Dimensional Weight Calculator

Carrier Divisors and Rounding Rules (October 2026)

Sources: USPS Domestic Mail Manual 223.1.4 and 253.1.2, revised July 12, 2026 (the USPS divisor changed to 139 then); UPS “Shipping Dimensions and Weight”; FedEx “What is dimensional weight” and the FedEx Service Guide effective January 5, 2026. Contract rates can use a different divisor, so check your agreement. For nonrectangular parcels USPS multiplies the volume by 0.785 before dividing.

How Billable Weight Is Calculated, Step by Step

1. Measure the longest side, then the other two, in inches, at the widest points including any bulge.
2. Round each side the carrier’s way: UPS and FedEx to the nearest inch (11.6 becomes 12, 9.4 becomes 9); USPS always up (9.4 becomes 10).
3. Multiply the three sides for cubic inches.
4. Divide by the divisor and round up: 1,728 ÷ 139 = 12.43 → 13 lb.
5. Compare with the actual weight, also rounded up, and pay for the larger.

Billable Weight by Box Size

The jump from the 12-inch cube to a 13 × 12 × 12 box shows the USPS threshold: one more inch takes the box over 1,728 cubic inches and the billable weight from 5 lb to 14 lb. The odd-sized box shows why rounding rules matter: the same 11.6 × 9.4 × 8.3 inch box is 864 cubic inches to UPS and FedEx but 1,080 to USPS, which still bills the actual weight because the box stays under 1 cubic foot. The very large box hits the USPS 70-lb price cap: its dimensional weight is 75 lb.

USPS Priority Mail Cubic Pricing

Commercial Priority Mail shippers can be charged by cubic tier instead of weight for small, heavy parcels: the piece must be 0.50 cubic foot or less, weigh 20 lb or less, and be no longer than 22 inches. For the tier, round each side down to the quarter inch, multiply and divide by 1,728. The DMM’s example: a 6-1/8 × 5-7/8 × 6-3/8 inch box becomes 6 × 5-3/4 × 6-1/4 = 215.6 cubic inches = 0.125 cubic foot, which is Tier 2 (0.101 to 0.20). The calculator shows the tier when a box qualifies.

Mistakes That Raise Shipping Costs

• Using a box “close enough.” Every inch on each side multiplies; a 14-inch cube is 59% more volume than a 12-inch cube.
• Ignoring bulges. Carriers measure the widest point; an overstuffed box measures larger than its flaps.
• Assuming USPS has no DIM weight. Above 1 cubic foot it does, at the same 139 divisor as UPS and FedEx since July 12, 2026.
• Quoting retail when you pay daily rates (or the reverse). UPS divides by 166 at retail counters and 139 on daily-rate accounts.
• Forgetting surcharges. Additional handling and large-package charges are triggered by length, girth and weight, not by DIM weight alone.

What This Calculator Does Not Cover

It does not quote prices, zones, fuel surcharges, residential or delivery-area fees, additional handling or large-package surcharges, contract divisors, or international metric rules (cm ÷ 5,000 for kilograms). For mixed nonrectangular shapes, tubes and USPS soft-pack cubic pricing, the carriers’ own calculators and your account’s rate sheet are the final word.

Frequently Asked Questions

How to Use This Calculator

1. Measure the box — length, width and height in inches at the widest points.
2. Enter the actual weight — what the scale shows, in pounds.
3. Choose the carrier and rate type — UPS daily or retail, FedEx, or USPS.
4. Read the results — rounded dimensions, cubic inches, dimensional weight, billable weight, the other carriers’ billable weight and any USPS cubic tier.