for it directly — no separate lookup needed to find the cheapest option. Compare parcel rates
USPS letter requirements
| Requirement | Value |
|---|---|
| Minimum length | 5 inches |
| Maximum length | 11.5 inches |
| Minimum height | 3.5 inches |
| Maximum height | 6.125 inches |
| Minimum thickness | 0.007 inch |
| Maximum thickness | 0.25 inch |
| Aspect ratio (L÷H) | 1.3 – 2.5 |
| Weight | Metered rate | Stamp rate |
|---|---|---|
| 1 oz | $0.78 (metered) | $0.82 (stamp, Jul 12+) |
| 2 oz | $1.07 (metered) | $1.11 (stamp) |
| 3 oz | $1.36 (metered) | $1.40 (stamp) |
| 3.5 oz (max) | $1.50 (metered) | $1.54 (stamp) |
| + Nonmachinable | +$0.49 | +$0.49 |
Maximum letter weight: 3.5 oz. Over 3.5 oz, the piece is a flat. Letters must be rectangular and uniformly thick. Square envelopes that meet size requirements still incur the nonmachinable surcharge if their aspect ratio falls outside 1.3–2.5.
USPS flats (large envelopes)
A flat exceeds at least one letter dimension but stays within flat maximums. The piece must be rectangular, uniformly thick (variance ≤ 0.25 inches), and flexible — it must bend at least 2 inches without damage when extended 5 inches from a surface (applies to pieces 10 inches or longer).
| Requirement | Value |
|---|---|
| Minimum length | Over 11.5 inches |
| Maximum length | 15 inches |
| Minimum height | Over 6.125 inches |
| Maximum height | 12 inches |
| Minimum thickness | Over 0.25 inch |
| Maximum thickness | 0.75 inch |
| Maximum weight | 13 ounces |
| Weight | Retail rate |
|---|---|
| 1 oz | $1.63 |
| 2 oz | $1.83 |
| 3 oz | $2.03 |
| 4 oz | $2.23 |
| 5 oz | $2.43 |
| 8 oz | $3.03 |
| 13 oz (max) | ~$4.03 |
Over 13 oz: The piece is no longer a flat — it must be mailed as a parcel. USPS Ground Advantage starts at ~$7+. If you have content that might reach 13 oz, review packaging before printing postage.
Exact classification thresholds: letter → flat → parcel
A mailpiece is classified as the lowest category whose requirements it meets. A piece becomes a flat if it exceeds any one letter dimension. It becomes a parcel if it exceeds any one flat dimension or weighs over 13 oz.
What makes a letter nonmachinable?
A nonmachinable letter cannot be processed through automated USPS mail-sorting equipment. The surcharge is $0.49 per piece, applied automatically. Unlike dimensional weight or zone pricing, this is a flat fee triggered entirely by the physical properties of the mailpiece.
Practical tip: If you can't avoid the nonmachinable surcharge, use a nonmachinable stamp ($1.27 as of 2026) rather than a standard stamp plus add-on postage. It's the same effective cost and avoids mail processing complications. Common situations: wedding invitations with wax seals, square greeting cards, anything with a coin or keyfob inside.
Canada Post Lettermail 2026
Canada Post classifies mail into standard Lettermail, non-standard/oversize Lettermail, and parcels. The threshold between Lettermail and parcels is weight (500g) and dimensions.
| Weight | Canada | USA | International |
|---|---|---|---|
| Up to 100g | $2.61 | $4.29 | $8.60 |
| 100–200g | $4.29 | $7.49 | $14.99 |
| 200–300g | $5.98 | $14.99 | $29.96 |
| 300–400g | $6.85 | $14.99 | $29.96 |
| 400–500g | $7.36 | $14.99 | $29.96 |
Non-standard size max: 380 × 270 × 20 mm (14.9" × 10.6" × 0.8")
The 500g cliff: when Canada Post Lettermail ends
Canada Post Lettermail has a hard weight ceiling of 500g and a maximum dimension of 380 × 270 × 20 mm. Exceed either, and the piece must be mailed as a parcel. The price jump is significant.
