How Numberwright Tow checks your rig
The exact logic, formulas, and legal definitions behind each towing calculator check.
The checks
A rig is legal and within the manufacturer's safe operating limits only when every rating passes simultaneously:
Australia checks
- Trailer vs ATM: Actual trailer weight (uncoupled) must not exceed ATM plate.
- Trailer axle vs GTM: Actual axle load (ATM minus TBM) must not exceed GTM plate.
- Towing capacity: Trailer weight must not exceed the lesser of vehicle towing capacity and towbar rating.
- Tow ball mass: TBM must stay below vehicle maximum ball load and towbar ball rating.
- Vehicle vs GVM: Laden vehicle plus tow ball mass must stay under GVM.
- Whole rig vs GCM: Laden vehicle plus laden trailer must stay under GCM.
- Rear axle rating: Coupled rear axle load must not exceed rear GAWR.
- Advisory TBM %: Tow ball mass is evaluated against the WA DoT recommended 8–10 % guideline.
USA & Canada checks
- Trailer vs Tow rating: Trailer actual weight must stay under tow rating.
- Hitch weight & tongue ratings: Trailer and tongue weight must stay under receiver limits.
- Payload: Passengers, cargo, accessories, and tongue weight must not exceed placard payload.
- Truck vs GVWR: Loaded truck with tongue weight must not exceed GVWR.
- Combination vs GCWR: Truck plus trailer gross combination weight must stay under GCWR.
- Rear axle vs GAWR: Rear drive axle load must not exceed rear GAWR.
- Advisory tongue %: 10–15 % for conventional trailers; 15–30 % for fifth-wheel/gooseneck (GMC guidance).
UK checks
- Caravan vs MTPLM: Laden caravan must stay under its MTPLM plate.
- Towing limit: Caravan weight must not exceed car handbook towing limit (braked or unbraked).
- Nose weight: Nose weight must not exceed car limit or towbar rating.
- Car vs MAM: Car laden weight with nose weight must stay under MAM.
- Gross train weight: Car plus caravan must not exceed gross train weight.
- 85 % guideline: Caravan as percentage of car kerbweight (Caravan and Motorhome Club guidance).
Largest trailer you can tow
The calculator determines the single governing limit that restricts the maximum laden trailer your current rig can pull:
• Tow capacity: Braked towing capacity or hitch/towbar trailer rating.
• Tow ball / tongue limit: Max ball load divided by assumed percentage (e.g. 10 % or 12.5 %).
• Vehicle payload / GVM margin: Remaining capacity before hitting GVM or payload, divided by assumed ball percentage.
• Combined GCM / GCWR margin: Remaining combination weight before reaching gross combination limit.
The smallest allowable weight among these limits is your real maximum trailer weight under current loading.
Weighbridge maths
On a public weighbridge, three simple readings provide complete weight separation: Reading A (tow vehicle on bridge with trailer coupled), Reading B (tow vehicle alone, unhitched), and Reading C (trailer wheels on bridge, still coupled).
Tow ball mass is the difference between hitched car and unhitched car (A − B). Gross trailer mass is C. Aggregate trailer mass is C + TBM. Combined rig weight is A + C.
On a 3-platform CAT scale in the US, weighing with weight-distribution bars engaged (Pass 1) and truck alone (Pass 2) yields GCW, total trailer weight, and an estimate of tongue weight. Adding Pass 3 with WD bars released provides the true tongue weight and front-axle load restoration percentage.
What we don't do
We don't store vehicle data or guess ratings; we don't give licence or legal advice.