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Kingpin setting & fifth wheel position calculator

Enter the trailer length, kingpin setting and where the tandems sit. Get the kingpin-to-rear-axle distance, a legal or too-long check against California's rule or your own limit, and how sliding the fifth wheel moves weight.

By / Updated October 2026

TRAILER

ft
in
in

Changes when you slide the tandems

in

Axle center to axle center

Limit to check

FIFTH WHEEL

lb
in

Negative = toward the drives

KINGPIN TO REAR AXLERESULT
Kingpin to rearmost axle
40 ft 0 in
Kingpin to tandem center
37 ft 12 in
Limit (to rearmost axle)
40 ft 0 in
Room to slide back
0 ft 0 in
Fifth wheel forward 6 in: steer
+653 lb
Drives
-653 lb
KPRALEGAL

Dashed line: the limit. Amber: the measured distance.

Legal with 0 ft 0 in to spare. Our dispatchers ask about your trailer before booking California and other KPRA-limited lanes, and every rate con comes to you to sign or turn down.

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Two settings decide a lot about a tractor-trailer: how far the trailer's rear axles sit from the kingpin, and where the fifth wheel sits on the tractor. The first decides whether the trailer is legal in states like California. The second decides how weight splits between the steer and the drives. This calculator handles both.

How it is calculated

Kingpin to rear axle (KPRA):

  • To the rearmost axle = trailer length − kingpin setting − distance from the rear of the trailer to the last axle
  • To the tandem center = KPRA to the rearmost axle − half the tandem spread

The result is compared with the limit you choose. California's limit is 40 feet to the rearmost axle for a semitrailer with two or more axles, 38 feet for a single axle.

Fifth wheel: the kingpin load splits between steer and drives by lever arm. Moving the fifth wheel forward by a few inches moves part of that load onto the steer axle, using an example tractor and loaded trailer.

A worked example

EXAMPLE / 53 FT VAN, 36 IN KINGPIN SETTINGEXAMPLE
Tandems back: 60 in from rear to last axle
45 ft KPRA
Tandems slid: 120 in from rear
40 ft KPRA
California limit, to rearmost axle
40 ft
With the tandems slidLEGAL

With the tandems all the way back, this trailer is five feet too long for California. Sliding them forward ten feet makes it legal, and also moves weight from the trailer axles onto the drives. Check that shift with the tandem slide calculator and the result with the axle weight calculator.

Setup tips

SettingEffect
Tandems forwardShorter KPRA, more weight on the drives, tighter turning
Tandems backLonger KPRA, more weight on trailer axles, wider off-tracking
Fifth wheel forwardMore weight on the steer, longer gap behind the cab
Fifth wheel backMore weight on the drives, lighter steering

Off-tracking, tire wear and handling also depend on alignment; see truck trailer alignment. Spread axle trailers measure and load differently; see spread axle trailer. Pulling someone else's trailer means checking its setup at every hook; that's part of power only dispatch.

Results are estimates. State limits, measuring points and local restrictions vary; check each state on the route.

SOURCES

Questions about this tool

01What is the California kingpin law?

A semitrailer with two or more axles may not have more than 40 feet from the kingpin to the rearmost axle, or 38 feet with a single axle. Cities and counties can restrict it to 38 feet on their roads. On a 53 ft trailer that usually means sliding the tandems forward.

02How do I measure kingpin to rear axle length?

From the center of the kingpin straight back to the center of the rearmost axle, or to the center of the tandem in states that measure that way. With the trailer length, kingpin setting and the distance from the rear of the trailer to the last axle, the calculator works it out.

03Do other states limit KPRA?

Many do, with limits commonly around 40 to 41 feet and different measuring points. Check the rule in each state on your route; the calculator lets you enter any limit and choose whether it's measured to the rearmost axle or the tandem center.

04What does sliding the fifth wheel do?

Moving the fifth wheel toward the cab shifts some of the trailer's kingpin weight from the drive axles onto the steer axle; moving it back does the opposite. It's how you fine-tune steer and drive weights without touching the trailer tandems.

California lanes, set up right

Our dispatchers check trailer setup before booking KPRA-limited lanes. Every rate con comes to you to sign or turn down.

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