How to weigh your rig at a weighbridge

A few weighbridge readings, hitched and unhitched, give you every figure you need to check GVM, GCM, ATM, GTM, axle loads and tow ball mass. Here is how to take them.

Why weigh at all

Brochure figures, tare plates and packing lists only estimate what your rig weighs today. A weighbridge measures it. Transport for NSW advises weighing your caravan and vehicle, and reweighing whenever you add weight.

Public weighbridges operate under the National Measurement Act 1960, and the National Measurement Institute publishes a map of them. Many are at transport depots, quarries and waste centres. Call ahead: hours, prices and what they will let you do vary. Ask whether the bridge is long enough for the whole rig, and whether you can weigh one axle at a time. Transport for NSW also suggests confirming the weighbridge is calibrated. Mobile weighing services exist too.

Before you go: load it as you travel

A reading only means something if the rig is loaded the way it will be on the road:

Full fuel, including any jerry cans you carry
Water tanks filled as you travel. Water and waste tanks are not part of a caravan's tare, so every litre is load.
Gas bottles, batteries and all fitted accessories
All gear packed, in the places it normally goes. Position changes axle loads and tow ball mass, not just the total.
Passengers. Take them with you, or weigh them separately and add them. Transport for NSW's instructions for GVM and GCM include the vehicle's occupants.

If you use a weight distribution hitch, weigh with it set up the way you tow. It moves load between the axles, so readings taken without it describe a different rig.

The readings

Each reading answers a different question.

#	What is on the weighbridge	What it tells you	Compare with
1	Whole rig, hitched (if it fits)	The combination	GCM
2	Vehicle only, hitched (trailer wheels off)	Vehicle including tow ball mass	GVM
2a	Vehicle front axle only, hitched	Front axle load	Front axle limit
2b	Vehicle rear axle only, hitched	Rear axle load	Rear axle limit
3	Trailer wheels only, hitched (vehicle off)	Trailer axle load	GTM
4	Trailer only, unhitched, on its jockey wheel, all on the bridge	The whole trailer	ATM
5	Vehicle only, unhitched	Vehicle without the trailer	For working out tow ball mass

If the bridge is too short for reading 1, add readings 2 and 3 together.

Working out tow ball mass

You can get tow ball mass two ways from these readings:

Reading 4 minus reading 3: the whole trailer less what its axles carry when hitched
Reading 2 minus reading 5: the vehicle hitched less the vehicle on its own

Both work only with any weight distribution hitch released for readings 2 and 3, because the hitch shifts load between the vehicle and the trailer's axles. Take those two readings twice: once released, to work out tow ball mass, and once set up as you tow, to check against the limits.

Jockey wheel mass is not tow ball mass

A common shortcut is to unhitch and put only the jockey wheel on a scale. RACQ describes measuring ball load this way. The WA Department of Transport says to weigh the trailer at the coupling point, with the coupling at the height it sits when hitched and loaded. Transport for NSW suggests a tow ball scale.

They differ because the jockey wheel and the coupling are at different points on the drawbar. The trailer pivots on its axles. The jockey wheel usually sits behind the coupling, closer to those axles, so the same trailer presses down harder on it. Invented figures: if the coupling is 4.0 m ahead of the axles and the jockey wheel 3.6 m, a jockey wheel reading comes out about 11% higher than the load at the coupling. A drawbar sitting at the wrong height adds more error.

A jockey wheel reading is a useful check. It is not the same figure as tow ball mass, so treat it that way.

After the weighbridge

Compare each reading with its limit from the vehicle's and trailer's plates and manuals. If something is over, move load before you remove it: shifting heavy items toward the trailer's axles, or forward of the vehicle's rear axle, often fixes an axle or tow ball problem without losing anything. See front and rear axle limits.

The rules on overloading, and how they are enforced, are set by each state and territory. Check with your state road authority for the rules that apply to you.

Model your own rig

LoadSavvy's Load check can start from your weighbridge readings and model what changes when you move or add a load, before you go back and weigh again. It is a planning tool, not a certified measurement. The weighbridge is the measurement.

Model your rig in the Load check →