Bridge Formula Weights Calculator

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The TruckScience Axle Weight Calculator now includes a Bridge Formula Weights Calculator.  To register for a free trial of the Axle Weight Calculator, click here now.

Why Bridge Formula?

Bridge Formula Law was enacted in order to protect highway bridges.  When it comes to bridges, axle spacing is as important as axle weight. The stress on bridge members as a shorter truck rolls across is much more than that caused by a long vehicle, even though both trucks may have the same total weight and individual axle weights. These formulae are designed to protect roads and bridges from the damage caused by the concentrated weight of shorter trucks, by limiting the weight-to-length ratio of a vehicle.

Where does Bridge Formula Apply?

Bridge formulae are used for truck size and weight regulations in the U.S., Canada (Ontario), Mexico, Europe, South Africa, Australia, and New Zealand.

What is Bridge Formula?

The minimum distance between the extreme axles of any two axle groups – for a given total gross mass on the axles within that distance – is controlled by the relevant bridge formula.  That is, allowable weight depends on the number of axles a vehicle has and the distance between those axles.

Federal Bridge Law

FMCSA (Federal Motor Carrier Safety Administration) regulation §658.17 states:

No vehicle or combination of vehicles shall be moved or operated on any interstate highway when the gross weight on two or more consecutive axles exceeds the limitations prescribed by the following formula:

W = the maximum weight in pounds that can be carried on a group of two or more axles to the nearest 500 pounds (230 kg).
L = spacing in feet between the outer axles of any two or more consecutive axles.
N = number of axles being considered.

Achieving More Weight

The longer and the more axles a truck has, the more weight that it can carry. The bridge formula defines this relationship. More weight can be carried by spreading weight over additional axles or by increasing the distance between axles.  In the example above, increasing the trailer wheelbase from 307in to 385in increased the permissible gross weight of the rig, from 75,500lb to the maximum allowable weight for this rig of 80,000lb.

What else is new this month?

New Body Types

We’ve added a catch-all body type of ‘Other Body’, and added sample bodies for sugarcane, skiploader, concrete boom pump and mixer. If you have a body type which we don’t yet provide a category for, we can add it to the ‘Other Body’ category for you now.

Usability Improvements

The edit dialog can now be popped out or dragged to an area of the screen where it won’t obscure your view of the drawing.


We’ve added tooltip information to many attributes, so help is at hand when you need it. We’ve also neatened up the Center of Gravity view, so you can control what you see.

The Irish Times

In other news this month, we were profiled in the ‘Inside Track’ feature in Ireland’s oldest daily broadsheet, The Irish Times.  Is it just me or did you just think about Turning Radius there? 🙂  Have a read of the article here.

More about the Axle Weight Calculator

You can watch a 2-minute video introduction to the Axle Weight Calculator on this page, and see more info about the app on our Axle Weight Calculator page.

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