Understanding the ATO's Prime Cost Formula in Detail

The ATO's prime cost formula is: Deduction = Asset Cost × (Days Held ÷ 365) × (100% ÷ Effective Life in Years). Each component has specific meaning. "Asset cost" is the full purchase price including GST if not registered, or the GST-exclusive price if the business is GST-registered and claims input tax credits. "Days held" is the number of days in the income year during which the asset was owned and used (or installed ready for use) for a taxable purpose. The division by 365 (or 366 in a leap year) pro-rates the deduction for partial years — an asset purchased on March 1 and held through June 30 of a financial year ending June 30 earns exactly 122 days of deduction rather than the full year.

For a $25,000 laptop with a 4-year ATO effective life, a full-year prime cost deduction is $6,250 per year (25,000 × 365/365 × 25%). Purchased on January 15 (166 days before June 30), the first-year deduction is only $2,842 (25,000 × 166/365 × 25%). The second year claims the full $6,250, and years three and four do the same. In the final partial year — if the asset completes its effective life — the remaining balance brings the total to exactly $25,000 over the ownership period.

Why Buildings and Structural Improvements Must Use Prime Cost

Under Australian tax law (ITAA 1997 Division 43), capital works including buildings, structural improvements, and environmental protection earthworks must be depreciated using the prime cost (straight-line) method at a rate of either 2.5% per year (for most buildings, giving a 40-year effective life) or 4% per year (for short-life buildings like certain temporary structures). Taxpayers have no choice to use diminishing value for these assets — the ATO explicitly prohibits it. This matters because a residential investment property constructed for $400,000 (excluding land) generates exactly $10,000 per year in Division 43 deductions for 40 years, with no front-loading benefit. Fitting out the building with plant and equipment — carpet, dishwashers, light fittings, window coverings — creates a separate set of shorter-life depreciating assets that can use diminishing value, which is why a quantity surveyor's depreciation schedule for a new property often shows two separate depreciation streams.

First-Year Pro-Ration: Planning Around Settlement and Installation Dates

Because prime cost deductions are pro-rated by exact days held, the settlement or installation date of an asset has a direct and calculable impact on the first year's deduction. An investor who settles on a $600,000 rental property (buildings component: $350,000) on July 1 (the first day of the Australian financial year) claims the full 2.5% deduction of $8,750 in that year. The same investor settling on June 28 — just three days into the income year ending June 30 — claims only $72 in Division 43 deductions for that year ($350,000 × 3/365 × 2.5%). Plant and equipment assets installed in the same late-June window face the same compression. For large acquisitions, aligning settlement with the beginning of the financial year maximises the first-year deduction and avoids losing nearly a full year of depreciation to poor timing.