The USPS postage rate calculator estimates shipping costs by service level based on package weight and dimensions. Results are estimates — visit USPS.com for exact current rates before purchasing postage, as prices update annually.
Package Details
Estimated Rates by Service
USPS Flat Rate Boxes (always the same price)
| Box Type | Dimensions | Retail Price | Online Price |
|---|---|---|---|
| Small Flat Rate Box | 8.69"×5.44"×1.75" | $10.65 | $10.40 |
| Medium Flat Rate Box (Top) | 11.25"×8.75"×6" | $17.20 | $16.85 |
| Medium Flat Rate Box (Side) | 14"×12"×3.5" | $17.20 | $16.85 |
| Large Flat Rate Box | 12.25"×12.25"×6" | $22.35 | $21.90 |
How to Estimate USPS Postage Rates
USPS offers several service levels with different price/speed tradeoffs. First Class Mail is cheapest for light packages under 13 oz. Priority Mail is the most flexible for heavier packages. Flat rate boxes eliminate weight as a factor.
Step 1: Weigh your package
Weigh the package after packaging — the final weight including box, padding, and contents. Round up to the nearest ounce. For packages near weight thresholds (13 oz for First Class, etc.), accuracy matters.
Step 2: Choose a service level
Match speed to need. First Class: under 13 oz, 1-5 days, lowest cost. Priority Mail: any weight up to 70 lbs, 1-3 days, most popular. Priority Express: guaranteed next-day or 2-day, highest cost. Media Mail: books, DVDs only, 2-8 days, very cheap.
Save with online postage
Buying postage online through USPS.com, Stamps.com, or shipping platforms like Shippo or ShipStation is typically 4-6% cheaper than retail. For Priority Mail, online rates save $0.40-$2.00 per shipment — worth it if you ship regularly.
Frequently Asked Questions
Is this postage calculator free?
Yes, completely free with no signup required. Note that USPS rates update annually — visit USPS.com for exact current rates before purchasing postage.
What is the difference between First Class and Priority Mail?
First Class Mail handles letters and packages up to 13 oz at the lowest rates (1-5 business days). Priority Mail covers packages up to 70 lbs with 1-3 business day delivery and free packaging. Priority Mail Express guarantees next-day delivery for the highest cost. For packages over 13 oz, Priority Mail is usually the cheapest fast option.
What are flat rate boxes?
USPS flat rate boxes allow you to ship at a fixed price regardless of weight (up to 70 lbs) — as long as it fits in the box. Small flat rate box: $10.40. Medium: $16.85. Large: $21.90. If your package is heavy (over 5-10 lbs), flat rate boxes almost always save money over weight-based Priority Mail.
What is Media Mail?
Media Mail is USPS's cheapest service for books, DVDs, CDs, printed music, and educational media. Rates start around $3.92 for 1 lb and increase slowly. Delivery takes 2-8 business days and USPS may open and inspect packages. Only eligible media types qualify — non-media items sent via Media Mail can be returned or charged the difference.
How do I calculate dimensional weight?
For large, light packages, carriers charge based on dimensional weight (DIM weight) rather than actual weight. DIM weight = (Length × Width × Height) ÷ 139 for USPS. If DIM weight exceeds actual weight, you pay the DIM rate. This especially affects large, lightweight packages like pillows or air-filled packaging.