Weighbridge & CAT Scale Calculator for Caravans and Trailers
Turn two or three weighbridge readings into tow ball mass, trailer weight and combined weight.
Australian weighbridge interpretation
260 kg
2,490 kg
2,750 kg
5,700 kg
Weigh the car on the bridge with the caravan coupled (A), the car alone (B) and the caravan's wheels on the bridge while coupled (C). Tow ball mass = A − B; ATM = C + (A − B); combined weight = A + C.
Worked example (AU)
Worked example (US CAT scale)
How it's calculated
Australia 3-reading method
• Tow ball mass (TBM) = A − B
• Gross trailer mass (GTM) = C
• Laden trailer (ATM) = C + (A − B)
• Combined weight = A + C
US CAT scale passes
• Gross combined weight (GCW) = steer₁ + drive₁ + trailer₁
• Truck alone = steer₂ + drive₂
• Trailer total = GCW − truck alone
• Tongue weight estimate (WD on) = (steer₁ + drive₁) − truck alone
• True tongue weight (Pass 3) = (steer₃ + drive₃) − truck alone
• Front-axle load restoration % = (steer₁ − steer₃) ÷ (steer₂ − steer₃) × 100
Sources
- RACQ — What you need to know about trailer and tow vehicle specifications
ATM, GTM, payload, ball load, towbar rating overrides, max towable = lesser of vehicle and towbar
- WA Department of Transport — Safe Towing Guide (PDF)
ATM, GTM, GVM (incl. hitch download), GCM, unbraked 750 kg, tow hitch download ≈ 8–10 % of ATM
- 49 CFR 571.110 (FMVSS 110) — Tire selection and rims; placard (Cornell LII)
Payload placard sentence; tongue weight counts as cargo
- GMC — Tongue weight
10–15 % conventional; 15–30 % fifth-wheel/gooseneck
Last reviewed: 2026-10-26
Built and maintained by Numberwright. The checks follow the definitions published by motoring bodies, transport departments and vehicle makers listed under Sources. Found a mistake? Contact details coming soon.
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