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Weight & Axle Load Calculator for Owner Operators

Check axle weights, gross weight and every bridge formula axle group on a 5-axle rig before you scale.

Axle Weights

lbs
lbs
lbs

Axle Spacing

Measure from the center of one axle to the center of the next. This tool is for a 5-axle tractor with a tandem trailer.

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ft
ft
ft

Enter axle weights and click Check Weights

Numbers tight? Let a dispatcher find better loads.

This tool is free. If it shows you are leaving money on the road, Grow Trucking finds loads, negotiates rates and does the paperwork for owner-operators and small fleets.

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What are the federal weight limits for a semi truck?

On the Interstate System, the federal limits are 80,000 lbs gross, 20,000 lbs on a single axle and 34,000 lbs on a tandem (two axles 40 to 96 inches apart). A truck can pass all three and still be illegal if it fails the Federal Bridge Formula. The formula limits the weight of every group of two or more axles in a row, based on the distance between the group's outer axles and how many axles it has.

  1. Bridge formula: limit (lbs) = 500 x ((L x N / (N - 1)) + 12N + 36), rounded to the nearest 500. L is the distance in feet between the outer axles of the group being checked. N is the number of axles in that group.
  2. Example: all 5 axles of a rig spread over only 40 feet may carry just 73,000 lbs. A 5-axle rig needs about 51 feet from first axle to last to reach 80,000 lbs.
  3. Check every group, not only the whole truck. The group that usually fails first is axles 2 to 5, the two drives and the two trailer axles. If they are 38 feet apart they may carry 67,500 lbs. Two tandems at 34,000 lbs each is 68,000, so a rig can pass both tandem limits and still be over.
  4. Slide the trailer tandem back to lengthen the gap between the rear drive axle and the front trailer axle. That raises the limit for axles 2 to 5, and it also moves weight onto the drive axles, so weigh again.
  5. A load is legal only if it clears the gross, the axle and the bridge formula limits for its real axle spacing.
Federal weight limits on the Interstate System (23 CFR 658.17)
LimitWeight
Gross vehicle weight80,000 lbs
Single axle20,000 lbs
Tandem axle group (axles 40 to 96 inches apart)34,000 lbs
Bridge formula (5 axles, 40 ft from first to last)73,000 lbs
Bridge formula (axles 2 to 5, 38 ft apart)67,500 lbs

States may not enforce a lower Interstate limit than these. But they can and do set their own limits on non-Interstate roads, and they set their own overweight fines, so this tool gives no fine figure. Permits change the numbers for oversize or overweight loads. The tool fits a 5-axle tractor with a tandem trailer. Confirm against the specific states and roads on your route.

Last reviewed October 2026. Sources: eCFR: 23 CFR 658.17, weight; FHWA: Bridge Formula Weights; eCFR: 23 CFR 658.5, definitions of single and tandem axle weight.

Frequently Asked Questions - Weight & Axle Load Calculator

What is the maximum legal weight for a semi-truck?
On the Interstate System, the federal maximum is 80,000 pounds gross vehicle weight, 20,000 pounds on a single axle, and 34,000 pounds on a tandem axle group. A truck can be under all three of those limits and still be illegal if it fails the Federal Bridge Formula, which limits weight based on axle spacing. Most steer axles are also rated by the manufacturer around 12,000 to 13,200 pounds, well under the 20,000-pound legal limit, so that rating usually caps steer weight in practice, not the law.
What happens if my truck is overweight at a scale?
Fines vary widely by state and by how far over the limit you are, and can range from modest penalties for a small overage to very large fines for a significant one. You can also be placed out of service until the weight is corrected, and repeat violations can affect your safety record. Check your specific state's overweight penalty schedule, since there's no single federal fine amount.
How do I calculate axle weights for my load?
Axle weight depends on where the load sits relative to the kingpin and the axles. Moving weight forward shifts more onto the steer axle; moving it backward shifts more onto the trailer tandems. The Federal Bridge Formula then checks whether your axle groups, given their spacing, can legally carry the resulting weight.
Can I slide my trailer tandems to fix an overweight axle?
Yes. Sliding the trailer tandems back, away from the kingpin, moves weight off the trailer axles and onto the drive axles. Sliding them forward does the opposite. A longer gap between the rear drive axle and the front trailer axle also raises the bridge formula limit for axles 2 to 5. How much weight each hole shifts depends on the trailer and the load, so weigh again after sliding rather than assuming a fixed amount per hole.
Why can I be legal on every axle and still be overweight?
Because of the Federal Bridge Formula. It limits the weight of any group of two or more axles in a row, based on how far apart the outer axles are. A typical rig with two 34,000 lb tandems carries 68,000 lbs on axles 2 to 5. If those four axles are 38 feet apart, the formula allows only 67,500 lbs, so the rig is over even though each tandem is legal. Sliding the trailer tandem back adds distance and fixes it.
What is a tandem axle?
Under the federal definition, it is two or more axles in a row whose centers are more than 40 inches and not more than 96 inches apart. Such a group may carry 34,000 lbs. Axles spaced farther apart than 96 inches are treated as separate single axles, with a 20,000 lb limit on each.

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Go Further With Weight & Axle Load Calculator

Guides, services and reference pages that go with this tool.

Terms explained in the Weight & Axle Load Calculator