How-to · USPS prices effective July 12, 2026 · Updated August 9, 2026
How to calculate USPS postage (and shipping cost)
Direct answer — calculate postage
Letters: a Forever stamp ($0.82) covers 1 oz; add $0.29 per extra ounce up to 3.5 oz. Packages: price = service + rated weight + zone (+ extras). Rated weight is the actual weight rounded up — or L × W × H ÷ 166 if the box is over 1 cubic foot and that is higher. Example: 5 lb in a 12×10×8 box to zone 4 costs $15.80 by Ground Advantage ($11.02 online) or $17.50 by Priority Mail.
First-Class Mail® postage (letters, flats, postcards)
Same price to any U.S. address. Square, rigid or lumpy letters add $0.49.
| Piece | Weight | Postage |
|---|---|---|
| Postcard | up to 6" × 4¼" | $0.65 |
| Letter (stamped) | 1 oz | $0.82 |
| Letter (stamped) | 2 oz | $1.11 |
| Letter (stamped) | 3 oz | $1.40 |
| Letter (stamped) | 3.5 oz | $1.69 |
| Large envelope | 1 oz | $1.69 |
| Large envelope | 4 oz | $2.56 |
| Large envelope | 8 oz | $3.72 |
| Large envelope | 13 oz | $5.24 |
Package postage by weight (12×10×8 box, zone 4)
Post Office price, online label price in brackets.
| Weight | Ground Advantage | Priority Mail | Priority Mail Express |
|---|---|---|---|
| 1 lb | $10.60 ($8.15) | $11.90 ($10.40) | $43.45 ($39.05) |
| 2 lb | $13.00 ($8.51) | $14.35 ($10.79) | $49.10 ($43.91) |
| 3 lb | $13.70 ($9.67) | $15.15 ($12.68) | $54.65 ($48.77) |
| 5 lb | $15.80 ($11.02) | $17.50 ($15.72) | $65.85 ($58.54) |
| 10 lb | $19.40 ($14.44) | $21.75 ($19.62) | $101.95 ($90.89) |
| 20 lb | $27.30 ($19.66) | $31.10 ($28.29) | $159.20 ($145.80) |
How rated weight works
- Actual weight — rounded up to the next pound (Ground Advantage also has 4, 8, 12 and 15.99 oz steps).
- Dimensional weight — only for boxes over 1,728 in³: L × W × H ÷ 166, rounded up.
- Rated weight — the higher of the two. Flat Rate and Media Mail ignore dimensional weight.
Example: 12×10×8 = 960 in³ — under 1 cubic foot, so no dimensional weight. 18×14×12 = 3,024 in³ ÷ 166 → 19 lb, so a 4 lb item in that box costs $26.40 by Ground Advantage to zone 4.
When Flat Rate boxes win
Flat Rate is one price to any zone, up to 70 lb. It wins for dense, heavy items going far: 10 lb in a 12×10×8 box to zone 4 is $19.40 by Ground Advantage, while a Medium Flat Rate Box is $24.80 and a Small Flat Rate Box $13.65 (if it fits). For light packages, Ground Advantage is cheaper.
Related postage questions
Boxes over 1 cubic foot: how USPS dimensional weight works. One price to any zone: USPS Flat Rate box prices.
Use the free USPS shipping calculator
Enter weight, box size and zone to compare every service at once: USPS shipping calculator. Related: dimensional weight, shipping zone calculator.