On the Interstate system, federal law caps a truck at 20,000 pounds on any single axle, 34,000 pounds on a tandem axle and 80,000 pounds gross, and any group of axles is further limited by the bridge formula. These limits are in 23 CFR 658.17 and 23 U.S.C. 127, and they include every enforcement tolerance. Off the Interstate, and where a state’s older limits were grandfathered, state law sets the numbers.
The four federal limits
| Limit | Figure | Rule |
|---|---|---|
| Single axle | 20,000 lb | 23 CFR 658.17(c): any one axle, including any one axle of a group |
| Tandem axle | 34,000 lb | 658.17(d) |
| Gross vehicle weight | 80,000 lb | 658.17(b), "except where lower gross vehicle weight is dictated by the bridge formula" |
| Any group of two or more consecutive axles | Bridge formula result | 658.17(e) and 23 U.S.C. 127(a)(2) |
658.17(a) applies these limits to "the National System of Interstate and Defense Highways and reasonable access thereto." Other federal-aid and state roads are under state weight law, which for many states is the same, and for some is different.
What counts as a single or tandem axle
The definitions in 23 CFR 658.5 are about spacing, not how many axles the truck has:
- Single axle weight is the total weight on the road from all wheels whose centers fall between two parallel vertical planes 40 inches apart across the full width of the vehicle.
- Tandem axle weight is the total weight from two or more consecutive axles whose centers fall between planes spaced more than 40 inches and not more than 96 inches apart.
So two axles 50 inches apart are a tandem limited to 34,000 pounds together. Two axles more than 96 inches (8 feet) apart are not a tandem; each is a single axle limited to 20,000, and the pair is limited by the bridge formula. A spread-axle trailer is the common example.
The bridge formula
The formula in 23 U.S.C. 127(a)(2), repeated in 658.17(e), is:
W = 500 [ LN / (N − 1) + 12N + 36 ]
where W is the overall gross weight allowed on any group of two or more consecutive axles, "to the nearest five hundred pounds"; L is the distance in feet between the outer axles of the group; and N is the number of axles in the group. The formula applies to every group, not just the whole truck: the steer and drive axles together, the drives and trailer axles together, all five axles, and so on. Each group must be at or under its figure.
One exception is written into the same sentence: "two consecutive sets of tandem axles may carry a gross load of 34,000 pounds each if the overall distance between the first and last axle is 36 feet or more." Without it, the formula would allow less than 68,000 pounds on a drive tandem and trailer tandem 36 feet apart, as the second example below shows. And "in no case shall the total gross weight of a vehicle exceed 80,000 pounds."
The bridge formula calculator runs every group for a given axle layout. A few values worked by hand from the formula, as examples:
| Axles in group (N) | Distance L (ft) | Formula result | Allowed (nearest 500 lb, subject to other limits) |
|---|---|---|---|
| 2 | 4 | 500 × (8 + 24 + 36) = 34,000 | 34,000 (also the tandem limit) |
| 3 | 10 | 500 × (15 + 36 + 36) = 43,500 | 43,500 |
| 4 | 36 | 500 × (48 + 48 + 36) = 66,000 | 68,000 under the two-tandem exception, if the group is two tandems |
| 5 | 40 | 500 × (50 + 60 + 36) = 73,000 | 73,000 |
| 5 | 51 | 500 × (63.75 + 60 + 36) = 79,875 | 80,000 |
The last two rows answer the question of why a short tractor-trailer cannot always gross 80,000: with 40 feet between the first and last axle, five axles are limited to 73,000 pounds. The spacing between the steer axle and the last trailer axle has to reach about 51 feet before the formula allows the full 80,000.
Steer axles and tire loads
There is no federal 12,000-pound steer axle limit. What 658.17(f) says is a limit on states: they may not limit steering axle weights "to less than 20,000 pounds or the axle rating established by the manufacturer, whichever is lower." States also may not limit tire loads to less than 500 pounds per inch of tire or tread width, and even that kind of limit may not be applied to steering axle tires.
The manufacturer’s axle rating and the tire’s rating still bind the vehicle. 49 CFR 393.75(g) bars operating on tires that carry more than the weight marked on the sidewall, except under a state special permit at reduced speed, never above 50 mph. A steer axle is therefore usually limited by the axle and tire ratings long before 20,000 pounds.
Tolerances: there are almost none
658.17(g) is strict: the single, tandem, gross and bridge formula weights "shall be inclusive of all tolerances, enforcement or otherwise," with one exception, a scale allowance when portable wheel-load weighers are used. The rule notes such scales are generally accurate within 2 or 3 percent, "but in no case shall an allowance in excess of 5 percent be applied." It also says that a fine schedule imposing no fine up to a threshold, "i.e., 1,000 pounds," is an unauthorized tolerance. On the Interstate, 34,100 pounds on a tandem is over.
States can have tolerances in their own law for their own roads. Florida, for example, builds a 10 percent scale tolerance into the weight tables for its state weight law (Florida Statutes 316.545(2)(a)). That does not change the federal Interstate limits.
State exceptions: grandfathered limits and the LCV freeze
The 80,000-pound figure is a cap on what states must allow and may allow on the Interstate, with history built in:
- 1956 limits. Under 658.17(i), the single, tandem and gross limits do not apply to weights "legally authorized under State law on July 1, 1956." The axle-group requirement does not apply to vehicles grandfathered under state axle-group tables or formulas on January 4, 1975. These grandfathered limits stay available to a state even if it adopts a lower limit for a while.
