Scale weight versus dimensional weight
Parcel carriers charge for the space a package takes as well as its weight. Dimensional weight (DIM weight, or volumetric weight) converts the package volume into a weight by dividing it by a divisor. The billable weight is the larger of the DIM weight and the actual scale weight, each rounded as the carrier's rules say. A light but bulky box, such as pillows or a lampshade, is billed on its size; a small heavy box is billed on its weight. This calculator applies the policy you enter and shows every step, so you can match it to your carrier service or contract.
The divisor and its units
The divisor has units: cubic inches per pound (in³/lb) or cubic centimetres per kilogram (cm³/kg). A smaller divisor gives a higher DIM weight. Values such as 139 in³/lb and 5,000 cm³/kg are common published examples, and 166 in³/lb appears in some services, but they are separate commercial policies, not conversions of each other: 139 in³/lb is about 5,022 cm³/kg. Check the divisor for your exact service, account and destination.
Rounding, step by step
- Each dimension is rounded first: up to the next whole unit, to the nearest whole unit (halves round up), or not at all.
- Volume = rounded length × width × height. Raw DIM weight = volume ÷ divisor.
- DIM weight and actual weight are each rounded up to the billed-weight step (1, 0.5 or 0.1).
- Billable weight = the larger of the two. Rounding happens only after the raw calculation.
Carriers word their rules differently, and the wording can change. As checked on 22 October 2026, FedEx's US page describes rounding each dimension to the nearest whole inch, FedEx Canada describes rounding up and both inch and centimetre divisors, and UPS publishes its own dimension and weight guidance. These are not interchangeable, which is why the policy here is entered rather than preset.
Worked example
A box of 12.1 × 10 × 8 inches weighing 3 lb, with a divisor of 139 in³/lb, dimensions rounded up and a 1 lb step. Length 12.1 rounds up to 13, so the volume is 13 × 10 × 8 = 1,040 in³. Raw DIM weight is 1,040 ÷ 139 = 7.482 lb, rounded up to 8 lb. The actual weight rounds to 3 lb, so the billable weight is 8 lb, driven by size. With no dimension rounding the volume is 968 in³ and the billable weight 7 lb, which shows how much the rounding rule matters.
The same box weighing 9.2 lb at 12 × 10 × 8 inches is billed at 10 lb on its actual weight. A divisor of 166 instead of 139 lowers the DIM weight of the 3 lb box to 6 lb. In metric, a 40 × 30 × 20 cm box of 2 kg with a 5,000 cm³/kg divisor has a raw DIM weight of 4.8 kg and a billable weight of 5 kg.
Compare two package sizes
Open “compare a second package” to test a different box with the same policy. Trimming the first box to 12 × 10 × 8 inches gives a raw DIM weight of 6.9065 lb and a billable weight of 7 lb, 1 lb less than the 12.1-inch box. The calculator reports the difference in billable weight only; what that is worth depends on your rate card. Never under-declare dimensions: carriers measure packages and can rebill.
Check the carrier's policy
This is a weight estimate under the policy you enter, not a shipping quote. It does not model rates, zones, surcharges for large or heavy packages, minimum charges, size or weight limits, customs or multi-package shipments. To fold shipping into product pricing, use the margin and markup calculator; to compare supplier pack sizes, the unit price calculator; and to plan when to reorder the stock you ship, the safety stock calculator.
FAQ
What divisor should I use?
The one in your carrier's current rate guide or your contract for that service. Defaults here are illustrative examples only.
Is billable weight the actual weight or the DIM weight?
Whichever is larger after rounding. Light, bulky packages are billed on DIM weight; small, heavy ones on actual weight.
How are dimensions rounded?
It depends on the carrier and region: up to the next whole unit, to the nearest whole unit, or not at all. Choose the rule your carrier states.
How does it work in centimetres and kilograms?
Switch to cm · kg and enter a divisor in cm³/kg, often 5,000 or 6,000. It is not a converted inch divisor, so check the metric policy separately.
Does this give the shipping price?
No. It gives a billable weight under the entered policy. The price depends on your carrier's rates, zone, service and surcharges.
Why do different calculators give different DIM weights?
They use different divisors, dimension rounding or weight steps. This calculator shows each choice so you can match one specific policy.