The numbers on your trailer's plate
Check the trailer's identification plate, registration information and manufacturer specifications for its ratings. These terms explain the calculation:
| Term | Means | Typical 6×4 |
|---|---|---|
| Tare | The weight of the empty trailer | 200–275 kg |
| ATM (aggregate trailer mass) | The most the trailer may weigh with its load, including the part carried on the tow ball | 750 kg |
| GTM (gross trailer mass) | The maximum loaded mass supported through the trailer's wheels when coupled; the tow ball carries part of the total mass | Check rating |
| Payload | What you can put in it: ATM minus tare | 475–550 kg |
estimated load (m³) = min( tray volume, payload ÷ ( density (t/m³) × 1,000 ) )
trips = ⌈ order (m³) ÷ estimated load (m³) ⌉
The estimate requires positive dimensions, density and available payload. Brake requirements depend on the trailer's ratings and the applicable rules, including GTM in Australia's VSB1. Check the current guidance and use the correct braked or unbraked towing limit from your vehicle manufacturer.
Why a 6×4 trailer holds less soil than you think
A 1.8 × 1.2 m tray with 30 cm sides holds 0.648 m³ level with the top. With a 500 kg model payload, these assumed densities give the following estimates:
| Material | Assumed density | Estimated load | Depth in tray | Limited by |
|---|---|---|---|---|
| Moist garden soil / topsoil | 1.3 t/m³ | 0.38 m³ | 18 cm | Weight |
| Sand, gravel, blue metal | 1.5 t/m³ | 0.33 m³ | 15 cm | Weight |
| Road base | 1.7 t/m³ | 0.29 m³ | 14 cm | Weight |
| Scoria | 1.0 t/m³ | 0.50 m³ | 23 cm | Weight |
| Compost | 0.6 t/m³ | 0.65 m³ | 30 cm (full) | Space |
| Bark or wood-chip mulch | 0.35 t/m³ | 0.65 m³ | 30 cm (full) | Space |
For this example, a level tray of soil at 1.3 t/m³ weighs about 842 kg before the trailer's tare is added, exceeding the 500 kg payload by about 342 kg. Tray appearance cannot establish a permitted load. Give the loader operator the applicable weight limit and arrange a measured part load.
Your car has a limit too
The trailer's plate is only half the story. Your vehicle's manufacturer sets a maximum towing capacity, and a lower unbraked figure for trailers without brakes. Some small cars and hatchbacks are rated below 750 kg unbraked. If yours is, the car's figure is the limit on the loaded trailer, not the trailer's ATM. Enter it in the calculator and the payload drops to match.
- Tow ball load. Follow the vehicle and trailer manufacturers' specified loading and tow ball limits. Measure the actual load; this calculator does not determine weight distribution or a suitable tow ball percentage.
- Vehicle payload. The tow ball weight counts against your car's own carrying limit, together with passengers and anything in the boot.
- Combined limits. Check axle and tyre ratings, towbar limits, the vehicle's gross vehicle mass (GVM) and gross combination mass (GCM). A load within the calculator's estimate can still exceed one of these limits.
- Tyres and bearings. A small trailer carrying its full rating is working hard. Check the tyre pressures and wheel bearings before a day of soil runs.
In New Zealand
New Zealand uses its own towing and loading rules. Check the current NZTA guidance, your trailer's rated gross mass and the vehicle manufacturer's applicable towing limit. Enter the trailer's rated gross mass in the ATM field and the applicable vehicle limit in the towing field. Brake, licence and combination requirements still need separate checks.
Securing and covering the load
Restrain loose material so it cannot shift, fall or blow out. Choose a suitable cover and restraints for the material and trailer, and follow the applicable local rules and the National Transport Commission's load restraint guidance. A volume estimate does not check restraint or load distribution.
Worked example: 2 m³ of garden mix
Use a 1.8 × 1.2 × 0.30 m tray, 750 kg ATM, 250 kg tare and an assumed density of 1.3 t/m³. Assume the vehicle and all other limits allow the model's 500 kg payload. The order weighs 2 × 1,300 = 2,600 kg.
