Guides · LoadLegit · 2026-07-03

How to calculate truck axle weight from where the load sits

Most drivers find out their axle weights at the CAT scale — after the trailer is loaded and sealed. But axle weight is just physics, and you can compute it from where the freight sits. That's how you catch a problem before loading, when it's cheap to fix.

The model: a beam on two supports

A trailer is a beam supported at the kingpin and at the trailer tandems. Any weight in the trailer splits between those supports in inverse proportion to its distance from each:

weight_on_tandems = W × d_kingpin→CoG / d_kingpin→tandems
weight_on_kingpin = W − weight_on_tandems

The kingpin weight then flows to the tractor, splitting again between steer and drive axles the same way. Add the tare weights of tractor and trailer (which have their own CoGs) and you have the full picture: steer, drives, trailer tandems.

Worked example

40,000 lb of freight with its CoG 25 ft behind the kingpin, tandems 37 ft behind the kingpin: tandems carry 40,000 × 25/37 ≈ 27,000 lb; the kingpin carries the other 13,000 lb, which the tractor splits between steer and drives. Load the same freight with the CoG at 30 ft and the tandems now carry ≈ 32,400 lb — add trailer tare and you're flirting with the 34,000 lb tandem limit, from nothing but load placement.

Finding the load's CoG

For a mixed load, the combined CoG is the weighted average of each piece's position: multiply each item's weight by its distance from the trailer nose, sum, divide by total weight. Tedious by hand — trivial for software. LoadLegit's 3D packer computes total weight and CoG automatically as you place boxes, then checks every limit including the bridge formula.

Already loaded and scaled? Punch the ticket numbers into the free calculator. Over on the tandems? Here's how the tandem slide fixes it.