Capital Gains Tax Property Calculator

What actually goes in a property cost base

The cost base is the starting point for the whole calculation. Get it wrong and you either overpay, or you understate a gain and meet an ATO amendment later.

What counts: the contract purchase price, stamp duty and mortgage registration fees, legal and conveyancing fees on both the purchase and the sale, buyer's agent and selling agent commission, title searches and building inspections at purchase, and capital improvements you paid for and never deducted, meaning a new kitchen, an extension or a retaining wall.

What does not count: loan interest and bank fees, council and water rates and body corporate levies, unless you genuinely never claimed them as a deduction, which almost nobody with an investment property can say honestly. Repairs and maintenance are revenue expenses, not capital ones. Depreciation you already claimed gets clawed back separately.

The most common omission is stamp duty. On a $900,000 purchase in New South Wales that is roughly $35,000. Leave it out and you have inflated your gain by exactly that, and handed over about $8,000 you did not owe. Keep every receipt, because the ATO can ask for records going back to the original purchase, and a rough recollection is not a cost base.

The main residence exemption and the six-year rule

If a property was your main residence the entire time you owned it, the gain is fully exempt. No CGT, nothing to report. It gets interesting when you move out.

Move out and leave it vacant, and you can keep treating it as your main residence indefinitely. There is no time limit at all. You could live overseas for a decade, come back, sell, and still claim the full exemption, as long as it never earned income. Move out and rent it, and you get six years. That is the six-year rule, and past six years the exemption starts shrinking day by day.

Concretely: you buy in Melbourne in July 2018, live there until June 2021, then move to Sydney for work and rent the place out. Sell by June 2027 and the six years are covered, so the whole gain is exempt. Sell in July 2027 and you start losing days.

Three things people get wrong. The clock resets if you move back in, so a fresh six-year period applies to each separate absence. You cannot treat two properties as your main residence at once, except for up to six months while you are genuinely moving house. And the property has to have been your home first, so if you rented it out from day one and moved in later, those early years are taxable no matter what. When the exemption is partial, the split is done by days: the gain multiplied by non-main-residence days divided by total days owned. It does not matter which years the price actually rose in.

The market value reset, the rule nobody claims

This is the one that costs people tens of thousands. If you lived in a property as your main residence, then first rented it out after 20 August 1996, and you would have had a full exemption had you sold the day before, the law treats you as having acquired it at its market value on the day you first rented it. Your cost base resets. Every dollar of growth from while you lived there vanishes from the calculation.

Say you bought in Brisbane in 2005 for $400,000, lived there until 2015, then rented it out when it was worth $750,000, and you sell in 2027 for $1,100,000. Without the reset your cost base is $400,000 and your gross gain is $700,000. With it, your cost base is $750,000 and the gain is $350,000. After the 50% discount that is $175,000 less taxable income, which at a 47% top marginal rate including the Medicare levy is about $82,000 of tax you simply do not pay.

The catch is evidence. You need a market valuation dated to the day you first rented it out. If you did not get one then, a retrospective valuation from a qualified valuer is still acceptable, just harder to defend if the ATO pushes back. An agent's appraisal is not a substitute. The calculator above has a field for this value, and it is the one field worth chasing paperwork for.

Timing, and why the year you sell changes the bill

CGT is not a flat tax on the gain. The discounted gain is added to your other income for the year and taxed at your marginal rate, which makes timing a genuine lever. Retire in October 2026 and drop from $180,000 to $40,000, and selling in the new financial year rather than the old one can cut the effective rate on the gain by fifteen percentage points or more.

The contract date, not settlement, decides which year the gain falls in. Sign on 25 June 2027 and settle on 20 July 2027 and the gain belongs to 2026-27, not 2027-28. That cuts both ways, and people plan around the wrong date constantly.

Order matters too. Capital losses come off the gross gain before the 50% discount, not after. A $50,000 loss against a $200,000 gain leaves $150,000, which discounts to $75,000 of taxable gain. Do it in the wrong order and you would expect $50,000, so the real bill lands higher than you planned for. A large concessional super contribution in the same year can pull some of the gain back down a bracket, which is worth modelling with a tax agent on your actual numbers rather than guessing.