How-to · Planning model · Updated August 9, 2026
How to calculate USPS postage (and shipping cost)
Direct answer — calculate postage
To calculate postage with USPS: letters use First-Class Mail® ounce prices (Forever stamp $0.82 for 1 oz as of July 2026 retail; +$0.29 each added ounce up to 3.5 oz; postcards $0.65). Packages: postage ≈ service + billable weight + zone (+ extras). Billable weight is usually the higher of actual pounds or dimensional weight (L × W × H) ÷ 166. Planning example for a 12×10×8 in box, zone 4: Priority Mail at 5 lb ≈ $13.54; Ground Advantage at 5 lb ≈ $7.95. Always confirm on usps.com.
First-Class Mail® postage (letters / postcards)
Retail First-Class Mail® stamped letters / postcards — July 2026 price-file style (Forever stamp $0.82). Confirm on usps.com.
| Piece | Weight | Postage |
|---|---|---|
| Postcard (single) | Card rate | $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 |
| Letter (metered) | 1 oz | $0.78 |
| Nonmachinable letter surcharge | Add-on | $0.49 |
Priority Mail® postage by weight (zone 4)
Custom box 12×10×8 in. Planning model — not Postal Explorer.
| Scale weight | Billable | Est. postage |
|---|---|---|
| 1 lb | 6 lb | $13.54 |
| 2 lb | 6 lb | $13.54 |
| 3 lb | 6 lb | $13.54 |
| 5 lb | 6 lb | $13.54 |
| 10 lb | 10 lb | $17.27 |
| 20 lb | 20 lb | $26.59 |
USPS postage by service (zone 4)
Custom box 12×10×8 in unless noted. DIM divisor 166 in this model. Not official postage.
| Service / package | Weight | Billable (model) | Est. cost |
|---|---|---|---|
| Priority Mail (custom) | 5 lb | 6 lb | $13.54 |
| Priority Mail (custom) | 10 lb | 10 lb | $17.27 |
| Ground Advantage (custom) | 5 lb | 6 lb | $7.95 |
| Priority Small Flat Rate box | 5 lb example | Flat rate | $9.75 |
| Priority Medium Flat Rate box | 20 lb example | Flat rate | $16.25 |
How billable weight works
- Actual weight — scale pounds, usually rounded up for postage.
- Dimensional weight — (L × W × H) ÷ 166 for many USPS retail parcels.
- Billable — typically the higher of the two (unless Flat Rate packaging).
Example: 12×10×8 → 960 ÷ 166 ≈ 6 lb DIM in this model. If your scale weight is lower, DIM can still set the price.
USPS shipping boxes tip
Free Priority Mail Flat Rate boxes win when the flat price undercuts weight-rated Priority for your zone and weight. Model Small Flat Rate ≈ $9.75; Medium Flat Rate ≈ $16.25 in this guide's bands — still confirm current retail charts.
Use the free USPS postage calculator
Adjust dimensions, zone, service, and extras: USPS postage & shipping cost calculator. Related: dimensional weight, freight cost estimator.