USPS Flat Rate Box Sizes and Rate Guide
Direct answer: USPS Priority Mail Flat Rate boxes come in Small, two Medium formats, Large, and Large Board Game sizes. Flat Rate is best when the official USPS box fits the product tightly and the current USPS rate beats a normal weight-zone or dimensional-weight shipment. If fit, protection, branding, or carton control matters more, use your own Packrift box and price the shipment normally.
Which flat rate box is cheapest: the Small Flat Rate Box always carries the lowest flat rate price, followed by the two Medium formats at the same mid-tier rate, with the Large and Board Game boxes at the top of the range. If your item fits the Small box, that is the cheapest flat rate path; confirm the current rate on the official USPS chart before buying postage.
USPS Flat Rate Box Sizes Chart
Use this chart for size planning, then confirm current USPS rates and mailing rules on the official USPS Priority Mail page before buying postage.
| USPS Flat Rate box | Outside dimensions | Inside dimensions | Best fit | Packrift own-box path |
|---|---|---|---|---|
| Small Flat Rate Box | 8 11/16 in x 5 7/16 in x 1 3/4 in | 8 5/8 in x 5 3/8 in x 1 5/8 in | compact, dense items such as small electronics, parts, samples, and documents. | box size calculator |
| Medium Flat Rate Box 1, top-loading | 11 1/4 in x 8 3/4 in x 6 in | 11 in x 8 1/2 in x 5 1/2 in | office supplies, shoes, and small mixed-item shipments that need depth. | boxes and mailers |
| Medium Flat Rate Box 2, side-loading | 14 in x 12 in x 3 1/2 in | 13 5/8 in x 11 7/8 in x 3 3/8 in | flatter items, folded apparel, books, supplies, and side-loading packouts. | corrugated boxes |
| Large Flat Rate Box | 12 1/4 in x 12 in x 6 in | 12 in x 11 3/4 in x 5 1/2 in | larger dense orders, gifts, computer accessories, and heavier ecommerce shipments. | dimensional weight calculator |
| Large Flat Rate Board Game Box | 24 1/16 in x 11 7/8 in x 3 1/8 in | 23 11/16 in x 11 3/4 in x 3 in | wide, shallow items such as board games, flat kits, frames, and long retail boxes. | vinyl record box sizing guide |
How to Use the Rate Part of the Decision
- Check the current USPS rate for the exact Flat Rate package you plan to use; Packrift does not republish live USPS postage tables.
- Compare the Flat Rate option against a normal own-box shipment when the order is light, bulky, or close to a dimensional-weight threshold.
- Use the Packrift dimensional weight calculator when your own box is larger than one cubic foot.
- Use the box size calculator when you know the product dimensions but not the carton size.
- Use USPS postage tools for live postage before shipping.
When Flat Rate Wins vs When Your Own Box Wins
| Shipping situation | Start with | Why |
|---|---|---|
| Dense item that fits with little wasted space | USPS Flat Rate box | The official box can simplify postage when the fit is clean and the current USPS rate is favorable. |
| Light but bulky ecommerce order | DIM-weight check | Dimensional weight can change the economics of using your own carton. |
| Product needs better fit, protection, or neutral packaging | Packrift corrugated boxes | Your own box gives better size control and avoids forcing the item into an official Flat Rate carton. |
| Recurring warehouse replenishment | Bulk quote | Buying repeat carton sizes is usually a procurement decision, not a one-off postage decision. |
Packrift Paths After You Measure
- Boxes and mailers for general ecommerce shipping cartons and mailer options.
- Corrugated boxes for own-box shipments that need size control.
- How to measure a box for shipping if the length, width, and height inputs are unclear.
- Box size calculator for translating item dimensions into carton dimensions.
- Reorder packaging by SKU when your team already knows the box it needs.
FAQ
What are the USPS Priority Mail Flat Rate box sizes?
The main USPS Priority Mail Flat Rate boxes are Small, two Medium box formats, Large, and Large Board Game. Use the outside and inside dimensions in the chart to decide whether the official USPS box fits before checking the current USPS rate.
Are USPS Flat Rate box prices based on weight?
USPS Flat Rate pricing uses the official USPS Flat Rate packaging size rather than zone-by-zone package dimensions, as long as the shipment follows USPS Flat Rate rules and weight limits. Always confirm the current rate on USPS before shipping.
Can I use my own Packrift box with a USPS Flat Rate label?
No. USPS Flat Rate labels require USPS Flat Rate packaging. Use Packrift boxes when you want your own box size, stronger fit control, branded or neutral packaging, or a normal Priority Mail rate based on weight, zone, and dimensions.
When should an ecommerce shipper use its own box instead of Flat Rate?
Use your own box when the product does not fit cleanly in a USPS Flat Rate box, needs more protection, creates too much void space, or ships cheaper under normal weight-zone or dimensional-weight pricing.
USPS Priority Mail is the source for current USPS Flat Rate details. Packrift helps teams choose and reorder non-USPS boxes when official Flat Rate packaging is not the right operational fit.