Daniel ran a $120,000 trading account for three years, averaging 22% gross annual returns. By every measure, he was succeeding. Then his accountant handed him a tax bill he hadn't planned for — $31,000 in a single year. He'd been sizing positions on gross profit, never net. One oversight compounded across three years nearly wiped the account entirely.
The core problem most active traders miss is that the ATO treats frequent trading activity as a business, not passive investing. That distinction determines whether you're assessed under capital gains tax rules or as ordinary income. Ordinary income means your trading profits stack on top of your salary — pushing many traders into the 45% marginal rate bracket before the Medicare levy even enters the equation.
The maths here is unforgiving. On a $50,000 account risking 1% per trade — $500 — a trader in the 45% bracket keeps only $275 of every winning dollar. The Kelly Criterion formula, f* = (bp − q) / b, requires honest inputs. If your edge produces a 55% win rate with a 1.5:1 reward-to-risk ratio, Kelly suggests roughly 23% of capital. But recalculate using after-tax reward — 1.5 × 0.55 = 0.825 net — and optimal Kelly drops significantly. Sizing on gross figures means chronic over-leveraging.
Practical structures traders use include quarterly tax provisioning — pulling 35–45% of net trading profit into a separate account each quarter before it gets redeployed. Some traders operate through a company structure at the 25% base rate, though this requires genuine business infrastructure and specialist advice. Deductible expenses — data feeds, platform fees, trading education — reduce assessable income legitimately. For deeper grounding on these mechanics, the capital gains tax framework on Investopedia explains the core distinctions clearly, the Kelly criterion on Wikipedia shows how sizing formulas absorb real-world variables, and Investopedia's breakdown of fixed ratio position sizing offers an alternative framework suited to after-tax capital preservation.
Every dollar lost to unplanned tax is a dollar that never compounds. Build the ATO into your position sizing model before your first trade of the financial year — not after your accountant calls.
This content is for educational purposes only and does not constitute financial product advice. Past performance is not indicative of future results. Profit Logic Ltd (ACN 688 669 936) accepts no responsibility for errors or omissions in this content or anywhere on this website. Always seek advice from a licensed financial adviser before making investment decisions.