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When shipping packages via the United States Postal Service (USPS), Flat Rate Boxes offer a convenient option. They provide a single, flat fee regardless of package weight (up to 70 lbs) or destination within the United States.
What Are USPS Flat Rate Boxes?
USPS Flat Rate Boxes are part of the Priority Mail service. You pay a fixed rate based on the box type you choose rather than by distance or exact weight (as long as it’s under 70 lbs). This makes shipping costs predictable and often more economical for heavier items.
Key Features:
- A single, flat fee for each box size
- Free USPS-branded boxes and envelopes
- Priority Mail delivery in 1–3 business days
- Tracking and basic insurance included
Why Choose USPS Flat Rate Shipping?
Shipping with USPS Flat Rate offers several advantages:
- Predictable Costs: You know exactly how much you’ll pay regardless of destination or weight (up to 70 lbs).
- Ease of Use: No need to weigh your parcel (other than ensuring it doesn’t exceed 70 lbs), and packaging materials are free.
- Fast Delivery: Flat Rate Boxes ship via Priority Mail, with most packages arriving in 1–3 business days.
- Business Benefits: E-commerce sellers can simplify shipping costs and access commercial rate discounts by printing labels online.
Types of USPS Flat Rate Boxes
USPS offers several Flat Rate Box options to accommodate different package sizes and shapes:
Small Flat Rate Box
Best For: Small items like jewelry, small electronics, or other compact products.
Medium Flat Rate Box (Top-Loading)
Best For: Bulkier items, clothing, shoes, or multiple smaller items consolidated into one box.
Medium Flat Rate Box (Side-Loading)
Best For: Wider or longer items that might not fit neatly into a top-loading box.
Large Flat Rate Box
Best For: Large or heavier items, multiple items packed together, or shipping to military bases overseas (with special APO/FPO/DPO discount).
Other Flat Rate Options
Flat Rate Envelopes: Several variations exist (standard, padded, legal). Perfect for documents, smaller apparel items, or objects that can be safely padded.
USPS Flat Rate Box Dimensions
Understanding the exact measurements of each Flat Rate Box helps determine what can fit into each size.
Small Flat Rate Box Dimensions
- External Dimensions: 8 5/8″ x 5 3/8″ x 1 5/8″
- Internal Dimensions (Approx.): 8 5/8″ x 5 3/8″ x 1 5/8″
- Shape & Appearance: Rectangular, shallow depth
- Best Uses: DVDs, video games, small electronics accessories, or collectible items
Medium Flat Rate Box Dimensions (Top-Loading)
- External Dimensions: 11 1/4″ x 8 3/4″ x 6″
- Internal Dimensions (Approx.): 11″ x 8 1/2″ x 5 1/2″
- Shape & Appearance: Top-loading, with flaps opening on the top
- Best Uses: Clothes, toys, small shipments of bulk items
Medium Flat Rate Box Dimensions (Side-Loading)
- External Dimensions: 14″ x 12″ x 3 1/2″
- Internal Dimensions (Approx.): 13 5/8″ x 11 7/8″ x 3 3/8″
- Shape & Appearance: Side-loading, wider, more rectangular shape
- Best Uses: Books, board games, or flat items that need more length than height
Large Flat Rate Box Dimensions
- External Dimensions: 12 1/4″ x 12 1/4″ x 6″
- Internal Dimensions (Approx.): 12″ x 12″ x 5 1/2″
- Alternate Large Box (Game Board Version): 24 1/16″ x 11 7/8″ x 3 1/8″
- Best Uses: Bulkier items, multiple items shipped together, large gift boxes, or board games for the longer version
Flat Rate Envelopes Dimensions
- Standard Envelope: 12 1/2″ x 9 1/2″
- Padded Envelope: 12 1/2″ x 9 1/2″ (with bubble padding)
- Legal Envelope: 15″ x 9 1/2″
Tips for Fitting Items Efficiently
- Measure Before You Pack: If your item is close to the outer dimensions of a box, measure carefully—especially if your shipment has awkward corners or angles.
