Big Rig Daily · Free Driver Tools

Truck Weight and Axle Calculator

Enter your steer, drive and trailer axle weights from the scale to compare them with the federal Interstate limits, then add your axle spacing to run the bridge formula. Everything runs in your browser.

Planning aid only. This calculator is a planning aid only, not legal advice. It uses the federal Interstate limits in 23 CFR 658.17. State limits, grandfathered weights, enforcement tolerances, non-Interstate roads and oversize or overweight permits vary, so check the rules for every state on your route. The certified scale ticket governs, along with your state and permit rules.

How the truck weight calculator works

The federal weight rule for the Interstate System, 23 CFR 658.17, sets three main limits: 80,000 pounds gross vehicle weight, 20,000 pounds on a single axle and 34,000 pounds on a tandem axle. The calculator adds your steer, drive and trailer weights to get the gross weight, then compares each one with those limits and shows how far under or over you are.

A tandem is two or more axles whose centers are more than 40 inches and not more than 96 inches apart, according to the Federal Highway Administration (FHWA). Choose single if your drive or trailer group is one axle. The steer axle is also limited by the axle and tire ratings, which can be lower than 20,000 pounds.

The bridge formula check

The federal bridge formula limits how much weight any group of two or more consecutive axles can carry, based on how many axles are in the group and how far apart the outer axles are:

W = 500 (LN / (N - 1) + 12N + 36)

W is the maximum weight in pounds on the group, rounded to the nearest 500 pounds. L is the distance in feet between the outer axles of the group, and N is the number of axles in the group. FHWA's Bridge Formula Weights guide says the groups that usually matter most on a five-axle tractor-trailer are the steer axle through the last drive axle, the first drive axle through the last trailer axle, and the whole truck from steer axle to last trailer axle.

There is one well known exception. Two consecutive tandems may carry 34,000 pounds each when the distance from the first axle of the first tandem to the last axle of the second tandem is 36 feet or more. The calculator applies that rule and never allows more than the 80,000 pound gross limit.

Measure distances from axle center to axle center, in feet. The bridge formula lookup at the bottom of the calculator works for any group, so you can check other axle combinations too.

Before you roll

• Weigh on a certified scale and keep the scale ticket.
• Slide the tandems or the fifth wheel to shift weight between axles, then reweigh.
• Check the posted limits and permit rules for every state on your route, since some roads and bridges have lower limits.

Weight distribution is part of the CDL general knowledge test. The CDL general knowledge test study guide covers what the test expects you to know, and the free CDL practice test lets you check yourself.

Frequently Asked Questions

What is the maximum legal weight for a semi truck?

On the Interstate System, the federal limit in 23 CFR 658.17 is 80,000 pounds gross, 20,000 pounds on a single axle and 34,000 pounds on a tandem axle, unless the bridge formula sets a lower limit. Some states have higher grandfathered limits or permits, and some roads have lower posted limits.

How do I use the bridge formula?

Count the axles in the group (N) and measure the distance in feet between the outer axles (L). Work out 500 times (L times N divided by N minus 1, plus 12 times N, plus 36), then round to the nearest 500 pounds. FHWA's example: three axles spread 20 feet apart may carry 51,000 pounds.

Can my tandems each carry 34,000 pounds?

Yes, under federal law two consecutive tandems may carry 34,000 pounds each if the first and last axles of the two tandems are at least 36 feet apart. The gross weight still cannot go over 80,000 pounds on the Interstate System without a permit.

Is this calculator a substitute for a scale ticket?

No. It is a planning aid only, not legal advice. The certified scale ticket shows your real weights, and state rules and permits decide what is legal on your route.