A worked example
You sign a contract to buy a unit on 20 May 2019 for $480,000, paying $17,000 stamp duty and $1,500 conveyancing, and later spend $3,500 on a new kitchen benchtop (a capital improvement). The cost base is $502,000. You sign a contract to sell on 11 April 2025 for $640,000, paying $13,000 agent's commission and $2,000 in legal fees: capital proceeds of $625,000.
- Capital gain: $625,000 − $502,000 = $123,000.
- Owned for nearly six years, so the 50% discount applies: $61,500.
- Net capital gain for 2024–25: $61,500.
- With other taxable income of $95,000, taxable income becomes $156,500. The first $40,000 of the gain is taxed at 30% ($12,000) and the next $21,500 at 37% ($7,955), plus 2% Medicare levy ($1,230): about $21,185.
How the calculation works
- Capital proceeds. What you received for the asset, less selling costs such as brokerage or the agent's commission.
- Cost base. What it cost you: the purchase price plus incidental costs of buying it (brokerage, stamp duty, legal fees), and for property, capital improvements.
- Gain or loss. Proceeds minus cost base, for each parcel sold.
- Losses. Capital losses carried forward from earlier years and losses made this year are taken off this year's gains. The ATO says you can choose which gains they reduce; taking them off gains that do not get the discount first gives the lowest result, and that is what the calculator does.
- Discount. Australian-resident individuals halve what is left of each gain on an asset owned for at least 12 months, counting from the day after acquisition to the day before the CGT event.
- Net capital gain. The total goes in your tax return and is taxed at your marginal rate along with your other income. A net capital loss cannot reduce other income; it carries forward.
The cost base for property
The cost base has five elements. For a typical investment property: (1) the purchase price; (2) incidental costs of buying and selling, such as stamp duty, conveyancing, buyer's and selling agents' fees, and advertising; (3) costs of owning it that you could not deduct, such as rates, insurance and interest while it was not income-producing (for example vacant land); (4) capital expenditure to increase or preserve its value, such as renovations and extensions; and (5) costs of defending your title. Anything you have claimed as a tax deduction, including capital works deductions, is excluded or reduces the cost base. The property cost base guide goes through each.
Your home
A home that was your main residence for the whole time you owned it, on land of 2 hectares or less, and never used to produce income is exempt. A property you never lived in is fully taxable. Anything in between (you rented it out for some years, moved out and used the six-year rule, ran a business from part of it, or it became your home part-way through) needs a partial calculation that depends on dates and use. The calculator handles the two clear cases and tells you when to ask an accountant.
Timing a sale
Because the contract date sets the financial year, signing in late June or early July can move the gain between years. If your income will be lower next year (for example after retirement), a July contract may mean less tax. Do not let tax timing override the right sale, but it is worth knowing.
Records to keep
Keep the purchase and sale contracts, settlement statements, stamp duty and legal invoices, receipts for every improvement, and any depreciation schedule, for five years after the sale.
Checking the result before you lodge
A calculator is only as good as the figures you give it. Before copying anything into your tax return, run through these checks:
- Holdings reconcile. The units left over after every sale should match what your broker, share registry or exchange says you still hold. If they do not, a buy, DRP or split is missing, and the cost base of some sale is wrong.
- Dates are trade or contract dates. Settlement dates move sales across 30 June and across the 12-month mark.
- Costs are on the right side. Purchase costs belong to the buy; selling costs belong to the sale. Entering both on one side gives the same gain but the wrong cost base for the parcels that are left.
- Losses brought forward match last year's return. The figure at label V of last year's return is this year's starting point.
- Pre-filled data. The ATO pre-fills some share and crypto data from brokers and exchanges. It often lacks cost bases, so treat it as a list of sales to account for, not as the answer.
If a disposal looks wrong, open its parcel breakdown in the year view: each line shows the parcel, its acquisition date, days held and cost base, which is usually enough to spot the missing or mistyped transaction.
Where the rules come from
The discount, the 12-month counting rule, the treatment of losses and the tax rates are taken from the ATO's published guidance, listed under sources below with the date they were last checked. Rates are stored per financial year and updated each July. Where a rule could not be confirmed against an official page, it is marked as unverified in the calculator's notes and in the schedule, so you and your accountant know what to check.