There are five standardized PO box sizes. They’re designated sizes 1 through 5, with 1 being the smallest and 5 the largest.
Size 5 isn’t pictured, but it’s roughly twice as wide as size 4.
PO Box Sizes Overview
PO box sizes are standardized across the entire Post Office. Below are the actual dimensions of USPS PO boxes as well as estimated capacities.
| Size | Width | Height | Depth | Capacity |
| 1 (XS) | 3″ | 5.5″ | 14.75″ | 10-15 standard envelopes or 2 magazines |
| 2 (S) | 5″ | 5.5″ | 14.75″ | 10-15 letters or 5 magazines |
| 3 (M) | 11″ | 5.5″ | 14.75″ | Many large envelopes and magazines stacked flat |
| 4 (L) | 11″ | 11″ | 14.75″ | 2 shoeboxes with room for 10-15 letters |
| 5 (XL) | 22.5″ | 12″ | 14.75″ | Multiple packages, letters, and magazines |
The large PO box sizes are considerably more expensive, so if you want to save some money pick the smallest size you think you’ll need. Learn more in our article on PO box costs.
Availability of Larger Sizes
Many post offices do not offer the large or extra large PO box sizes (sizes 4 and 5).
When using the PO Boxes Online rate tool, you can search search all post offices within 50 mile radius. Depending on your location, you may be able to find an alternate post office that offers the larger sizes.
If you are unable to find a large PO box in your area, you may want to consider a UPS mailbox. Click here to read our article on UPS mailboxes.
What happens if my box fills up?
If your PO box overflows, postal workers will usually store the extra mail in back and leave a notice in your box. However, “repeat offenders” may be asked to
According to the D910 Post Office Box Service Guidelines:
When mail for a customer’s post office box(es) exceeds the capacity of the box(es) on 12 of any 20 consecutive business days (excluding Saturdays, Sundays, and national holidays), the customer must use caller service, change to a larger box, or use one or more additional boxes (subject to availability) to which mail will be addressed.
How are large packages handled?
If a package is too large to fit in a PO box, one of two things will happen.
- A postal worker will leave a notice in the box. Simply bring the slip to the front window and a worker will retrieve your package.
- Your mailbox will contain a key to a larger storage locker containing your package.
Please note that the Post Office doesn’t normally accept packages from third-party carriers like UPS or FedEx (see below).
UPS and FedEx Shipping Limitations
It’s important to know that even if you have a large PO box, non-USPS carriers such as UPS, FedEx, and DHL will usually not deliver to a USPS post office box. For the most part, these services will only deliver to a physical home or business address.
In other words, a PO box is for USPS mail only. One exception is shipments sent through FedEx SmartPost, which works directly with the Post Office and is able to PO boxes.