- Use Space Wisely: Fill gaps with bubble wrap or packing paper to prevent items from shifting. For non-fragile goods, you can use clothing or other soft items as padding.
- Consider Orientation: Sometimes rotating an item or removing retail packaging can help it fit in a smaller box.
- Mind Fragility: Even if your item fits, leave enough space for adequate cushioning if it’s breakable.
Weight Limits and Restrictions
A major advantage of USPS Flat Rate Boxes is shipping any weight up to 70 lbs for the same price. However, keep these points in mind:
- You cannot exceed 70 lbs
- Packages must close securely without altering the box shape
- Over-stuffing can cause the box to bulge, which may be classified as “modified” and affect pricing
- Hazardous materials or restricted goods still follow USPS regulations regardless of shipping service
How Much Do Flat Rate Boxes Cost?
Flat Rate Box pricing changes periodically. For exact costs, check the USPS Price List or use the USPS Postage Calculator. General price ranges (as of 2023-2025):
- Small Flat Rate Box: Typically $10–$11 at retail, less with online/commercial pricing
- Medium Flat Rate Box: Usually $16–$18 at retail, often cheaper with online or commercial base pricing
- Large Flat Rate Box: Generally $20–$23 at retail; special rates for APO/FPO/DPO addresses
Pro Tip: Print labels through the USPS website or third-party shipping software (PayPal, ShipStation, or Pirate Ship) to access Commercial Base rates, which can save 10–30% compared to retail prices.
When to Use — and When Not to Use — Flat Rate Boxes
When to Use:
- Heavy Items: If your shipment is dense or close to 70 lbs, Flat Rate often provides major savings compared to zone-based rates.
- Distant Zones: Sending a package from East Coast to West Coast (or vice versa)? Flat Rate can be cheaper if your item is moderately heavy.
- Regular/Established Size Items: If your products consistently fit in a Medium or Large Flat Rate Box, it simplifies your shipping operations.
When NOT to Use:
- Lightweight Shipments: If your item weighs less than 1–2 lbs and you’re not shipping it too far, other USPS services (Priority Mail Weight & Zone-Based or First-Class Package if under 1 lb) may be cheaper.
- Oddly Shaped/Large Items: If the item won’t fit comfortably in any Flat Rate Box, you may need a larger, non-Flat Rate box.
- Very Short Distances: If you’re shipping locally (within the same region), zone-based Priority Mail rates can be lower than a Flat Rate Box cost.
FAQ: USPS Flat Rate Box Dimensions
Can I modify a Flat Rate Box to fit my item better?
No. Altering a Flat Rate Box (cutting or reshaping) can invalidate the Flat Rate pricing, and USPS may charge you based on weight and zone instead.
How strict is USPS about bulging boxes?
USPS requires the box to close normally (without expansions or duct-taping extended flaps). Overstuffed boxes can be flagged and reclassified.
Do all Flat Rate Boxes use the same Priority Mail shipping label?
Each type (Small, Medium, Large) has a specific label that you select when purchasing postage. Ensure you pick the correct packaging type at checkout.
Are there additional costs for Flat Rate Boxes?
The listed Flat Rate price generally includes postage, tracking, and basic insurance up to $100 (commercial) or $50 (retail). Additional services, like Signature Confirmation or more insurance, cost extra.
Can I reuse a Flat Rate Box?
Yes, if it’s still in good shape. But if it’s marked as a Flat Rate Box, it must be used for Flat Rate shipping only. You can’t use a pre-branded USPS box for other shipping services.
USPS Flat Rate Boxes provide a dependable, easy-to-use shipping solution for many mailing needs. Their consistent pricing structure and variety of box sizes help simplify your logistics, whether you’re mailing personal items, running a small business, or sending care packages to loved ones.
By knowing the exact dimensions of each Flat Rate Box, you can avoid overstuffing or buying the wrong shipping materials. With proper planning, USPS Flat Rate shipping can save you both time and money—especially for heavier, long-distance packages.
Our articles make government information more accessible. Please consult a qualified professional for financial, legal, or health advice specific to your circumstances.
