The short answer
Divide the residual (ex GST) by the residual percentage. The residual is set as a percentage of the amount financed, so reversing it recovers the amount financed exactly.
Example (illustrative)
Residual $15,471.50 ex GST at 28.13% → $15,471.50 ÷ 0.2813 = $55,000.
The residual percentages to expect
| Lease term | Minimum residual (ATO guideline) |
|---|---|
| 1 year | 65.63% |
| 2 years | 56.25% |
| 3 years | 46.88% |
| 4 years | 37.50% |
| 5 years | 28.13% |
Keep everything on the same GST basis
This is where most hand calculations — and some published ones — go wrong. The financier claims the GST on the car, so the amount financed, the finance payment and the residual should all be ex GST. Mix an inc-GST figure with ex-GST ones and the rate moves by whole percentage points.
- If the residual is shown inc GST, divide by 1.1 first.
- Use the ex-GST finance payment, not the inc-GST one, where the quote shows both.
- Do not use the drive-away price as the amount financed unless the quote confirms it is ex GST and nothing else was added or taken off.
- Check the recovered amount against any "amount financed" or "vehicle finance" figure on the quote — they should agree closely.
When the amount financed is printed but looks high
Some quotes capitalise fees, brokerage or add-ons into the amount financed. That is money you are paying interest on but not money spent on the car. If a printed amount financed is noticeably above the residual-derived figure, ask the provider what the difference is — that is where the cost is.