The standard flat-rate packaging of the three US carriers in one place: USPS Flat Rate box dimensions inside and out, UPS Simple Rate cubic-inch tiers, FedEx One Rate packaging, plus a calculator that tells you which tier your box falls into.
Three carriers, three different logics: USPS prices by the specific box, UPS by cubic volume in your own box, FedEx by its own packaging type. All dimensions below were read from the carrier pages on July 17, 2026.
Only USPS-produced Flat Rate boxes get the flat price [5]. One price up to 70 lbs, any US destination [4]. Dimensions from the Postal Store product specifications [1][2][3]:
| Box | Outside (in) | Inside (in) | Inside volume |
|---|---|---|---|
| Small Flat Rate Box | 8-11/16 x 5-7/16 x 1-3/4 | 8-5/8 x 5-3/8 x 1-5/8 | 76 in³ |
| Medium Flat Rate Box (FRB-1, top-loading) | 11-1/4 x 8-3/4 x 6 | 11 x 8-1/2 x 5-1/2 | 589 in³ |
| Medium Flat Rate Box (FRB-2, side-loading) | 14-1/8 x 12 x 3-1/2 | 13-5/8 x 11-7/8 x 3-3/8 | 593 in³ |
| Large Flat Rate Box | 12-1/4 x 12 x 6 | 12 x 11-3/4 x 5-1/2 | 882 in³ |
Flat Rate envelopes: any USPS-produced Priority Mail envelope smaller than 9.5 x 12.5 in qualifies for the Flat Rate Envelope price [5]. Flaps must close within the normal folds [4].
Your own box on Priority Mail is priced by weight and zone instead: max 70 lbs, max 108 in combined length plus girth [4]. Above 1,728 cubic inches, dimensional weight applies at L x W x H / 139, with fractions of an inch rounded up [4]. Nonstandard surcharges: $4.50 for 22 to 30 in length, $21.00 over 30 in, $35.00 over 3,456 cubic inches; USPS Flat Rate packaging is exempt from all of them [4].
Your own packaging, five tiers by volume, up to 1,728 cubic inches total [6]:
| Tier | Volume range | Common box sizes (in) | Fits roughly | Starting at |
|---|---|---|---|---|
| Extra Small | 1 to 100 in³ | 4x4x4, 6x4x4, 8x6x2 | sunglasses, t-shirt, mug | $12.20 |
| Small | 101 to 250 in³ | 6x6x6, 8x6x5, 12x9x2 | hardcover book, ream of paper | $15.80 |
| Medium | 251 to 650 in³ | 8x8x8, 12x9x6, 13x11x2 | shoebox, 3-ring binder | $18.95 |
| Large | 651 to 1,050 in³ | 10x10x10, 12x12x7, 15x11x6 | basketball, 14-inch laptop | $24.85 |
| Extra Large | 1,051 to 1,728 in³ | 12x12x12, 16x12x9, 18x12x6 | small appliance, tennis racket | $30.95 |
Speeds: next-day, second-day, three business days or five-day delivery; declared value $100 included [6].
FedEx packaging only. Envelopes must weigh under 10 lbs; boxes, paks and tubes under 50 lbs [7]. Dimensions and the FedEx 2Day flat rates (effective February 2, 2026, local zone) [7]:
| Packaging | Dimensions (in) | 2Day rate |
|---|---|---|
| FedEx Envelope | 9-1/2 x 12-1/2 (reusable 9-1/2 x 15-1/2; padded 9-3/4 x 11-1/2) | $12.75 |
| Extra Small Box | 7-1/4 x 2-1/2 x 4-11/16 | $13.40 |
| FedEx Pak | 10-1/4 x 12-3/4 or 12 x 15-1/2 (padded 11-3/4 x 14-3/4) | $16.30 |
| Small Box | 10-7/8 x 1-1/2 x 12-3/8 or 8-3/4 x 2-5/8 x 11-1/4 | $19.55 |
| Medium Box | 11-1/2 x 2-3/8 x 13-1/4 or 8-3/4 x 4-3/8 x 11-1/4 | $24.35 |
| Large Box | 12-3/8 x 3 x 17-1/2 or 8-3/4 x 7-3/4 x 11-1/4 | $32.05 |
| Extra Large Box | 11-7/8 x 10-3/4 x 11 or 15-3/4 x 14-1/8 x 6 | $42.75 |
| FedEx Tube | 6 x 6 x 38 | $42.75 |
An overstuffed envelope (over 3 in thick) or pak (over 5 in) is re-rated as a box by cubic volume [7].
The checker compares your outside dimensions with the USPS inside dimensions, which is the honest direction: a 12 x 9 x 6 box does not go into a box that is 12 x 11.75 x 5.5 inside.
Four boxes. Small: 8-11/16 x 5-7/16 x 1-3/4 in outside. Medium top-loading: 11-1/4 x 8-3/4 x 6 in. Medium side-loading: 14-1/8 x 12 x 3-1/2 in. Large: 12-1/4 x 12 x 6 in. All ship up to 70 lbs at one price. Dimensions are from the USPS Postal Store product pages.
Yes. UPS Simple Rate works with your own packaging up to one cubic foot (1,728 cubic inches). The price depends on which of five volume tiers the box falls into, from Extra Small (1 to 100 cubic inches) to Extra Large (1,051 to 1,728 cubic inches).
You pay by weight and zone. Maximum combined length and girth is 108 inches and maximum weight is 70 lbs. Above 1 cubic foot (1,728 cubic inches), dimensional weight applies: length times width times height divided by 139, rounded up.
UPS Simple Rate accepts your own box up to 1,728 cubic inches. The comparison table UPS publishes lists FedEx at up to 1,452 cubic inches with FedEx packaging and USPS at up to 1,200 cubic inches; the biggest single USPS Flat Rate box measures 882 cubic inches inside.