- Longer combination vehicles. 23 U.S.C. 127(d) defines an LCV as a tractor with two or more trailers operating on the Interstate above 80,000 pounds, and allows only the LCV types a state had in actual lawful operation on or before June 1, 1991. Appendix C to 23 CFR 658 lists, state by state, the frozen weights and lengths for these combinations.
- State-specific provisions in the statute. 23 U.S.C. 127 (2023 edition of the U.S. Code) contains a long list, including: Maine and Vermont may apply their state weight laws on their Interstate highways (127(a)(11)); Louisiana may permit up to 100,000 pounds for sugarcane during harvest, up to 100 days a year (127(a)(9)); Wyoming may allow additional LCV types up to 117,000 pounds that meet the axle and bridge limits (127(d)(1)(C)); Idaho and North Dakota may allow vehicles up to 129,000 pounds on Interstate segments where state law authorizes them, if they meet the axle and bridge limits (127(t), (u)); and specified segments in several states keep the limits that applied before they became Interstate.
That is why "axle weight limit by state" and "bridge law by state" have no single answer. For the Interstate, start with the federal limits and check the state’s grandfathered and LCV provisions. For other roads, the state’s weight statute and its permit office are the reference.
Exceptions that add weight for certain vehicles
| Vehicle | What the statute allows | Source |
|---|---|---|
| Idle reduction unit (APU) | Gross and axle limits increased by the unit’s weight, up to 550 lb, with proof it works | 23 U.S.C. 127(a)(12); 23 CFR 658.17(n) still shows the older 400 lb figure |
| Natural gas or battery-electric power unit | Up to 2,000 lb over the limit on the power unit, maximum 82,000 lb gross | 23 U.S.C. 127(s) |
| Emergency vehicles (fire apparatus) | States may not enforce less than 24,000 lb single steer, 33,500 single drive, 62,000 tandem, 52,000 tandem rear drive steer, up to 86,000 gross | 23 U.S.C. 127(r) |
| Heavy-duty tow and recovery | Weight limits do not apply while towing a disabled vehicle to the nearest appropriate repair facility, if the tow vehicle weighs at least as much as the disabled one | 23 U.S.C. 127(m) |
| Fluid milk products | Treated as a load that cannot easily be divided (nondivisible) | 23 U.S.C. 127(a)(13) |
On the APU figure: the statute as printed in the 2023 U.S. Code says 550 pounds, while the CFR paragraph still says 400. The statute controls; confirm with the state if you rely on the extra weight.
Overweight permits
658.17(h): "States may issue special permits without regard to the axle, gross, or Federal Bridge Formula requirements for nondivisible vehicles or loads." A nondivisible load is one that cannot reasonably be dismantled or divided, like a single large machine. Divisible loads, such as bulk grain or steel coils that could be split, generally cannot be permitted over the federal limits on the Interstate, with narrow exceptions such as the grandfathered state permits and the emergency permits in 23 U.S.C. 127(i), which let a state permit overweight divisible loads delivering relief supplies after a presidentially declared major disaster, for no more than 120 days after the declaration.
Permit costs, routes and escort rules are state law. Each state’s permit office sets them; there is no federal fee.
Worked example: a standard 5-axle tractor-trailer
Example, applying 658.17. Steer axle 12,000 lb, drive tandem 34,000 lb, trailer tandem 34,000 lb: 80,000 gross. The axles are: steer, drive tandem with axles 4 feet 3 inches apart, and trailer tandem with axles 4 feet 1 inch apart. From the first drive axle to the last trailer axle is 38 feet, and from the steer axle to the last trailer axle is 52 feet.
- Steer axle: 12,000 is under 20,000, and assume under the axle and tire ratings. Legal.
- Each tandem: 34,000 on axles 40 to 96 inches apart. Legal.
- Drive plus trailer tandem (4 axles, 38 ft): the formula gives 500 × (152/3 + 48 + 36) = 500 × 134.67 = 67,333, which rounds to 67,500, but the two-tandem exception allows 34,000 each at 36 feet or more. 68,000 on the group is legal.
- All five axles (52 ft): 500 × (65 + 60 + 36) = 80,500, capped at 80,000. Legal at 80,000.
Same truck with the trailer tandems slid forward so the drive-to-trailer spacing is 33 feet: the two-tandem exception no longer applies, and the four-axle group is limited by the formula to 500 × (44 + 48 + 36) = 64,000 pounds. The load is now 4,000 pounds over on that group even though every axle and the gross are within limits. This is the most common way a legal gross weight still gets a ticket.
Common mistakes
- Assuming 80,000 is always allowed. The gross is also limited by the bridge formula over the outer axles.
- Looking only at axle groups, not combinations of groups. Every run of consecutive axles is checked.
- Counting on a tolerance. On the Interstate, the limits include all tolerances except a portable scale allowance of up to 5 percent.
- Quoting a 12,000-pound federal steer limit. There isn’t one; the steer axle is limited by the 20,000-pound single axle rule and the manufacturer’s axle and tire ratings.
- Treating a divisible load as permittable. Federal permits are for nondivisible loads.
For how scales enforce these limits and who has to stop, see weigh stations explained; for station locations, the weigh station directory.