Flat Rate Box Sizes and Prices

USPS prints the Small Flat Rate Box at 8-5/8" x 5-3/8" x 1-5/8" inside and 8-11/16" x 5-7/16" x 1-3/4" outside ($14.65), two Medium Flat Rate Boxes ($25.80) and the Large Flat Rate Box at 12" x 11-3/4" x 5-1/2" inside and 12-1/4" x 12" x 6" outside ($36.10) in Price List Notice 123 effective October 4, 2026.

Maintained by Archie Chang · Sources checked against USPS text on November 2, 2026 · Methodology & sources

Not an official source. FedRateCalc is independent and is not affiliated with the U.S. Postal Service (USPS). USPS.com and your Post Office are the authority for prices and mailing rules. The prices here are the retail prices in USPS Price List Notice 123, effective October 4, 2026, read from an Internet Archive copy on November 2, 2026; commercial prices are listed separately in Notice 123 and are not shown. Figures marked as arithmetic are our own calculations from the table.

Flat Rate boxes by size

Flat Rate boxes sorted by inside volume, with the sizes printed in Notice 123 (volume is our arithmetic: the three inside dimensions multiplied, rounded to a whole cubic inch)
Box	Inside	Outside	Retail price	Inside volume, cubic inches
Small Flat Rate Box	8-5/8" x 5-3/8" x 1-5/8"	8-11/16" x 5-7/16" x 1-3/4"	$14.65	75
Medium Flat Rate Box 1	11" x 8-1/2" x 5-1/2"	11-1/4" x 8-3/4" x 6"	$25.80	514
Medium Flat Rate Box 2	13-5/8" x 11-7/8" x 3-3/8"	14-1/8" x 12" x 3-1/2"	$25.80	546
Large Flat Rate Box	12" x 11-3/4" x 5-1/2"	12-1/4" x 12" x 6"	$36.10	776
APO/FPO/DPO Flat Rate Box	12" x 11-3/4" x 5-1/2"	12-1/4" x 12" x 6"	$34.65	776

The table shows the printed sizes and does not say what fits in a box. When a box holds an item depends on the item, so check it against the inside size, and USPS.com and your Post Office are the authority. For the sealing rule, USPS.com says:

“When sealing a Flat Rate Envelope or Box, the container flaps must be able to close within the normal folds.”

USPS.com Priority Mail page, read November 2, 2026

Flat Rate envelopes by size

Flat Rate envelopes sorted by length times width, with the sizes printed in Notice 123
Envelope	Size as printed	Retail price	Note
Flat Rate Envelope	12-1/2" x 9-1/2" or smaller	$13.90	—
Padded Flat Rate Envelope	12-1/2" x 9-1/2"	$15.00	Packaging only available at USPS.com.
Legal Flat Rate Envelope	15" x 9-1/2"	$14.25	Packaging only available at USPS.com.

“Packaging only available at USPS.com.”

USPS Price List Notice 123, effective October 4, 2026, Flat Rate—Domestic footnote 1

For the DMM rule on which envelopes qualify:

“All USPS-produced Priority Mail envelopes smaller than the EP14F envelope (9.5 inches by 12.5 inches) are eligible for the Priority Mail Flat Rate Envelope price whether or not they are marked “Flat Rate Envelope.””

USPS Domestic Mail Manual 123, section 1.4.1

How the boxes are named

Notice 123 prints one price for the Medium Flat Rate Boxes with two sizes under it. USPS.com lists them as “Medium Flat Rate Box — 1 (top-loading)” and “Medium Flat Rate Box — 2 (side-loading)”, and DMM 123 calls them Medium Flat Rate Boxes (FRB-1) or (FRB-2). The numbers 1 and 2 in the table above match them by their inside sizes, which are the same on both pages.

When we checked on November 2, 2026, two outside sizes on the USPS.com Priority Mail page differed from Notice 123: USPS.com printed the Small Flat Rate Box as 8 1/16″ x 5 7/16″ x 1 3/4″ outside and the second Medium box as 14″ x 12″ x 3 1/2″ outside, while Notice 123 prints 8-11/16" x 5-7/16" x 1-3/4" and 14-1/8" x 12" x 3-1/2". This page shows Notice 123. Check the box in your hand or the USPS.com page before you rely on an outside size.

“Priority Mail Flat Rate Boxes are as follows:
Small Flat Rate Box to domestic, APO/FPO, and DPO destinations.
Medium Flat Rate Boxes (FRB-1) or (FRB-2) to domestic, APO/FPO, and DPO destinations.
Large Flat Rate Box to domestic destinations.
Large Flat Rate Box and “special version of this box” identified with the additional logo: “Americasupportsyou.mil.” to APO/FPO and DPO destinations. This box is priced less to mail than the conventional domestic Large Flat Rate Box. If the special version of the APO/FPO Flat Rate Box is used for non-APO/FPO and DPO destination addresses, domestic or international Large Flat Rate Box prices will apply.”

USPS Domestic Mail Manual 123, section 1.4.2

Other Flat Rate items on USPS.com

Also listed on the USPS.com Priority Mail page, with the price and sizes as USPS.com prints them (not in the Notice 123 Flat Rate table)
Item	Price on USPS.com	Size as printed
Window Flat Rate Envelope	$13.90 at Post Office & Online	10″ x 5″ 12 1/2″ x 9 1/2″
Small Flat Rate Envelope	$13.90 at Post Office	10″ x 6″
Large Flat Rate Board Game Box	$36.10 at Post Office	Outside: 24 1/16″ x 11 7/8″ x 3 1/8″ Inside: 23 11/16″ x 11 3/4″ x 3″

Prices for the Flat Rate envelopes and boxes are on the Priority Mail Flat Rate prices page.

Frequently asked questions

What size is the Small Flat Rate Box?

Notice 123 prints 8-5/8" x 5-3/8" x 1-5/8" inside and 8-11/16" x 5-7/16" x 1-3/4" outside, at a retail price of $14.65.

What are the Medium Flat Rate Box sizes?

Notice 123 prints two sizes under one price of $25.80: 11" x 8-1/2" x 5-1/2" inside (11-1/4" x 8-3/4" x 6" outside) and 13-5/8" x 11-7/8" x 3-3/8" inside (14-1/8" x 12" x 3-1/2" outside). DMM 123 calls them Medium Flat Rate Boxes (FRB-1) or (FRB-2).

What is the size of the Large Flat Rate Box?

Notice 123 prints 12" x 11-3/4" x 5-1/2" inside and 12-1/4" x 12" x 6" outside, at a retail price of $36.10.

What size is the APO/FPO/DPO Flat Rate Box?

Notice 123 prints the same sizes as the Large Flat Rate Box, 12" x 11-3/4" x 5-1/2" inside and 12-1/4" x 12" x 6" outside, at a retail price of $34.65.

What sizes are the Flat Rate envelopes?

Notice 123 prints 12-1/2" x 9-1/2" or smaller for the Flat Rate Envelope ($13.90), 15" x 9-1/2" for the Legal Flat Rate Envelope ($14.25) and 12-1/2" x 9-1/2" for the Padded Flat Rate Envelope ($15.00).

Do the flaps have to close?

USPS.com says “When sealing a Flat Rate Envelope or Box, the container flaps must be able to close within the normal folds.”