What Size Trailer Do I Need?
Most people pick a trailer by looking at the biggest thing they want to carry. That's the wrong end of the problem. A trailer you can't legally tow is worse than one that's slightly too small, so the honest order is: what can your car pull → what weight do you need → what size gives you that.
Independent guide based on typical Australian trailer specifications, VSB 1 and Queensland TMR guidance. Confirm figures against your tow vehicle's handbook and the trailer's compliance plate.
Start with the car, not the trailer
Three numbers from your vehicle decide everything, and the lowest one wins:
| Number | Where to find it | What it limits |
|---|---|---|
| Braked / unbraked towing capacity | Owner's manual or placard | The loaded trailer's total mass (ATM) |
| Tow ball download limit | Tow bar rating plate | The weight pressing down on the ball, usually ~10% of ATM |
| GCM (gross combination mass) | Manufacturer's spec | Car + passengers + load + trailer, all together |
The one that catches people out is the tow ball limit. A car rated to tow 2,000 kg but with a 150 kg ball limit can't legally pull a 2,000 kg trailer loaded normally, because a 2,000 kg trailer puts roughly 200 kg on the ball. The tow bar, not the engine, is the real ceiling.
Note that most vehicles quote two towing figures: a low unbraked number (often 750 kg) and a higher braked one. If the trailer has no brakes, only the unbraked figure applies.
Then work out the weight you need
Payload = ATM − Tare. The trailer's own weight eats into what you can carry, so a bigger trailer does not give you proportionally more payload.
Typical empty weights by trailer type:
| Trailer type | Typical tare |
|---|---|
| Box / cage trailer (domestic, camping) | ~300 kg |
| Multi-roller boat trailer | ~400 kg |
| Car carrier (flat top) | ~650 kg |
| Tandem cage or tipper (commercial) | ~700 kg |
So a 750 kg ATM box trailer carries about 450 kg: not 750 kg. Work out the heaviest load you realistically carry, add the tare, and that's the ATM you need.
Then pick the size
Common Australian box trailer sizes, with the deck measured internally:
| Size | Deck (mm) | Deck (m) | Suits |
|---|---|---|---|
| 6×4 | 1800 × 1200 | 1.8 × 1.2 | Tip runs, small garden loads, occasional light use |
| 7×5 | 2100 × 1500 | 2.1 × 1.5 | The default all-rounder: moving house, mowers, bikes |
| 8×5 | 2400 × 1500 | 2.4 × 1.5 | Heavier and longer loads, trade use, tandem builds |
| 10×6 | 3000 × 1800 | 3.0 × 1.8 | Machinery, bulk landscaping, commercial work |
These are nominal sizes: a true 7 ft × 5 ft is 2134 × 1524 mm, but Australian builders round to 2100 × 1500. Measure the actual trailer before planning a tight load.
Roughly, weight maps to size like this:
| ATM you need | Usual answer | Indicative price (AUD) |
|---|---|---|
| Up to 750 kg | 6×4 single axle | $800 – $1,500 |
| 750 – 1,500 kg | 7×5, single or tandem | $1,500 – $3,000 |
| 1,500 – 2,500 kg | 8×5 tandem | $2,800 – $4,500 |
| 2,500 – 3,500 kg | 8×5 tandem, commercial build | $4,000 – $8,000 |
Prices are indicative only and move with steel prices, build quality and inclusions.
The thresholds that force your hand
Two weight lines change what the trailer must have, and both are worth knowing before you buy:
- Over 750 kg ATM: the trailer must have brakes.
- Over 2,000 kg ATM: brakes on all wheels, plus a breakaway system that applies them if the trailer separates.
That's why 750 kg is such a common rating: it's the last point where a trailer can legally have no brakes at all. Crossing it adds cost to the trailer and often means a tandem axle. See our brake requirements guide for the detail.
Single or tandem?
Above about 750 kg you have a choice. Heavy-duty single-axle trailers exist up to roughly 1,600 kg ATM and stay light, cheap and easy to reverse or push around by hand. Tandem axles take over for heavier loads: more stable at highway speed, far safer if a tyre lets go, but heavier, dearer to maintain and harder to manoeuvre in a driveway.
Our single vs dual axle guide compares them properly.
Three mistakes worth avoiding
- Buying for the one big job. If you move house once and do tip runs monthly, size for the tip runs. Hiring a bigger trailer for one weekend is cheaper than storing and towing an oversized one for years.
- Sizing by space when the load is dense. Soil, sand, pavers and bricks hit the payload long before they fill the deck. A 7×5 runs out of legal weight at roughly a third of a cubic metre of soil. See what a 7×5 can carry.
- Forgetting where you'll keep it. An 8×5 tandem does not fit down every side passage or into every garage, and it's a lot harder to push by hand than a 6×4.
Quick FAQ
What size trailer do I need to move house? A 7×5 with a cage handles a one-bedroom move in a few trips. For a full house, more trips in a 7×5 usually beats buying something bigger, or hire a larger trailer for the day.
Is a 6×4 or a 7×5 better? A 7×5 for most people: the extra 300 mm of length and 300 mm of width is the difference between a queen mattress fitting flat and not. Choose a 6×4 only if storage space or budget is tight, or your tow vehicle is small.
What size trailer can I tow without brakes? Any trailer up to 750 kg ATM, and only if your vehicle's unbraked towing capacity covers it. In practice that's a 6×4, or a light 7×5.
Do I need a bigger trailer for a ride-on mower? Usually a 7×5 is enough, but measure the mower first, because some ride-ons are longer than 2.1 m with the catcher fitted, and then you want an 8×5.
What size trailer for a boat? Boat trailers are sized to hull length, not in feet-by-feet: roughly a 5.0–5.5 m multi-roller for hulls to 5.5 m, and 6.0–7.0 m tandem for hulls up to 7 m.
Get a number instead of a guess. The Size Recommender turns what you carry into a recommended size and price range, and the Compliance Auditor checks the ATM, braking and tow ball figures behind it, with no sign-up.
Independent reference only. Sizes, weights and prices are typical values; confirm against your tow vehicle's handbook and the trailer's compliance plate, and verify with TMR or a certified RPEQ engineer before purchasing, building or registering a trailer.