Depreciation can mean two different things

For a buyer or owner, depreciation usually means the fall from purchase price to what the vehicle could sell for today. That is a market question. Make, model, variant, age, kilometres, service history, condition, colour, location, supply and demand can all affect a real transaction. A fixed percentage cannot observe those facts.

For Australian tax, decline in value is a statutory calculation for a depreciating asset used for a taxable purpose. It creates an adjustable value and may support a deduction after private use and other rules are applied. It does not promise that a dealer or private buyer will pay the adjustable value. The two numbers can diverge sharply.

This page displays both ideas side by side. The custom market path compounds the entered annual loss and applies the entered resale floor. The prime cost and diminishing-value paths do not use that floor, because inserting a user resale target into a tax formula would blur the concepts.

How each projection is calculated

The Australian Taxation Office describes prime cost as a constant decline over effective life. For assets whose relevant start time is on or after 10 May 2006, the general diminishing-value formula uses base value multiplied by days held divided by 365 multiplied by 200% divided by effective life. Actual tax work proceeds income year by income year, using opening adjustable value, additions to cost and exact days.

The calculator uses a continuous elapsed-years simplification so comparisons are easy. It does not model part-year additions, GST credits, balancing adjustment events, luxury car limits, low-value pooling, accelerated measures, private-use changes or an adjusted prime-cost formula. Those exclusions are why the tax-labelled figures are an educational cross-check only.

The custom path is deliberately editable. A new vehicle may lose value differently from an older vehicle, and a single rate across the whole holding period may be unrealistic. Run several rates to create a range, then replace the range with current comparable evidence before selling, insuring or refinancing.

Read retained value, loss and spread together

The large result is the custom market scenario, not a guaranteed valuation. Retained percentage divides that result by original cost. The accumulated loss is the difference between cost and value. The annualised display is the rate you entered, not an internally discovered rate.

The spread takes the highest of the three projected values and subtracts the lowest. A large spread signals that the method choice dominates the answer. It is a reason to investigate, not to choose whichever number is most convenient. A small spread only shows the assumptions happen to align.

Choose assumptions that can be defended

Original cost

For personal planning, start with the drive-away amount actually paid and separately record accessories or later improvements. For tax, cost can be modified by GST treatment, the car limit, second-element costs and other rules. Do not assume the finance balance or total repayments equal tax cost.

Elapsed years

Use the time since acquisition for a rough ownership scenario. Tax calculations use start time and days held in each income year, which can differ from a simple anniversary. A value of 4.5 means four and a half years in this model.

Effective life

An effective-life assumption controls both statutory-style paths. The ATO permits relevant choices between a Commissioner’s determination and self-assessment in applicable circumstances, subject to rules. Confirm the current determination for the specific asset and acquisition facts rather than adopting the default shown here.

Market decline and floor

Estimate a rate from comparable observations across time, not from one optimistic advertisement. The floor prevents the custom scenario falling below an entered salvage or long-term resale assumption. It is not a guaranteed minimum and should not exceed original cost.

Use depreciation in the whole ownership budget

Depreciation is often a large but invisible vehicle cost. Add finance interest, registration, insurance, fuel or charging, tyres, servicing, repairs, parking and selling costs. A vehicle with a low weekly repayment can still have a high total cost if it loses value quickly or carries a large balloon.

Compare the projected resale amount with the loan payout at the same future date. If the payout is higher, selling may require cash to clear the secured debt. Moneysmart warns that a sale price may not cover car-loan debt. Ask the lender for an actual payout figure because future interest rebates, fees and payment timing matter.

For a used-car transaction, a Personal Property Securities Register search can show whether a security interest is registered and can provide stolen or written-off information for a VIN search. It does not value or mechanically inspect the vehicle. Independent inspection and documentary checks remain important.

For business or work use, keep acquisition documents, finance records, odometer evidence, a logbook where relevant, expense records and sale documentation. The taxable-use percentage entered here is merely a multiplier. The method available and deductible amount depend on the taxpayer, use and current law.

Build a low, central and high resale range

One forecast hides uncertainty. Run the calculator three times with deliberately different custom decline rates and record each market result. A lower depreciation rate creates an optimistic resale case; a higher rate creates a conservative case. Keep original cost, elapsed period and floor constant so the effect of the rate is visible.

Then test the scenarios against real listings for the same generation, engine or battery, transmission, trim, body style and model year. Adjust for kilometres and documented condition instead of comparing a base model with a premium variant. Dealer retail asking prices can include warranty, preparation and margin, while a private sale or trade-in may sit lower. Treat those channels separately.

Future technology and regulation can also move values. Fuel prices, manufacturer incentives, model replacements, battery condition, safety expectations, recall history and supply shortages can change demand. The calculator does not forecast those events. Refresh the evidence close to the decision date.

If the purpose is insurance, read the policy definition of agreed value or market value and ask the insurer how accessories and modifications are treated. A calculated resale scenario is not automatically the insured amount or a settlement entitlement. If the purpose is a financial statement or business sale, apply the accounting policy and obtain professional advice where material.

Finally, keep nominal dollars and real purchasing power distinct. The results are nominal vehicle amounts and do not adjust for general inflation. A car that sells for the same dollar amount years later has still lost purchasing power. Use the separate inflation calculator when that comparison matters.