The whole thing is one lever
A trailer is a beam resting on two supports: the axle and the ball. Anything you set on it splits its weight between those two supports in proportion to where it sits. Put an item directly over the axle and the axle takes all of it. Put it exactly over the ball and the ball takes all of it. Anywhere in between, the ball takes a share equal to the distance from the axle divided by the distance from the axle to the coupler.
That is the entire calculation. An item of weight W sitting a distance d ahead of the axle centreline, on a trailer whose coupler is L ahead of that axle, puts W multiplied by d divided by L onto the ball. Sum that over everything on the deck, add whatever the empty trailer already puts on the ball, and you have your tongue weight. Something behind the axle produces a negative number and takes weight off the ball, which is why a heavy item loaded at the tail is far more damaging than its weight suggests.
The useful consequence is that the sensitivity is easy to read. A 500 lb item on a trailer with a 110 inch coupler length changes the tongue weight by about 55 lb for every foot you slide it. A 2,000 lb item changes it by 218 lb per foot. That is why the answer to a badly balanced trailer is almost always to move the one heavy thing rather than to shuffle the small ones.
Ten to fifteen percent, and why the bottom of that band matters more
The figure usually quoted for a conventional bumper-pull trailer is 10 to 15 percent of the loaded trailer weight on the ball. Treat that as a working target rather than a rule handed down from anywhere; the trailer manufacturer publishes a figure for their trailer and that one wins. But the band is not symmetric in its consequences, and this is the part worth internalising.
Too much tongue weight is a capacity problem. It comes out of the tow vehicle payload, it lands mostly on the rear axle, it lifts the nose and it degrades steering and braking. All of that is bad and all of it is visible before you leave the driveway, because the truck squats and you can see it.
Too little tongue weight is a stability problem, and it is invisible at low speed. A trailer whose centre of mass sits at or behind the axle has no restoring tendency. A gust, a truck passing, a lane change, a crown in the road, and the trailer begins to yaw. If the loading is light enough on the tongue, each swing is bigger than the last. The oscillation builds, it couples into the tow vehicle, and the driver discovers the problem at the moment there is nothing useful left to do about it. Every year this puts trailers and the vehicles towing them into ditches and into oncoming traffic. It is the single reason to care about the number this page produces.
Getting the distances right
Two measurements cause most of the error. The first is the coupler-to-axle distance on a tandem trailer: use the midpoint between the two axles, not the front one. The second is where the item actually balances. People measure to the front face of a load because it is the easy edge to find, and for anything long and dense that puts the answer out by half the item length, which on a 6 foot crate is three feet of error and can be several hundred pounds of tongue weight.
For a uniform box, drum or pallet, the centre of gravity is the geometric middle and measuring to it is easy. For anything with concentrated mass at one end — a machine with an engine on it, a mower with the deck forward, a boat, a car — find where it balances rather than assuming. On a vehicle you are loading, most of the mass sits over the engine, and a front-engine car loaded nose-first behaves nothing like the same car loaded backwards.
The calculator will let you enter a position that is off the deck, and flags it when the suggested move goes there. That flag is worth reading rather than dismissing, because it means one item cannot fix the balance and you need to rearrange more than one thing.
What the deck will not tell you
| Factor | Effect on this arithmetic |
|---|---|
| Fuel, water, propane on the trailer | Real weight in a fixed position — enter each tank as its own item |
| Tank contents sloshing | Not modelled. Moving liquid shifts the balance dynamically, and a part-full tank is worse than a full one. |
| Weight-distributing hitch | Redistributes tongue load between the tow vehicle axles. It does not change the tongue weight, and it does not fix a rearward load. |
| Coupler height and tow vehicle squat | Changes the trailer attitude and therefore the real load split slightly. Level is what the numbers assume. |
| Suspension, tires, coupler condition | Not modelled at all. A correct tongue weight on worn equipment is still a bad trailer. |
Once the load is where you want it, weigh it. A tongue scale is cheap, a public scale costs a few dollars, and either of them settles the question in a way that a set of measured distances never quite can. Then take the result across to the towing and payload calculator, because tongue weight is payload on the tow vehicle and it has to fit inside a rating there as well. If you want to know what the trailer axles are doing with the rest of it, that is the trailer axle load calculator.
Questions people ask
How do I measure tongue weight without a scale?
The direct method is a tongue-weight scale that sits under the coupler, which is the cheapest reliable answer and worth owning if you tow regularly. Without one, a public scale gives it to you in three weighings: the tow vehicle alone, the whole rig, and then the rig with the trailer coupled but its wheels off the platform. The differences give you actual tongue weight and actual axle loads at the same time, which is more information than the tongue scale gives you. The bathroom-scale-and-lever arrangements that get passed around online can work, but the ones that use a beam and a pivot multiply small measurement errors along with the force, and a household scale is being used well outside what it was built for. Whatever method you use, measure at the coupler height the trailer will actually tow at, because a trailer that sits nose-high or nose-low reads differently.
Why does the calculator ask for the empty tongue weight separately?
Because the trailer itself is not balanced over its axle, and the amount it is out by varies enormously between designs. A utility trailer with a long tongue, a jockey wheel, a toolbox and a spare on the A-frame might sit at 10 to 12 percent of its own weight on the ball before anything goes on the deck. A tilt deck or a car hauler with the axles set well back can be higher. Ramps stored at the tail pull the other way. There is no way to derive that number from the trailer dimensions, so it has to be measured once with the trailer empty, and then it applies to every load you ever put on it. Do it once with a tongue scale and write it on the trailer.
Does a weight-distributing hitch let me load the trailer more toward the back?
No, and this is a common and dangerous misreading of what the hardware does. A weight-distributing hitch uses spring bars to transfer part of the tongue load forward onto the tow vehicle front axle and rearward onto the trailer axles, which restores the front axle load, the steering feel and the headlight aim that a heavy tongue took away. It works on the distribution of an existing tongue weight. It does nothing whatsoever about a trailer whose centre of mass is too far back, because that is a property of the trailer and its load and it exists whether or not there is a hitch attached. Sway control devices, similarly, add friction that damps small oscillations; they do not create the restoring force that adequate tongue weight provides. If the loading is wrong, fix the loading.
What tongue weight percentage should a fifth wheel or gooseneck run?
Higher, and this page is not the right tool for it. Fifth wheel and gooseneck arrangements put the pivot over or ahead of the tow vehicle rear axle rather than behind it, which changes the stability problem completely and allows a much larger share of the trailer weight to sit on the tow vehicle — commonly a good deal more than the 10 to 15 percent quoted for a bumper pull. The consequence is that pin weight eats tow vehicle payload very quickly and the binding constraint moves to the truck rear axle rating. The moment arithmetic on this page still describes where the weight goes on the trailer, but the target percentage does not carry across, and the trailer manufacturer figure is the one to use. Check the trailer plate and the towing supplement together.
The trailer swayed once and then settled. Is that fine?
It is a warning rather than a result. Sway that damps out on its own means the loading is marginal rather than adequate — the same trailer at a higher speed, on a worse crown, with a passing truck, or after somebody moved a toolbox, is the trailer that does not settle. Treat a single episode as the instruction to stop, look at where the weight is sitting, and move it forward. Speed matters here as well: the speed at which oscillation stops damping and starts growing is a real threshold for a given loading, and it is often not far above normal highway speed. The honest answer to a trailer that swayed is to re-load it, weigh the tongue, and inspect the tires and the coupler while you are stopped anyway.