Trailer load calculator

The numbers on your trailer's plate

Trailers built for Australian roads carry an identification or compliance plate that shows their weight ratings. Three of them decide how much soil you can carry:

The 750 kg figure is common because Australian rules let a single-axle trailer of up to 750 kg run without brakes. Heavier trailers must have brakes. Larger box trailers (7×5, 8×5) are sold both ways, so read the plate rather than assuming.

Why a 6×4 trailer holds less soil than you think

A 6×4 tray with 30 cm sides holds 0.65 m³ level with the top. With a 500 kg payload, this is what it can legally carry:

The practical rule: with soil, sand or gravel, a legally loaded 6×4 trailer looks about half full. Filled level with the sides, it is about 350 kg over with soil and nearly 500 kg over with sand or gravel. Many landscape yards know this and will refuse to overload a small trailer. Tell the loader operator your payload in kilograms and ask for a part bucket.

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. Aim for about 10 % of the loaded trailer's weight on the tow ball. Put heavy loads just forward of the axle, never behind it, or the trailer will sway.
• Vehicle payload. The tow ball weight counts against your car's own carrying limit, together with passengers and anything in the boot.
• 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's rules are set out differently. NZ Transport Agency Waka Kotahi requires brakes on trailers above 2,000 kg. For unbraked trailers it recommends that the loaded trailer weigh no more than three-quarters of the towing vehicle's unladen weight, and within the vehicle maker's towing limit. Any car and trailer must also be able to stop within 7 m from 30 km/h. Use the trailer's rated gross mass as the "ATM" in the calculator, and enter the lower of your car's rated limit and the three-quarters guide as the towing limit.

Securing and covering the load

A load of loose soil or mulch has to be restrained so that nothing can fall or blow off. In Queensland, New South Wales, Victoria, Tasmania, South Australia and the ACT, the law refers to the National Transport Commission's Load Restraint Guide for Light Vehicles. In practice that means a tarp or fine cargo net tied down over the whole tray. Mulch and dry compost blow out of an open trailer within a few hundred metres, and fines for insecure loads apply everywhere.

Worked example: 2 m³ of garden mix

1. Trailer: 6×4, ATM 750 kg, tare 250 kg, so payload 500 kg.
2. Garden mix at 1.3 t/m³: safe load 500 ÷ 1,300 = 0.385 m³ per trip.
3. Trips: 2 ÷ 0.385 = 5.2, so 6 trips, each loaded about 18 cm deep.
4. Compare that with a delivery fee of around $70–$120. Unless the yard is around the corner, six return trips cost more in time and fuel than the truck. Collecting only pays for small top-ups under about half a cubic metre.

Trailer questions