towlimits only states a rule if we can point to where it comes from. This page lists those sources and sets out the formula behind every calculator, so you can check our working.
Sources
- Vehicle Standards Bulletin 1 (Revision 6): trailers with an ATM of 4.5 tonnes or less (August 2026), Australian Government, Department of Infrastructure
- Towing a trailer or caravan, NSW Government (Transport for NSW)
- Safety chains and shackles for trailers under 4.5 tonnes ATM (CI-119C), Department of Transport, Western Australia
- Light trailer requirements: compliance with VSB1, Transport Tasmania
- A guide to trailer brakes, RACQ
- How much is your vehicle designed to carry or tow?, NRMA
The national rules for trailers up to 4.5 tonnes ATM are in VSB 1. States and territories enforce them and publish their own guidance, and some add rules of their own, for example on learner and provisional drivers towing. Where we quote a state rule, we name the state.
We last checked every rule against these sources on 12 October 2026. If you spot something out of date, please email hello@towlimits.com with a link to the source and we'll fix it.
How each calculator works
All masses are in kilograms. Results are rounded to the nearest kilogram for display. Calculations run in your browser and nothing you type is sent to us.
Towing calculator
- Vehicle with trailer hitched = loaded vehicle + towball download, compared with GVM
- Combined = loaded vehicle + loaded trailer, compared with GCM
- Loaded trailer compared with towing capacity, and with the trailer's ATM and the towbar rating if entered
- Trailer axle load = loaded trailer − towball download, compared with the trailer's GTM if entered
- Towball download compared with the vehicle's maximum and the towbar's towball rating if entered
A limit shows amber from 95% to 100% and red above 100%.
How much can I tow?
The smallest of: towing capacity; GCM − loaded vehicle; (GVM − loaded vehicle) ÷ towball percentage; maximum towball download ÷ towball percentage; and the towbar rating if entered.
Payload calculator
Payload = GVM − kerb weight. If you enter tare instead, we add fuel to fill the tank from the 10 litres included in tare, at about 0.84 kg per litre for diesel and 0.74 kg per litre for petrol. Remaining = payload − accessories − people − cargo − towball download.
Towball weight calculator
Towball percentage = towball download ÷ loaded trailer × 100. Under 5% shows as light and over 15% as heavy, the edges of the ranges published by Transport for NSW (about 5 to 10%) and NRMA (about 10 to 15%). The legal limits are the vehicle's and towbar's maximums in kilograms.
Caravan payload calculator
Payload = ATM − tare. Remaining = payload − extras fitted after the tare − water (1 kg per litre) − gas − gear.
Weighbridge calculator
GTM = whole rig − vehicle hitched. Towball download = vehicle hitched − vehicle unhitched. Trailer weight = GTM + towball download.
Brake and chain requirements
Brakes by GTM (VSB 1 Revision 6): none required up to 750 kg; at least one axle up to 2,000 kg, with over-run brakes allowed; every wheel, controlled from the vehicle, over 2,000 kg, plus a breakaway system. Chains and couplings by ATM as set out on the requirements page. If you don't enter GTM, we estimate it as 90% of ATM and say so.
What we don't do
- We don't publish manufacturer figures for specific vehicles yet. They vary between variants and model years, so use your own vehicle's plate and manual.
- We don't cover heavy trailers over 4.5 tonnes ATM, which fall under heavy vehicle rules.
- We don't give legal or engineering advice. For modifications, talk to your state transport authority or a licensed vehicle engineer.