Semi Truck Axle Weight Limits in the US
Every loaded tractor-trailer in the US answers to the same three questions at the scale house: what does each axle group weigh, what does the whole rig weigh, and is the weight spread over enough distance. This guide covers the federal limits behind those questions in plain language — the numbers themselves, the bridge formula that sits underneath them, and the state-level wrinkles that catch people out.
The federal numbers
On the Interstate system, federal law sets three caps: 20,000 lb on a single axle, 34,000 lb on a tandem (two axles spaced closer than 96 inches apart), and 80,000 lb gross for the whole vehicle. Those are the legal ceilings; no state can force a lower limit on Interstate highways, and none of them let you exceed these without a permit.
For the standard five-axle tractor-trailer, those caps turn into the working numbers every driver knows:
- Steer axle — about 12,000 lb. Legally a single axle could carry 20,000 lb, but a typical tractor's steer tires are rated for roughly 6,000 lb each, and many states enforce the tire rating. Around 12,000 lb is the practical limit for most trucks — check your own tire sidewalls.
- Drive tandem — 34,000 lb. The two axles under the back of the tractor.
- Trailer tandem — 34,000 lb. The two axles under the rear of the trailer.
- Gross — 80,000 lb. Note that 12,000 + 34,000 + 34,000 is exactly 80,000: a fully loaded rig has no slack. If one group runs light, that capacity does not transfer to another group past its own cap.
An important detail: axle limits and the gross limit are enforced separately. A rig grossing a legal 76,000 lb can still be in violation because 35,500 lb of it sits on the drives. Getting under 80,000 is the easy part; getting each group under its own number is what load planning is actually about.
The federal bridge formula, in plain language
Underneath the flat numbers sits the federal bridge formula. Its job is to protect bridges from concentrated weight: the same pounds are harder on a bridge span when they press down over a short distance than when they are spread out. So the formula ties the weight any group of axles may carry to two things — how many axles are in the group and how far apart the first and last of them are. More axles and more spacing earn you more legal weight; fewer axles packed close together earn you less.
That is why an 80,000 lb gross is only legal on a combination with enough overall wheelbase, and why short combinations cap out below 80,000 even with the axles individually legal. It is also one reason sliding your trailer tandems is not purely a weight-distribution move: sliding them back stretches the distance between axle groups, which the bridge formula rewards, while some states cap how far back they may go (see the kingpin-to-tandem notes in our guide to sliding tandems). For everyday van and reefer work the flat limits are the ones you will bump into first; the bridge formula mostly bites on short-wheelbase setups, multi-axle permits and spread-axle trailers.
Where the weight actually sits
A trailer is essentially a beam resting on two supports: the kingpin (which feeds the tractor's drive tandem through the fifth wheel) and the trailer tandems. Freight loaded toward the nose presses on the drives; freight toward the doors presses on the trailer tandems; the steer axle mostly carries the tractor itself. Two loads with identical gross weight can scale thousands of pounds apart on the drives purely because of where the pallets sit. That is the whole game of load planning — and it is what a trailer load planner with an axle weight calculator lets you check before the truck is loaded, instead of at the scale.
State variance and permits
Off the Interstate system, states set their own rules, and they differ more than most people expect: some allow heavier gross weights on state routes, some enforce kingpin-to-tandem length limits (California's roughly 40-foot rule is the best known), and axle limits on local roads can be lower during spring thaw in northern states. Oversize/overweight permits change the math entirely — divisible loads generally cannot be permitted heavier, while non-divisible loads can, with conditions. Treat the federal numbers as the baseline that is true on the Interstate, and check the specific states on your lane before you count on anything beyond them.
What being overweight costs
Fines vary widely by state — some charge a flat amount, many charge per pound over, and the rate typically climbs with the size of the overage. Beyond the fine, an overweight rig may be put out of service until the load is reworked or partially off-loaded, which usually costs more in time and rework than the ticket itself. The conservative habit is simple: plan the load with headroom on every axle group, and verify on a certified scale before heading down the road. These figures are commonly cited working numbers, not legal advice — the scale ticket and your state's statutes are what govern.