How the gain is worked out
Cost base is what you paid for the units, plus the brokerage when you bought and when you sold.
Capital gain is what you sold them for, less the cost base. If that is below zero, it is a capital loss.
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The 12-month discount
If you held the units for at least 12 months, only half the gain is taxed. The count leaves out the day you bought and the day you sold, so shares bought on 1 July 2025 qualify if sold on 2 July 2026 or later.
Capital losses come off your gains first. The discount is then applied to what is left.
Selling from 1 July 2027
The rules change on 1 July 2027. The 50% discount is replaced by indexation and a 30% minimum tax, and growth before that date keeps the discount. The calculator shows the gain for a sale after that date, but not the new tax treatment, because that needs the shares' value on 1 July 2027 and inflation figures that are not published yet.
Read how the 2027 changes work, in plain English.
More than one sale
When you bought the same shares more than once, which units a sale came from changes the cost base and the 12-month test. Portfoli works this out for every sale you make, with the oldest shares sold first, and shows every step for each financial year.
Start free, upload the file your broker gives you, and see your gains for each year.
General information for Australian resident individuals, based on the ATO's rules for capital gains on shares. It doesn't take into account your circumstances, and Portfoli isn't a registered tax agent. Check your figures with one before you lodge.