The basic formula
Dimensional weight(Length × Width × Height) ÷ divisor
Billable weightGreater of actual weight or dimensional weight
Measure the outside of the packed box, not the product inside it. Multiply the three dimensions to get cubic volume, divide by the divisor required by your service, and then compare the result with the package's actual scale weight.
A worked example
Package: 20 × 14 × 12 inches, weighing 18 pounds
Volume: 3,360 cubic inches
Using divisor 139: 3,360 ÷ 139 = 24.17 pounds before the carrier's final rounding rule
Planning result: dimensional weight is greater than 18 pounds, so it controls the billable-weight estimate.
What the divisor changes
A smaller divisor produces a larger dimensional weight. That is why the same box can produce different estimates under different services or contracts. Never assume a divisor from memory: use the value in the current service guide or negotiated agreement.
Four details to verify before buying postage
- Whether each outside dimension is rounded before multiplication.
- Which divisor applies to the service, destination, and package type.
- How the final dimensional and billable weights are rounded.
- Whether oversize, additional-handling, fuel, residential, or zone charges also apply.
When actual weight controls
If the scale weight is greater than the calculated dimensional weight, actual weight generally controls this part of the rate calculation. Dense items often fall into this category. Large lightweight goods—pillows, lampshades, plastic parts, or loosely packed products—are more likely to be dimensionally billed.
Check your package now
Compare multiple common divisor profiles and your own contract divisor in one view.
Open the calculatorThis guide provides planning information, not a carrier quote. Carrier tariffs, service guides, contracts, measurement methods, and surcharges control the final charge.