USPS Flat Rate Box Sizes: Dimensions & Fit Guide
USPS Priority Mail Flat Rate boxes come in Small, Medium, and Large sizes, with two different Medium box shapes. Choosing the right one starts with its inside dimensions: your item and protective packaging must fit without forcing the box closed.
Use the USPS Flat Rate box size chart below to compare dimensions in inches and centimeters. This guide covers U.S. domestic shipping.
USPS Flat Rate box size chart
Measurements follow USPS Postal Explorer. Centimeter conversions are approximate.
| Box | Inside dimensions (inches) | Outside dimensions (inches) | Inside dimensions (cm) | Outside dimensions (cm) |
|---|---|---|---|---|
| Small | 8⅝ × 5⅜ × 1⅝ | 8¹¹⁄₁₆ × 5⁷⁄₁₆ × 1¾ | 21.9 × 13.7 × 4.1 | 22.1 × 13.8 × 4.4 |
| Medium, top-loading | 11 × 8½ × 5½ | 11¼ × 8¾ × 6 | 27.9 × 21.6 × 14.0 | 28.6 × 22.2 × 15.2 |
| Medium, side-loading | 13⅝ × 11⅞ × 3⅜ | 14⅛ × 12 × 3½ | 34.6 × 30.2 × 8.6 | 35.9 × 30.5 × 8.9 |
| Large | 12 × 11¾ × 5½ | 12¼ × 12 × 6 | 30.5 × 29.8 × 14.0 | 31.1 × 30.5 × 15.2 |
Source: USPS Flat Rate Quick Reference.
USPS’s Priority Mail overview lists the side-loading Medium box’s outside length as 14 inches, while Postal Explorer and the USPS store list 14⅛ inches. The inside measurements agree. Measure your actual box if you need its precise exterior size.
Which USPS Flat Rate box fits your item?
Small Flat Rate box dimensions
The Small box measures 8⅝ × 5⅜ × 1⅝ inches inside. Its shallow depth is the main limitation: protective wrapping can make an otherwise small item too thick.
Medium Flat Rate box dimensions: two shapes
The Medium box comes in two configurations:
- Top-loading: 11 × 8½ × 5½ inches inside.
- Side-loading: 13⅝ × 11⅞ × 3⅜ inches inside.
The top-loading version offers more depth. The side-loading version provides a longer, wider space for flatter items. Both have the same Flat Rate postage when purchased through the same pricing channel.
Large Flat Rate box dimensions
The Large box measures 12 × 11¾ × 5½ inches inside. It provides a larger footprint than the top-loading Medium, but the same interior depth.
Small, Medium, and Large boxes have different postage prices. Empty space within a selected box does not increase its Flat Rate postage. Check USPS’s current prices and compare other eligible shipping options before choosing.
How to measure an item for a shipping box
Measure the item at its widest points, including handles, protrusions, and any packaging that will remain around it.
For equal cushioning on opposite sides:
Required inside dimension = item dimension + twice the cushioning thickness
Apply this calculation separately to length, width, and height. If different sides need different protection, add each side’s actual allowance instead.
Compare your totals with the box’s inside dimensions. If the item can safely change orientation, try swapping its length, width, and height. Leave enough clearance to insert it without compressing necessary cushioning or forcing the flaps closed.
Example: Will a small accessory fit?
Suppose a non-fragile accessory measures 6 × 3 × ½ inches. With an illustrative ½ inch of protective material on every side, it needs:
- Length: 6 + ½ + ½ = 7 inches
- Width: 3 + ½ + ½ = 4 inches
- Height: ½ + ½ + ½ = 1½ inches
Those dimensions fit within the Small Flat Rate box. However, only ⅛ inch of depth remains, so check the actual packed item.
The allowance demonstrates the arithmetic; it is not a protection recommendation for every accessory.
Example: Which Medium Flat Rate box fits a book?
A book measuring 8 × 5½ × 1½ inches, with an illustrative 1 inch of cushioning on every side, needs 10 × 7½ × 3½ inches of inside space.
It will not fit in the Small box. The top-loading Medium fits, but the side-loading Medium does not: its 3⅜-inch depth is less than the padded book’s shortest dimension. Swapping axes will not solve that problem.
A generic box may also work. Compare postage before deciding.
Check protection as well as fit
A mathematical fit does not establish adequate protection. Choose cushioning and box strength for the item’s weight, fragility, and shape, and prevent movement inside the package.
Fragile items may require more cushioning or double-boxing. Follow item-specific packing guidance rather than copying an example’s allowance. FedEx’s breakables guide provides one reference for fragile-item protection.
For another resource while planning your packaging, visit Box Size Fit. Check any suggested dimensions against your actual item, protective materials, and box.
USPS Flat Rate packaging rules
Flat Rate postage requires the corresponding official USPS Flat Rate packaging. A Priority Mail-branded box is not automatically a Flat Rate box.
The flaps must close along their normal folds. You may reinforce seams with tape, but cannot enlarge or reconstruct the box to fit the contents. The standard domestic weight limit is 70 pounds; destination-specific restrictions can also apply. See USPS’s official rules.
If you compare Flat Rate with shipping in your own box, measure the finished parcel’s exterior and weigh the complete package. Other services may price by weight, destination, dimensions, or a combination.
Frequently asked questions
Are USPS Flat Rate boxes free?
The packaging is free; postage is separate. Order supplies through the USPS store or check availability at a Post Office.
Is the smallest Flat Rate box always the cheapest option?
No. It may be the least expensive suitable Flat Rate box, but another eligible service using your own packaging may cost less. Compare options for your actual shipment.
Are USPS box dimensions inside or outside measurements?
USPS publishes both for these Flat Rate boxes. Use inside measurements to check fit. Outside measurements describe the exterior parcel. Always check which measurements a listing provides.
How much padding should I add?
Use protection suited to your item. There is no single allowance that works for everything. The examples above explain the calculation, not universal packing requirements.