Each full model load is 500 ÷ 1,300 = 0.384615… m³. The 0.648 m³ tray is larger, so weight limits this example. Calculate trips using the unrounded volume: ⌈2 ÷ (500 ÷ 1,300)⌉ = 6 trips.
| Trips | Volume per trip | Material weight per trip |
|---|---|---|
| First 5 trips | About 0.385 m³ | 500 kg |
| Final trip | About 0.077 m³ | 100 kg |
| Order total | 2 m³ | 2,600 kg |
That is about 17.8 cm deep for a full model load and 3.6 cm for the last one, if spread evenly across this tray. These rounded depths explain the calculation; use measured weights and your supplier's actual density for loading. Need to convert a tonne quote? Use the cubic metres to tonnes calculator.
When does delivery cost less?
These are fictional AUD quotes, not supplier prices: collection and delivered material each cost 60 AUD/m³, each return trip costs 10 AUD, and delivery costs 70 AUD for the whole order. Assume the same tax basis and material quantity in both quotes.
| Scenario | Trips | Collection | Delivered |
|---|---|---|---|
| 500 kg payload, no hire charge | 6 | 180 AUD | 190 AUD |
| 500 kg payload, 40 AUD total hire | 6 | 220 AUD | 190 AUD |
| 500 kg vehicle towing limit, no hire charge | 11 | 230 AUD | 190 AUD |
Without hire, collection costs 2 × 60 + 6 × 10 = 180 AUD, only 10 AUD below delivery. Adding 40 AUD total hire raises collection to 220 AUD, so delivery costs 30 AUD less. A 500 kg vehicle towing limit leaves 500 − 250 = 250 kg payload and requires 11 trips: 120 + 110 = 230 AUD, making delivery 40 AUD less.
Your time, loading time, vehicle wear and unentered charges are excluded. Reproduce these scenarios in the load calculator and quote comparison, then replace every example price with your own quote. An incomplete quote produces no cost comparison; enter 0 only for a charge that is genuinely zero.
Trailer questions
How much soil can a 6×4 trailer carry?
About 500 kg, which is 0.35–0.4 m³ of moist soil, for a typical 750 kg ATM trailer weighing 250 kg. Check your own trailer's plate: a heavier trailer or a car with a low towing limit reduces it further.
Can a 6×4 trailer carry a cubic metre of mulch?
A 1.8 × 1.2 m tray with 30 cm sides holds 0.648 m³ level, so one cubic metre exceeds the volume modelled here. At an assumed 0.35 t/m³ it weighs 350 kg, but actual density and all weight limits still need checking. A cage changes the dimensions and tare; confirm its ratings and suitable restraint before recalculating.
How do I find my trailer's tare weight?
It is stamped on the compliance plate, usually on the drawbar, and on the registration papers. If the plate is missing or the trailer has been modified, weigh it empty at a public weighbridge; many landscape yards and transfer stations have one.
Is it illegal to overload a box trailer?
Keep within the trailer, vehicle and towing equipment ratings and the rules where you drive. Overloading can make braking and handling unsafe. Check the current transport authority guidance; this calculator cannot establish legal compliance.
Sources and method
- Department of Infrastructure, Vehicle Standards Bulletin 1 (Revision 6), August 2026 publication – trailer ratings, definitions and construction requirements. Current VSB1 publication
- NZ Transport Agency Waka Kotahi, Guide to safe loading and towing for light vehicles. nzta.govt.nz
- National Transport Commission, Load Restraint Guide for Light Vehicles 2018. ntc.gov.au
- South Australian Government, Buying a safe caravan – vehicle, towing equipment and manufacturer limits. mylicence.sa.gov.au
- Australian Native Landscapes, Landscape product weight and volume – supplier examples and density variation with moisture, grade and source. Its listed garden mix and topsoil densities differ; 1.3 t/m³ here is a planning assumption, not a measured supplier specification. ANL weight and volume guidance
Method: limit volume by both tray space and the entered weight allowance, round the trip count up, and use the remainder for the final trip. Costs use that trip count and round cash amounts to cents. The worked example and density presets are illustrative; no supplier quote, towing inspection or physical load test was performed.
Written by DueRiva · Last reviewed 26 October 2026 · Report an error