How much can a semi or B-double carry?
Legal gross mass for a 6-axle semi and a 9-axle B-double under GML, HML and WA rules from August 2026, and how to turn that into payload for your own truck.
The short answer
A standard 6-axle semi can weigh 43.5 t in total under general mass limits, and a 9-axle B-double 64.5 t. Take away the empty weight of the truck and trailers to get the payload: a semi weighing 15.5 t empty can carry about 28.0 t.
Gross limits, 2026
These are the most a combination may weigh in total, loaded, including the truck. They follow the national changes from 1 August 2026.
| Combination | General (GML) | Complying steer (GML) | Higher mass (HML) |
|---|---|---|---|
| 6-axle semi | 43.5 t | 44.0 t | 45.5 t |
| 9-axle B-double | 64.5 t | 65.0 t | 68.0 t |
The general limit is the lowest of three figures: the regulation's table for that vehicle, the sum of the axle group limits, and the "reduced mass amount" (the sum less 0.5 t for each dual tandem and 1 t for each dual tri, plus 1 t, or 2 t when that total is over 55 t). For the 6-axle semi the limit comes from the regulation's table; for the B-double it comes from the reduced mass amount.
Axle groups: the limit most people hit first
| Axle group | GML | HML |
|---|---|---|
| Steer | 6.0 t | 6.0 t |
| Drive tandem | 17.0 t | 17.0 t |
| Trailer tri-axle | 21.0 t | 22.5 t |
A load that is legal in total can still overload one group. A deck of drinks pallets packed to the front of the trailer can push the drive group over while the trailer group has room to spare. That is why DepotOps plans axle weights, not just gross.
Western Australia
WA sets its own limits. As of right, a 6-axle semi may weigh 42.5 t. Under the Prime Mover, Trailer Combinations Order, the same combination can run at 44.5 t on the approved network, or 45.0 t with steer tyres at least 295 mm wide.
From gross to payload
Payload = legal gross − empty weight of the whole combination (with fuel, driver and gear). Weigh the empty combination once on a weighbridge rather than trusting the brochure. If your prime mover has a GCM rating below the legal limit, the rating applies instead.
Common questions
What is the maximum gross weight of a 6-axle semi-trailer in Australia?
Under general mass limits it is 43.5 t with a standard steer axle, or 44.0 t with a complying steer axle. Under higher mass limits (HML) on approved routes it is 45.5 t. In WA the statutory limit is 42.5 t.
How much can a B-double carry?
A 9-axle B-double has a general mass limit of 64.5 t and 68.0 t under HML. Take away the empty weight of the whole combination: at 24.9 t empty, that leaves about 39.6 t of payload under GML.
Why can I not load up to the gross limit?
Each axle group has its own limit too. If the load sits too far forward or back, the steer, drive or trailer group goes over before the gross does. Plan where the load sits, not just how much it weighs.
Do these limits apply in WA and the NT?
WA does not apply the Heavy Vehicle National Law and sets its own limits; they are in the table above. The NT also sits outside the national regulator for most heavy vehicle rules. Check with the local authority there.
Sources
- NHVR — General mass limits information sheet (1 August 2026) — checked 2026-09-25
- Heavy Vehicle (Mass, Dimension and Loading) National Regulation (in-force text) — checked 2026-09-30
- Main Roads WA — Prime mover, trailer combinations operating conditions — checked 2026-09-30
A guide, not legal advice. Your permit, notice, registration or accreditation can set different limits; check them before you rely on a figure here.