Caravan Tow Safety (85% Rule) Calculator
Sanity-check a caravan-and-tow-vehicle match against the well-known 85% guideline and the 8–10% tow-ball download rule. These rules of thumb help you pick a stable, safe combination before you worry about the legal GVM, GCM and tow-rating limits.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the caravan's loaded ATM and the tow vehicle's kerb mass to get the tow ratio.
- Optionally enter the actual tow-ball download to check it against the recommended 8–10% band.
- Read the verdicts — aim for a tow ratio at or below 85% and a ball weight of about 8–10% of ATM.
How it works
Tow ratio = caravan ATM ÷ tow-vehicle kerb mass × 100. At or below 85% the tow vehicle stays the heavier, more stable partner; 85–100% is workable but needs care; over 100% means the van outweighs the vehicle and is not recommended. Tow-ball download % = ball download ÷ ATM × 100, with 8–10% the sweet spot — too little promotes sway, too much overloads the rear axle and coupling.
Worked example
Worked example. A caravan with an ATM of 2500 kg behind a vehicle with a 2300 kg kerb mass gives a tow ratio of 2500 ÷ 2300 × 100 = 108.7% — over 100%, so not recommended. A 225 kg ball download is 225 ÷ 2500 × 100 = 9%, right in the ideal 8–10% band.
Common mistakes
- Treating the 85% figure as a legal limit — it is only a stability guideline; the law is set by tow rating, GVM, GCM and ball rating.
- Comparing the van's ATM to the vehicle's GVM instead of its kerb mass when applying the 85% rule.
- Ignoring ball weight distribution — a van loaded tail-heavy can drop below 8% ball weight and sway even if the tow ratio looks fine.
Frequently asked questions
Is the 85% rule a legal requirement?
No. It is an industry rule of thumb for towing stability, not law. Your legal obligations are to stay within the vehicle's braked towing capacity, GVM, GCM and maximum tow-ball download. A combination can pass the 85% guideline yet still be illegal on one of those limits, or exceed 85% and still be legal — always check the ratings too.
Why does tow-ball download matter so much?
Ball weight controls trailer stability. Around 8–10% of ATM keeps the trailer nose-heavy enough to resist sway. Below about 8% the trailer can fishtail dangerously; much above 10% overloads the tow vehicle's rear axle and tow-ball rating and lightens the front, reducing steering grip.
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- GVM & GCM Compliance Calculator
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