Kingpin setting and fifth wheel position calculator

This kingpin setting calculator returns the kingpin to rearmost axle (KPRA) distance for your trailer setup and checks it against the California limit or another state's limit that you enter.

Some states limit how far the trailer's rear axle may be from the kingpin, because a long gap makes the trailer cut corners wider and harder to manage on tight roads. California is the best known. A 53 foot trailer with the tandem slid all the way back can be over the limit, so drivers set the tandem before entering the state.

How it works

KPRA = trailer length - kingpin setback - distance from the trailer rear to the centerline of the rearmost axle

Enter the trailer length in feet, the kingpin setback from the trailer front (often about 36 inches, but measure yours), and the distance from the back of the trailer to the rearmost axle. Choose the California rule or enter another state's limit.

The California rule

Caltrans summarizes the California rule this way: the kingpin to rearmost axle distance of a semitrailer may not exceed 40 feet for semitrailers with two or more axles, or 38 feet for those with one axle. Some routes carry additional restrictions, posted by signs.

Source: Caltrans, Vehicle Lengths, checked October 2026

Other states set their own limits, and many are different from California's. The calculator lets you enter a state's limit; the default for another state is only an EXAMPLE. Check the state's rule for the route you will run.

A worked example

A 53 foot trailer has its kingpin 36 inches from the front. With the tandem slid back, the rearmost axle centerline sits 96 inches from the rear.

• KPRA: 636 - 36 - 96 = 504 inches, or 42 feet. Over California's 40 foot limit by 2 feet.
• Slide the tandem forward 24 inches, so the rearmost axle is 120 inches from the rear: 636 - 36 - 120 = 480 inches, exactly 40 feet.

Moving the tandem forward also moves weight from the trailer axles toward the tractor's drive axles. Check axle weights at a scale after any slide.

Kingpin setting vs fifth wheel position

These are two different adjustments:

• The trailer tandem slide changes KPRA and shifts weight between the trailer axles and the tractor.
• The fifth wheel slide on the tractor shifts weight between the steer axle and the drive axles, and changes overall length and turning.

Both affect axle weights, so they are usually set together at a scale. Our guide to the king pin on a truck explains the kingpin and fifth wheel coupling, and the 18 wheeler truck tire position chart shows how the axles and positions are numbered.

Before you slide

Check that the locking pins are fully engaged after every slide, and do a tug test. A tandem that is not locked can slide under braking. The pre-trip inspection checklist includes the coupling checks.

Related tools

Brakes on the trailer axles carry more of the load when the tandem is forward; the brake adjustment limit checker checks pushrod stroke against the federal limits. Speed matters for handling a long trailer too; the speed limiter impact calculator shows the time and fuel trade at different speeds.

Axle weights after a slide

Any slide changes weight on the axles. Federal and state limits apply to each axle group as well as to gross weight, and a load that is legal with the tandem in one position can be over on the drives or the trailer tandem in another. After setting the tandem for KPRA, weigh the truck at a certified scale. If the drives are heavy, sliding the fifth wheel can move weight to the steer axle, within the steer axle and tire ratings. Small adjustments, weighed each time, are better than big guesses.

Measuring the setup

Measure once and mark it. Many drivers paint or tag the tandem hole that gives a legal KPRA for their trailer, so they can set it quickly before entering a state with a limit. Re-measure if you change trailers, because kingpin setback and tandem rails differ between trailers.

Where we fit

We plan loads for 26 ft box trucks and larger, including 53 foot trailers into states with KPRA rules. We find and negotiate the load, you say yes, counter or no, and the broker sends the rate confirmation to you to sign. Single-truck carriers can read how we dispatch owner-operators.