The actual math (so you can sanity-check it)
Your trailer is a beam resting on two supports: the kingpin, held up by the tractor's fifth wheel, and the tandem center, held up by the trailer's own axles. Everything the trailer weighs — the box, the floor, the freight — sums to one resultant weight, call it W, acting at one point some distance c behind the kingpin. You never have to find W or c. They cancel, which is the entire reason this can be solved from a scale ticket and a tape measure.
Take moments about the kingpin. The trailer is sitting still and not rotating, so the load's moment about that point equals the tandems' moment about the same point: W × c = T × d, where T is what the tandems scale and d is the kingpin-to-tandem-center distance. Rearrange and you have T = W·c ÷ d. Read that slowly, because it contains the whole answer: tandem weight is inversely proportional to how far back the tandems sit. A support farther from the pivot needs less force to hold the same load — a longer lever arm doing the same job.
Now slide. Pulling the pins doesn't move the freight's center of gravity; it moves the support. W and c are untouched, so d is the only term that changes. Slide n holes of spacing s rearward and the distance becomes d + n·s, giving Tnew = W·c ÷ (d + n·s), which is the same as T × d ÷ (d + n·s). Slide forward and the sign flips to d − n·s. That substitution is the calculator, in one line.
Whatever leaves the tandems arrives at the kingpin, because the two reactions still have to add back up to W. Say that plainly: sliding never removes a pound from the truck. It only chooses which group carries them. If your gross is over 80,000, sliding cannot touch it, and the tool says so instead of pretending a slide is a fix.
A worked example, step by step
Tandems 35,200 — 1,200 over the 34,000 tandem limit — drives 32,400, kingpin to tandem center 36 ft, 6-inch holes. The calculator tries one hole at a time and stops at the first position where both groups land at or under 34,000. Here is what it steps through:
- One hole rearward, d becomes 36.5: 35,200 × 36 ÷ 36.5 = 34,718. Still over.
- Two holes, d = 37: 35,200 × 36 ÷ 37 = 34,249. Still over.
- Three holes, d = 37.5: 35,200 × 36 ÷ 37.5 = 33,792 lb. Under. The transfer is 35,200 − 33,792 = 1,408 lb, and it goes to the tractor, putting the drives near 32,400 + 1,408 = 33,808.
Now notice how narrow that is. At three holes the tandems read 33,792 and the drives 33,808 — 208 lb of total margin, split between two groups. Take a fourth hole and d becomes 38: the tandems fall to about 33,347, but the drives climb to about 34,253, and you have swapped one overage for the other. On this load, three holes isn't the best of several workable positions. It is the only workable position.
That squeeze is why the tool marks any predicted group above 33,500 lb — anything within 500 lb of the limit — as tight rather than presenting it as a comfortable result. A number that lands 200 lb under a limit is a prediction, not a clearance.
Why there is no universal pounds-per-hole figure
Subtract Tnew from T for a single hole and the transfer works out to T·s ÷ (d + s), which for the first hole is very close to T·s ÷ d. Both s and d are properties of your particular trailer, and T is a property of today's load, so the per-hole answer is yours, not the industry's. Three concrete setups, all run through the formula above:
- 4-inch holes, tandems 42 ft behind the kingpin, carrying 30,000 lb — the first hole moves about 236 lb.
- 6-inch holes, tandems 36 ft back, carrying 34,000 lb — about 466 lb.
- 6-inch holes, tandems already well forward at 24 ft back, carrying 34,000 lb — about 694 lb.
Two of those three sit between 250 and 500 lb. The third is nearly three times the first, on identical math, purely because d is smaller. That is the trap in any flat per-hole figure: it goes furthest wrong exactly when you are sliding hardest, with the tandems far forward and every hole biting deeper than the flat number claims.
The same inverse relationship is why your measurement of d is worth getting right. Rerun the example with d guessed at 33 ft instead of 36 and three holes predicts 33,670 rather than 33,792. Guess 39 and you get 33,896. Roughly a hundred pounds of answer rides on a three-foot guess, and the tool has no way to tell that you estimated. Measure it once and write it in the permit book.
The limits the tool compares you against
The three ceilings in the results table come from the federal rules for the Interstate system. FHWA states them briefly: single axles are limited to 20,000 pounds; axles spaced more than 40 inches and not more than 96 inches apart — the definition of a tandem — are limited to 34,000 pounds; and gross vehicle weight is limited to 80,000 pounds. On a conventional tractor-trailer your drive group and your trailer group both meet that spacing definition, which is why both are measured against the same 34,000 and why a slide is a trade between two equals.
Your steer axle is a single axle, so the federal ceiling above it is 20,000 — but that ceiling is not necessarily the number that binds you. The lower of your front axle rating and your tire rating can sit well below it, and those figures are stamped on the equipment rather than written in the regulation. The calculator asks for the steer weight only to total your gross; it makes no judgment about the steer group itself. Off the Interstate system, states set their own limits and permits can raise or lower any of these, so treat the three numbers on this page as the baseline case, not the universal one.
