Calculate kingpin to rear axle (KPRA) distance, check it against California’s 40 ft king pin law or another limit, or find slider holes to move forward.
Kingpin to Rear Axle Formula
The calculator works three ways. When you enter trailer dimensions, it finds the kingpin to rear axle distance from the trailer length, the kingpin setback, and the rear overhang:
KPRA = L - (S + O) / 12
When you check a measured distance against a limit, it reports how much room you have:
Margin = (Limit - KPRA) * 12
When you plan a tandem slide, it finds required forward holes. Weight transfer appears only if you supply a loading-specific per-hole estimate:
Holes = max(0, roundup( (KPRA - Limit) * 12 / H )) Weight shift = Holes * W
- KPRA = kingpin to rear axle distance, in feet
- L = trailer length, in feet
- S = kingpin setback, from the front of the trailer to the center of the kingpin, in inches
- O = rear overhang, from the measurement point required by the selected rule to the rear of the trailer, in inches
- Limit = the state or route KPRA limit you must meet, in feet
- Margin = clearance in inches, where a negative value means you are over the limit
- H = slider hole spacing, in inches
- W = weight shifted per hole, in pounds
- Holes = number of holes to slide the tandems forward, rounded up
The first mode turns a tape measure or a spec sheet into a KPRA number. Because the trailer length is in feet and the two offsets are in inches, the formula divides the offsets by 12 before subtracting. The second mode compares that number to a limit and returns the inches of margin, so you can see at a glance whether you pass and by how much. The third mode is the planning step: it divides the amount you are over the limit by the rail hole spacing, rounds up to a whole hole, and multiplies the holes by an approximate per-hole weight to show how the load moves between your drives and your trailer tandems.
KPRA Distance References
States and routes use different axle reference points and vehicle conditions. California Caltrans lists 40 ft for two or more rear axles and 38 ft for a single axle, measured to the center of the rearmost axle. Other numerical choices are reference values; confirm the rule and endpoint for the exact vehicle and route. See Caltrans truck lengths and routes.
| Reference | Distance | Measurement point and scope |
|---|---|---|
| California, 2 or more rear axles | 40 ft | Center of rearmost axle; vehicle and route conditions apply |
| California, single rear axle | 38 ft | Center of that axle; vehicle and route conditions apply |
| Numerical references | 41 or 43 ft | Verify endpoint and route; former state groups mixed axle and group references |
| Numerical references | 45 ft 6 in or 42 ft 6 in | Verify endpoint and highway class |
| Numerical reference | 40 ft 6 in | No statewide legal classification is inferred |
Sliding the rear axle assembly forward shortens the distance to a fixed point on that assembly and changes axle loads. A distance-only calculation does not establish route legality, bridge-weight compliance or available rail travel.
| Rail hole spacing | KPRA change per hole | Approx weight shift per hole |
|---|---|---|
| 4 inch rail | 4 in | User-supplied estimate |
| 6 inch rail | 6 in | User-supplied estimate |
Hole spacing alone does not determine weight transfer. Enter a loading-specific estimate if useful, and re-weigh after moving. Blank weight input omits this output.
Example Problems
Example 1. You have a 53-foot dry van with a 36-inch kingpin setback and an 84-inch overhang measured from the same rear reference point. The kingpin to rear axle distance is:
KPRA = 53 - (36 + 84) / 12 = 53 - 10 = 43 ft
Against California’s 40-foot limit, 43 feet is 3 feet, or 36 inches, too long.
Example 2. You need that trailer with a 43-foot KPRA to meet the 40-foot California limit. You must lose 3 feet, which is 36 inches. On a 6-inch rail:
Holes = 36 / 6 = 6 holes forward New KPRA = 43 - (6 * 6) / 12 = 40 ft
Sliding forward 6 holes gives a calculated 40-foot distance to the chosen reference. If a loading-specific estimate is 400 lb per hole, estimated transfer is 2,400 lb from the drives to the trailer tandems. Verify rail travel, pin engagement, measured KPRA and scale weights after moving.
Frequently Asked Questions
What is a legal kingpin to rear axle distance? There is no universal limit or measurement point. California options use the center of the rearmost axle. Other routes can use a group center or a particular axle and impose vehicle, trailer-length, route or weight conditions. Being within a selected distance is not legal clearance.
How do you measure kingpin to rear axle? Use the center of the kingpin and the exact rear reference point named in the applicable rule. California uses the center of the rearmost axle; a tandem midpoint is different. Use the same reference for a measured KPRA, rear overhang and comparison limit.
How do you shorten your KPRA to get legal? Slide the trailer tandems forward. Each hole you move forward shortens the kingpin to rear axle distance by the rail spacing, usually 6 inches, so a few holes can cover a few feet quickly. The same move shifts weight off your drives and onto your trailer tandems, so plan the slide and your scale weights together, then weigh again before you leave the yard.
