USPS Box Sizes: Free Priority Mail & Flat Rate
USPS offers free boxes for Priority Mail and Flat Rate shipping. Know your options to maximize savings.
What free box sizes does USPS offer?
USPS free boxes: Flat Rate Small (8.69x5.44x1.75"), Medium (11x8.5x5.5" or 14x12x3.5"), Large (12x12x5.5"). Priority Mail boxes in various sizes for weight-based shipping. Order free at usps.com/shop.
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| Carrier | Service | Price | Delivery Time | |
|---|---|---|---|---|
| USPS | Small Flat Rate | Free box / $10.40 | 8.69x5.44x1.75" | |
| USPS | Medium Flat Rate | Free box / $17.10 | 11x8.5x5.5" | |
| USPS | Large Flat Rate | Free box / $22.45 | 12x12x5.5" | |
| USPS | Priority Mail Box | Free box | Weight-based pricing |
Small Flat Rate
Medium Flat Rate
Large Flat Rate
Priority Mail Box
* Prices shown are discounted ShipWave rates. Actual rates vary by exact dimensions, weight, and origin/destination.
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Flat Rate Box Sizes
Small: 8.69" x 5.44" x 1.75" - $10.40 - Best for small, heavy items.
Medium: 11" x 8.5" x 5.5" (top-loading) or 14" x 12" x 3.5" (side-loading) - $17.10
Large: 12" x 12" x 5.5" - $22.45 - Up to 70 lbs.
Flat Rate is same price regardless of weight—great for heavy items.
When to Use Flat Rate vs Priority Mail
Use Flat Rate: Heavy items, long distances (Zone 5+), predictable pricing needs.
Use Priority Mail (weight-based): Lightweight items, short distances, when your item doesn't fit Flat Rate boxes well.
Always compare rates—neither is always cheapest.
Pro Tips to Save More
Order Free Boxes Online
Get boxes delivered free at usps.com/shop. No need to visit the post office.
Compare Before Shipping
Flat Rate isn't always cheapest—especially for lightweight packages.
Don't Waste Flat Rate Boxes
Flat Rate boxes must use Flat Rate service. Don't use them for Ground Advantage.
Frequently Asked Questions
- No. Flat Rate boxes can ONLY be used with Priority Mail Flat Rate service. Using them for other services may result in additional charges.
- All Flat Rate boxes have a 70 lb limit. Practically, you'll rarely fit 70 lbs in Small or Medium boxes.
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Related Shipping Guides
Flat rate shipping charges one price regardless of weight - perfect for heavy, dense items. Compare all flat rate options and learn when to use them.
Shipping Box Size Guide: Choose the Right BoxSelecting the right box size saves money and protects items. Use our guide to find the perfect fit.
Shipping Box Sizes: Complete GuideChoosing the right box size affects shipping cost and product safety. Understand standard sizes.
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