Axle Weight & Bridge Formula Calculator
Enter your CAT scale weights and instantly check your gross, steer, and tandem axles against federal limits. Catch an overweight axle before the chicken coop does.
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Key Takeaways
- Federal maximums: 80,000 lbs gross, 20,000 lbs single axle, 34,000 lbs tandem group.
- Gross weight is the sum of every axle group and cannot exceed 80,000 lbs on the interstate without a permit.
- The Federal Bridge Formula caps weight on any axle group based on the distance between its outer axles.
- Fix an overweight axle by sliding your trailer tandems to shift weight between the drives and trailer groups.
- States may set lower limits and issue permits - always weigh at a certified CAT scale and check your state rules.
O TruckingFree Axle Weight & Bridge Formula Calculator
Axle Group Weights
Enter weights as read off your CAT scale ticket (5-axle rig)
Federal limit: 20,000 lbs
Federal limit: 34,000 lbs
Federal limit: 34,000 lbs
These are federal (interstate) maximums. Some states set lower limits and issue overweight/permit exceptions — always check your state DOT rules and any permits. This is an estimate only; weigh at a certified (CAT) scale before you roll.
Federal Truck Weight Limits Explained
The 80,000 lb gross weight limit
On the Interstate Highway System, a standard tractor-trailer is capped at 80,000 lbs gross vehicle weight without a special permit. Gross weight is the total of everything the truck puts on the ground: the tractor, trailer, fuel, and your load. Even if every individual axle is legal, you can still be overweight on gross - and vice versa.
Single axle limit: 20,000 lbs
A single axle - like your steer axle - is limited to 20,000 lbs. Steer axles rarely approach this on a loaded van, but heavy hood tractors, full fuel tanks, and front-heavy freight can push you close.
Tandem axle group limit: 34,000 lbs
A tandem axle group - two axles spaced close together, such as your drive axles or your trailer axles - is limited to 34,000 lbs. On a typical 5-axle rig you have two tandem groups: the drives and the trailer tandems. Each is measured separately at the scale.
The Federal Bridge Formula
The Bridge Formula protects bridges by requiring heavier loads to be spread over more axle spacing. The formula is:
Where W is the maximum weight (rounded to the nearest 500 lbs, capped at 80,000), L is the distance in feet between the outer axles of the group, and N is the number of axles in the group. The greater the spacing (larger L), the more weight the group is allowed to carry. Use the Advanced section of the calculator above to run it for any axle group.
How to fix an overweight axle by sliding tandems
The most common scale problem is not being over gross - it is being heavy on one axle group and light on another. Because your gross is legal, you just need to move weight between the groups by sliding the trailer tandems:
- Drives too heavy? Slide the trailer tandems forward (toward the nose) to shift weight off the drives and onto the trailer axles.
- Trailer tandems too heavy? Slide them rearward to shift weight off the trailer and onto the drives.
- A common rule of thumb: each tandem hole moves roughly 250-400 lbs between the two groups - reweigh after adjusting.
- If your gross is over 80,000 lbs, sliding will not help. You must offload or redistribute freight.
FAQ
What is the maximum gross weight for a semi truck?▼
What is the tandem axle weight limit?▼
What is the Federal Bridge Formula?▼
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