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An LTL invoice bills 4,770 pounds on a shipment your dock scale recorded at 3,419. Nothing on the bill is wrong. The carrier never asserted that the freight weighed 4,770 pounds, it applied a density floor and rated the cube.
Your audit has no rule to test that line against. It compares billed weight to tendered weight, finds a gap of 1,351 pounds, and either flags a correct charge as an overbilling or releases it because the rate per hundredweight looks right. The invoice is using one word, weight, for at least three different quantities.
Weight on a freight invoice is a computed figure
What the scale says is an input. What gets rated is chargeable weight, and chargeable weight is whatever the governing tariff says it is. Three mechanisms produce most of it.
- A density floor. The carrier measures the space the shipment occupies, multiplies by a minimum density, and rates the product. Scale weight drops out entirely.
- Deficit weight. The shipment is rated at the minimum of a higher rate break because that band prices cheaper, and the pounds you did not ship appear as their own line.
- Dimensional weight. Volume divided by a published divisor, billed whenever it beats the scale. Standard in parcel and air, where the divisor is the whole argument.
All three are legitimate and all three are published. None lives in the document your team treats as the rating authority, and no two carriers write them the same way.
A density floor swaps your weight for the trailer's
PittOhio's rules tariff Item 605, Minimum Charge Based on Cubic Capacity and Density, sets two triggers. Freight with an average density under 3 pounds per cubic foot that requires at least 350 but less than 750 cubic feet of trailer space is rated on a calculated weight of 6 pounds per cubic foot, at the customer's class 125 rate. Freight under 6 pounds per cubic foot that requires 750 cubic feet or more is rated the same way.
The item carries its own example: a shipment found to occupy 795 cubic feet at a density of 4.3 pounds per cubic foot is rated on a calculated weight of 4,770 pounds. At 4.3 pounds per cubic foot, 795 cubic feet holds about 3,419 pounds of freight. At the 6 pound floor it becomes 4,770. The carrier bills 1,351 pounds of air by rule, and overrides the commodity's class on the way past.
Two further provisions in the same item settle more disputes than the thresholds. Cubic capacity is determined by totaling the cubic feet of all articles, pieces and packaged units under NMFC Item 110, so the number is assembled from handling units rather than measured across the load. And where packaging is insufficient, the height of the article is taken as 96 inches, the inside height of the trailer, however tall the freight actually stands.
Then the numbers move between carriers. Old Dominion's accessorial brochure places its Minimum Charge for Low Density Freight, Item 610-1, at under 3 pounds per cubic foot between 350 and 749 cubic feet, or under 6 pounds per cubic foot from 750 cubic feet up, and handles capacity loads separately under Item 390 at 20 linear feet or 20,000 pounds. Saia's 170-D rules tariff sets its lower tier at under 4 pounds per cubic foot across the same 350 to 749 band.
Three carriers, three threshold tables, one skid of insulation. The same freight stays under the rule on one routing guide position and gets recalculated on the next.
Deficit weight bills pounds you did not tender
The second mechanism runs the other way, and it usually works in your favour. Class rate scales price lower per hundredweight as weight rises, so a shipment sitting just under a break can cost more than the same freight rated at the minimum of the break above. Rating engines bump it.
IBM's documentation for deficit rating in Sterling walks the arithmetic through with sample breaks of 0 to 499, 500 to 999, and 1,000 to 4,999 pounds. A 495 pound shipment rated inside its own band prices at 4,950 dollars. Rated at the 500 pound minimum of the band above, it prices at 4,000. The engine returns the cheaper figure and shows the 5 pound difference as a deficit line.
Two consequences follow. The invoice carries a weight nobody tendered, so any check matching billed pounds against shipped pounds fails on that line by design. And the bump has to be tested in both directions: you are owed the lower of the two results, and a line rating 495 pounds inside the 0 to 499 band when the band above prices cheaper is an overcharge with clean arithmetic on its face.
Dimensional weight is the same idea with a published divisor
Parcel and air derive the deemed figure from volume rather than from a minimum density, and the divisor decides the bill. The USPS final rule for July 2026 domestic competitive products moves the dimensional weight factor to 139 and rounds length, width and height up to the nearest whole inch, across Priority Mail Express, Priority Mail, USPS Ground Advantage and Parcel Select. Volume at or under 1,728 cubic inches remains exempt.
Going from a 166 divisor to 139 lifts billable weight on an unchanged box by about 19 percent, and the rounding is not cosmetic. A carton measured at 18.1 by 14.2 by 10.3 inches bills as 19 by 15 by 11: that is 3,135 cubic inches against the 2,647 you shipped, or 23 billable pounds where the measured volume gives 20.
Air freight runs it in metric. The IATA TACT rules at section 3.9.4 charge a consignment on volume once its average dimensional figure exceeds 6,000 cubic centimetres per kilogram, which works out at 166 cubic inches per pound. The divisor USPS has just retired and the international air standard are the same ratio in different units.
| Mechanism | What sets it off | Where it is written | What to check |
|---|---|---|---|
| Density floor | Cube and density cross a carrier threshold | That carrier's rules tariff, not the NMFC | Measured cube, the threshold table for that lane, any class override |
| Deficit weight | Actual weight sits below a cheaper rate break | Rating engine logic on the rate scale | Both outcomes computed, the cheaper one billed |
| Dimensional weight | Volume over the divisor beats the scale | Carrier service guide or air waybill tariff | Contract divisor, rounding rule, dimensions used |
What Penny does with a chargeable weight line
Penny starts by asking which of the three produced the billed number, because a billed weight that differs from your tender is not yet a variance.
Where a floor applied, she rebuilds it on the carrier's own figures: cube totalled from the handling units, the minimum density and class in the tariff item governing that lane, and whether the thresholds were actually crossed. A shipment at 760 cubic feet and 6.2 pounds per cubic foot sits above the line and rates on its scale weight. She also tests the 96 inch substitution, which is conditional on packaging rather than automatic.
Where the line was bumped, she computes both bands and confirms the cheaper one was billed. Where volume drove the charge, she checks the divisor against your contract rather than the published default, since a negotiated divisor is a common term and rating systems do not always carry it.
Where your tender holds no measured dimension for the handling unit, she does not construct one. The line comes back marked needs-data with the field named, the behaviour described in an agent that says needs data. A cube you cannot evidence loses for the reason set out in a dimension you never recorded: the carrier measured, you asserted.
What to do next
Take a quarter of LTL invoices and pull every line where billed weight differs from tendered weight by more than five percent. Sort them three ways: density floor, deficit bump, and genuinely unexplained. The last group is usually the smallest, and the first is the one nobody has priced.
Then work the density floor group against the governing tariff item and ask whether the thresholds were actually met. That test separates a rule correctly applied from one applied to freight that never triggered it.
Bring that quarter to a walkthrough and we will run Penny across it live, naming the mechanism behind every weight on it.