| Scenario | Lettermail rate | Parcel rate (if over) |
|---|---|---|
| 490g envelope, domestic | $7.36 (oversize) | — |
| 510g envelope, domestic | — | ~$12.20 (Regular Parcel, 0.5 kg) |
| 490g envelope, USA | $14.99 (oversize) | — |
| 510g envelope, USA | — | ~$21.54 (Expedited USA, 0.5 kg) |
The math: A 490g domestic oversize Lettermail costs $7.36. A 510g domestic parcel starts at $12.20+ — a $4.84+ jump for 20 grams. Keeping a catalog, mailer, or kit under 500g is worth real money if it can be done without compromising the content.
Frequently asked questions about mail classification
- What is the difference between a letter and a flat in USPS mail?
- A USPS letter is a rectangular mailpiece between 5–11.5 inches long, 3.5–6.125 inches high, 0.007–0.25 inch thick, and weighing up to 3.5 ounces. A flat (large envelope) exceeds at least one letter dimension: over 11.5 inches long, over 6.125 inches high, or over 0.25 inch thick — up to maximums of 15×12 inches and 0.75 inch thick, weighing up to 13 ounces. Letters start at $0.82 (2026 stamp); flats start at $1.63.
- What is a nonmachinable letter surcharge?
- USPS charges a $0.49 nonmachinable surcharge on letters that cannot be processed through automated mail-sorting equipment. Triggers include square shape (aspect ratio outside 1.3–2.5); clasps, strings, or button closures; rigid contents such as keys, coins, or USB drives; uneven thickness from enclosed items; address parallel to the short side; or material too stiff to process. Common culprits: square greeting cards, wedding invitations with wax seals, and envelopes with coins or keyfobs inside.
- When does a USPS flat become a parcel?
- A USPS flat becomes a parcel when it exceeds any flat dimension: longer than 15 inches, higher than 12 inches, thicker than 0.75 inch, or heavier than 13 ounces. When a flat becomes a parcel, postage jumps significantly — a 14-ounce padded envelope must ship via USPS Ground Advantage (starting ~$7+), compared to ~$4.03 for a 13-ounce flat.
- How much does it cost to mail a large envelope in 2026?
- A USPS large envelope (flat) starts at $1.63 retail for the first ounce, adding $0.20 per additional ounce. A 2 oz flat costs $1.83, a 5 oz flat costs $2.43, and the maximum 13 oz flat costs approximately $4.03. Commercial/metered rates are slightly lower. Keeping contents to 13 oz and within 15×12×0.75 inches keeps you in flat postage rather than parcel rates.
- Is a 9×12 envelope a flat or a parcel?
- A 9×12 inch envelope is a flat (large envelope) as long as it weighs 13 ounces or less and is no more than 0.75 inch thick. It exceeds the letter height maximum of 6.125 inches (the 12" side) and the letter length maximum of 11.5 inches (the 9" side also happens to exceed the minimum flat dimension on the other axis). First-Class Mail flat rates apply, starting at $1.63 for 1 oz.
- What are Canada Post Lettermail size and weight limits?
- Standard Canada Post Lettermail dimensions are 140–245 mm long, 90–156 mm high, and 0.18–5 mm thick, weighing up to 50g. Non-standard/oversize Lettermail can be up to 380×270×20 mm and up to 500g. At 500g (or over 380×270×20 mm), Lettermail ends — the piece must be sent as a parcel, starting at approximately $12.20 for 0.5 kg via Regular Parcel.
Need to ship a parcel? Compare carriers first.
Once your item doesn't qualify for letter or flat rates, ship. by Covantra compares USPS, UPS, FedEx, Canada Post, Purolator, and more — with known surcharges and DIM weight calculated before you print.