Two of the tool's answers deserve a sentence each. "No slide needed" means both groups already read at or under 34,000 with the tandems where they are — it is a statement about two numbers on a ticket, not an endorsement of the setup. "Sliding can't fix this one" appears when both groups are over at once. That situation is unreachable by sliding for the reason established above: the total is fixed, so pulling weight off one group necessarily pushes it onto the other. The freight has to move inside the box, or come off it.
Fixing the drives can create a bridge-formula problem
This is the failure mode that catches people, and it only shows up in the other direction — when the drives are over and the fix is a forward slide.
Federal weight rules are not just three group numbers. FHWA's bridge formula, W = 500(LN/(N−1) + 12N + 36), caps the weight allowed on any group of two or more consecutive axles, where L is the distance in feet between the outer axles of the group and N is the number of axles in it. Your drive tandem and your trailer tandem together form a four-axle group, and there is a specific exception written for it: two consecutive sets of tandem axles may each carry 34,000 lb — 68,000 combined — provided the overall distance between the first and last axles of those tandems is 36 feet or more.
Watch what a forward slide does to that. Both trailer axles move forward, so the outer distance shortens by exactly what you slid: half a foot per 6-inch hole. Sitting at 36 ft, the exception applies and the group may carry 68,000. Slide two holes forward and L is 35 ft, the exception no longer applies, and the raw formula governs: 500 × (35 × 4 ÷ 3 + 48 + 36) = 65,333, which FHWA takes to the nearest 500 as 65,500. One foot of slide dropped that group's allowance by 2,500 lb, while each individual tandem can still be sitting at or under 34,000 and this page's calculator sees nothing wrong at all.
It sees nothing wrong because it never asks for that span. If a forward slide is what balances your drives, check the group distance separately — our bridge formula calculator tests every consecutive axle group rather than only the outer span.
What about the fifth wheel?
Sliding the fifth wheel applies the same lever idea to the tractor's wheelbase instead of the trailer's: forward loads the steers, rearward loads the drives. The total the tractor carries doesn't change at all — you are only choosing how to divide it between two groups. This page does not predict that division, because doing it honestly needs your tractor's wheelbase and fifth-wheel travel, and the form asks for neither. Move it in small increments and let the scale settle the argument.
Two different 36-foot measurements
Worth flagging before you confuse them, because both appear on this page and both happen to be 36 in the examples above. The d in the lever formula runs from the kingpin to the midpoint between the two trailer axles. The L in the bridge exception runs from the first drive axle to the last trailer axle. Different endpoints, different purpose, and they do not change by the same amount when you slide — d and L each shift by exactly the distance you slid, but they start from completely different baselines. Substituting one for the other will give you a confidently wrong answer.
To get d, measure from the kingpin straight back to a point halfway between the trailer's two axle centers, with the tandems where they sit right now. Do it once, note which hole the pins are in, and after that you can compute d for any other hole by adding or subtracting the spacing — you never need the tape again on that trailer. Pacing it off is better than guessing, and both are worse than measuring.
What this calculator cannot see
- A load that moves. The whole derivation rests on W and c staying put. Bulk liquid, loose pallets, a partial that walked forward under hard braking, freight a lumper restacked — once the center of gravity shifts, the lever arm you solved for is not the one under the trailer anymore.
- Side to side. A platform scale reports one number per axle group, not per wheel. A group can read 33,000 with one side carrying noticeably more than the other, and neither the ticket nor this page will show it. Tire and suspension ratings, though, are per side.
- Your steer rating. Steers are governed by the lowest of the axle rating, the tire rating, and whatever single-axle rule applies where you are — none of which the form asks for. That is why the steer figure here feeds the gross total and nothing else.
- Whether the pins seated. Every number above assumes the tandems ended up where you counted them. Pins that look set and are not are a mechanical problem arithmetic cannot detect.
- Route rules. Kingpin-to-rear-axle limits, overall length, and overhang can all bind before weight does, and none of them appear in this model.
- Anything past twelve holes. The search stops at twelve. Beyond that it tells you the freight itself needs to move rather than quietly extrapolating a slide you don't have room for.
The re-weigh is the only measurement
Everything on this page is a prediction built from three numbers you typed and one distance you probably estimated, pushed through a model that treats two axles several feet apart as a single support point and a mixed load as one weight at one point. Those simplifications are reasonable. They are not zero. The scale is a measurement; this is arithmetic about a measurement.
So use it the way it is meant to be used — to decide which way to pull the pins and roughly how far, so you spend one trip across the scale instead of four. Then go read the ticket. If the result came back where the tool said, you have learned something durable about your own trailer that will make the next load faster. If it didn't, the ticket is right and the model was wrong, and the size of the gap is a clue about which input to correct.
Nothing here tells you that you are legal, correctly loaded, or clear to roll. It tells you what your own numbers do when the support moves. What you do with that is between you, the scale, and the rules on your route.