The United States Postal Service (USPS) imposes size limits based on mail class, domestic versus international service, and whether you use USPS-provided packaging (Flat Rate) or your own boxes. Below is a comprehensive overview of the maximum box dimensions you can ship with USPS, organized by service category and highlighting the largest permissible package for each scenario.
1. Key Takeaway
For domestic shipments, the absolute maximum box size is governed by a combined length plus girth of 108 inches (130” for Ground Advantage retail) with a 70 lb weight limit. For international shipments, non-Flat-Rate packages similarly cannot exceed 108” length plus girth (79” for Priority Express International), and First-Class Package International Service is further restricted to 24” maximum length and 36” combined length + width + height.
2. Domestic Package Size Limits
2.1. Length + Girth Rule
- Standard maximum for most domestic parcels:
- Length (L) + Girth (2 × Height + 2 × Width) ≤ 108 inches
- USPS Ground Advantage (Retail) may measure up to 130 inches length + girth, but incurs oversized pricing.
2.2. Weight Limit
- All domestic mail classes: maximum weight 70 lb per mailpiece.
2.3. Examples of Oversized Parcel Dimensions
| Description | Length | Width | Height | Length + Girth | Service |
|---|---|---|---|---|---|
| Absolute maximum (Priority, Ground) | 42” | 18” | 24” | 42 + (2×18 + 2×24) = 42 + 84 = 126” (exceeds 108”) → Not allowed | – |
| Maximum allowed | 30” | 20” | 14” | 30 + (2×20 + 2×14) = 30 + 68 = 98” | Priority Mail, Ground |
| Ground Advantage retail max | 40” | 24” | 13” | 40 + (2×24 + 2×13) = 40 + 74 = 114” (under 130”) | Ground Advantage Retail |
3. Domestic Flat-Rate Boxes
USPS provides a variety of Flat-Rate boxes with specific dimensions; weight limit is 70 lb, but dimensions must match USPS-provided packaging exactly. The largest Flat-Rate box is:
- Large Flat-Rate Box: 12 1/4″ × 12″ × 6″ (Top-loading) or 11 7/8″ × 3 3/8″ × 13 5/8″ (Side-loading); weight up to 70 lb.
4. International Package Size Limits
4.1. First-Class Package International Service
- Maximum length: 24 inches
- Combined length + width + height: ≤ 36 inches
- Maximum weight: 4 lb (64 oz).
4.2. Priority Mail International (by weight)
- Combined length + girth: ≤ 108 inches
- Maximum weight: 70 lb (1120 oz).
4.3. Priority Mail International Flat-Rate Boxes
- Small Flat-Rate Box: any dimension within USPS-provided envelope/box
- Medium & Large Flat-Rate Boxes: maximum weight 20 lb; dimensions fixed by USPS packaging.
4.4. Priority Mail Express International
- Combined length + girth: ≤ 79 inches
- Maximum weight: 70 lb.
5. Measuring Length and Girth
- Length (L): the longest side of the package.
- Girth: measured around the thickest part, perpendicular to length: $ Girth = 2 (Width + Height) $
- Length + Girth: must not exceed service-specific maximum (108”, 130”, or 79”).
6. Choosing the Largest Box You Can Ship
- Domestic (own box):
- Up to 108” L + Girth; up to 70 lb.
- Retail Ground Advantage allows up to 130” L + Girth at oversized rates.
- International (own box):
- For most services (Priority Mail International): up to 108” L + Girth; up to 70 lb.
- For First-Class Package International: maximum 24” length, 36” total L + W + H; up to 4 lb.
- For Priority Mail Express International: up to 79” L + Girth; up to 70 lb.
7. Practical Recommendations
- When shipping large, heavy items domestically, use Priority Mail or Ground Advantage with your own packaging, ensuring L + Girth ≤ 108” (or 130” for retail ground).
- For international shipments, if your package exceeds 24” in length or 36” in combined dimensions, select Priority Mail International or Priority Mail Express International rather than First-Class.
- Consider USPS Flat-Rate packaging if weight is high but dimensions fit the provided box; Flat Rate can be more cost-effective despite dimensional constraints.
- Always measure twice-length, width, height-and calculate girth to avoid surcharges or rejection.
This comprehensive guide enables selection of the largest permissible USPS box by service type, ensuring compliance with USPS size and weight regulations for both domestic and international shipments.
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