Axle weight calculator: drag the load and see if it is legal
The total weight can be legal and the load still illegal: what gets fined at the weighbridge is the weight on each axle, and that depends on where the items sit. Enter the load, drag it along the deck and watch it move. If the items are identical and go stacked, set the tiers and the stack counts as one footprint carrying all of them. Three rulebooks in one tool: Spanish domestic, international (96/53/EC) and US federal, where the bridge formula is checked on every group of consecutive axles — which is where a legal-looking truck usually fails.
Items going on the vehicle
| Description | Length (mm) | Width (mm) | Height (mm) | Qty | Weight ea. (kg) | Tiers | Remove |
|---|
Why the total weight is not enough
A combination can run at 39,000 kg, under the maximum, and still put 13,500 kg on the driving axle. That is an offence, and you cannot see it on the paperwork: you can only see it by looking at where the load sits.
A semi-trailer rests on two points: the kingpin that couples into the tractor’s fifth wheel, and the tri-axle at the back. Every item splits its weight between those two supports according to where it sits, exactly like a plank resting on two bricks. A pallet directly over the tri-axle puts almost all of its weight there. The same pallet against the nose puts almost all of it on the kingpin, and from there onto the tractor.
That is why moving one item by a metre changes the numbers, and why this tool lets you drag it instead of asking you to type a position.
The arithmetic
For an item at distance x behind the kingpin, with the tri-axle at distance B behind the kingpin:
to the tri-axle = P × x / B to the kingpin = P × (B − x) / B
Everything reaching the kingpin is then split between the tractor’s two axles according to the wheelbase and how far forward the fifth wheel sits. Add the tares and you have the weight on each axle.
It is the statics of a beam on two supports. It does not model the suspension, the split between the three axles of the tri-axle group, or road camber.
The warning almost nobody gives: 25 % on the driving axle
As well as not going over, there is a minimum. Annex I of Directive 96/53/EC, point 4.1, requires that the weight on the driving axle or axles is not less than 25 % of the total laden weight. Shift everything backwards to relieve the tractor and you lose traction: the drive wheels spin in the wet and on a gradient, and it is illegal as well.
It is the classic mistake of anyone who only checks that no axle is over.
The United States is a different rulebook: the bridge formula
Switch the limits to United States and everything changes, including the units. The federal limits on the Interstate System are 20,000 lb on a single axle, 34,000 lb on a tandem and 80,000 lb gross. There is no 25 % minimum on the drive axle — that is a European rule and this tool does not apply it there.
What there is instead is the bridge formula, and it catches trucks that pass every other test. It limits the weight on any group of two or more consecutive axles according to how far apart they are:
W = 500 × ( L N/(N−1) + 12N + 36 )
W is pounds, rounded to the nearest 500. L is the distance in feet between the first and last axle of the group and N is how many axles are in it. The logic is a bridge, not a road: axles close together concentrate their load on a short span, and a short span carries less.
The trap is that it applies to every group, not just the whole truck. Your gross can be 79,000 lb, every axle can be under, and an interior group of three axles can still be over. That is why this tool lists every group and not just the total.
Two things soften it. A group of two axles less than 8 ft apart is a tandem, so 34,000 lb governs even where the formula would allow a little more. And two consecutive tandems may each carry 34,000 lb regardless of the formula if the first and last of the four axles are 36 ft or more apart — without that exception the ordinary five-axle rig would be illegal, so it is marked exempt in the table.
This is why American drivers slide the trailer tandems. Sliding them back moves weight off the drives and onto the trailer, and at the same time it lengthens L, which raises what the formula allows. Change the tandem spread and the kingpin-to-bogie dimension here and you can watch both effects at once.
These are the federal limits. States keep grandfathered higher limits on some roads, several run higher gross weights under permit, and bridge postings can be lower than any of this. The formula tells you whether you are legal on the Interstate; it does not tell you whether you are legal on the road you are actually on.
This is a planning aid, not a weighing. The tares and dimensions it ships with are typical values: yours are on the vehicle plate. The only figure that counts against a weighbridge is the weighbridge’s.
The case it loads with
Twenty Euro pallets at 800 kg each, 16,000 kg of freight, on a standard semi-trailer. The combination comes to about 30,500 kg with tares, well under the maximum, and the tool places it in a legal position for you.
Now break it. Grab the pallets and drag them towards the nose: the driving axle climbs past its limit while the total weight does not move by a single kilo. That is the fine you cannot see on the paperwork.
Then the other way, everything to the back: the warning that fires is the other one, the 25 % minimum on the driving axle. Between the two mistakes there is a good band, and that is exactly what the automatic placement looks for.
Also flip the switch at the top to Spain with the load in place. Something illegal internationally can be legal there, because Spain allows 12,000 kg on the driving axle instead of 11,500.
Where it stops being reliable
- These are European limits. Directive 96/53/EC for international traffic, and Spanish domestic law. This is not the US Federal Bridge Formula, and it does not cover US, Canadian or Australian limits.
- It is a model, not a weighbridge. Use it to plan and to throw out a bad plan before loading, not to certify anything.
- The default tares and dimensions are typical, not yours. A reefer trailer weighs far more than a curtainsider. Put your own plate figures in and the numbers get much better.
- The tri-axle is treated as one support. How the load splits between its three axles depends on the suspension and any lift axle, and that is no longer geometry.
- It does not model 3-axle tractor units. The trailing axle changes the rear split and needs more data than a form should ask for.
- Each item is assumed evenly loaded: its weight acts at its centre. For machinery with an offset centre of gravity, it does not hold.
Questions people actually ask
Why are the Spanish limits different from the European ones?
Because Directive 96/53/EC sets the maxima for international traffic, and each country may set its own for traffic within its borders. Spain raised its own with Orden PJC/780/2025: the driving axle went to 12,000 kg and the tri-axle to 27,000 kg, with the combination at 44,000 kg.
The practical consequence is ugly: a truck loaded to the Spanish maximum can be in breach the moment it crosses the border. That is why the switch is at the top.
Does the 25 % driving-axle rule always apply?
It is point 4.1 of Annex I of the Directive, and it refers to the total laden weight of the combination. Beyond the letter of it, it is mechanical common sense: with no weight over them, the drive wheels do not grip. You notice it first on a wet hill.
Where do I find the kingpin-to-tri-axle dimension?
On the trailer’s plate or its type approval. It usually runs between 7,000 and 8,100 mm, and it is the figure that varies most from trailer to trailer. Guess it and the answer is a guess too.
Does this work for a container on a skeletal trailer?
For the weight distribution, yes — treat the container as one long item, since what matters is where its centre of gravity falls relative to the kingpin. What this tool does not do is check door height or whether the freight fits inside the container: the mixed load calculator does that.
Can I rely on it to actually load a truck?
To decide where to put the pallets before you start, yes, and it will save you trouble. To certify that the vehicle is legal, no: only the weighbridge says that. Any tool that tells you otherwise is lying to you.
Is something missing from this calculator? Tell us here