Medium flat rate box dimensions
The Priority Mail Medium Flat Rate Box (Side-Loading) measures 13-5/8" x 11-7/8" x 3-3/8" on the inside (14-1/8" x 12" x 3-1/2" outside). In centimeters that is about 34.6 × 30.2 × 8.6 cm. Its volume is about 546 cubic inches. USPS also makes a medium box (top-loading) at 11" x 8-1/2" x 5-1/2" for the same price — pick the shape that matches your item.
Medium flat rate box price
| Where you buy postage | Price |
|---|---|
| Online label (USPS.com, shipping apps) | $22.02 |
| Post Office counter | $25.80 |
The price is the same to every address in the 50 states, whether it weighs 2 ounces or 70 pounds. Buying the label online saves $3.78 per package. These are the prices in USPS Notice 123, effective October 4, 2026; USPS updates them each January and July, and the packaging itself is free.
Medium flat rate box shipping cost
Here is what a parcel of the same size would cost by USPS Ground Advantage (online label) at different weights and distances. Green cells are where the Medium Box (Side-Loading) at $22.02 is cheaper.
| Weight | Ground, zone 2 | Ground, zone 5 | Ground, zone 8 |
|---|---|---|---|
| 8 oz | $7.34 | $8.24 | $8.95 |
| 1 lb | $8.08 | $9.29 | $11.22 |
| 2 lb | $8.48 | $10.50 | $13.42 |
| 5 lb | $10.41 | $14.53 | $20.24 |
| 10 lb | $12.91 | $17.81 | $26.39 |
| 20 lb | $18.11 | $26.68 | $42.14 |
Going cross-country (zone 8), the Medium Box (Side-Loading) beats Ground Advantage from about 10 lb. For a nearby address (zone 2), Ground Advantage is cheaper at every weight in the table, so flat rate only pays off for longer trips or when you want 1–3 day delivery. The calculator above does this comparison for your exact item and zone.
What fits in a medium box?
Typical things people send in it: a 13-inch laptop, folded jeans and hoodies, board games, picture frames.
From our presets, these fit: hoodie (11 × 9 × 2.5 in), laptop (13 × 9.5 × 1.5 in), phone (6.5 × 3.5 × 1.5 in), video game (6.75 × 5.25 × 0.5 in).
Too big for it: shoebox (no flat rate option — use Ground Advantage); 3 books (fits the Medium Box (Top-Loading)).
The item must fit with the flaps closing normally — USPS does not allow taping a flat rate box wider or taller to squeeze more in.
Frequently asked questions
How much is a medium box (side-loading) in 2026?
$22.02 with an online label and $25.80 at the Post Office counter (USPS Notice 123, effective October 4, 2026).
Is there a weight limit for the medium box (side-loading)?
Yes, 70 lb — the same limit as all Priority Mail. Up to that, weight doesn't change the price.
Where can I get a medium box (side-loading)?
Free at any Post Office or ordered free from the USPS online store. You must use the official USPS Flat Rate packaging to get the flat price.
How long does a medium box (side-loading) take to arrive?
It ships as Priority Mail, which USPS schedules for 1–3 business days. Delivery times aren't guaranteed.