A trader builds a system with a 1% stop-loss and a 2:1 reward-to-risk ratio. Backtests show 58% win rate. Live trading begins — and the account bleeds. Not from bad signals. From slippage. Every entry fills 0.12% worse than modelled. Every stop fills 0.09% worse. That is 0.21% of edge erased per trade, silently, invisibly, every single time.

At 200 trades per year, that 0.21% slippage drag compounds into roughly 42% of the original modelled edge — gone. On a $50,000 account risking 1R = $500 per trade, that is $4,200 annually handed to market makers and liquidity gaps. Most traders never isolate this number. They blame the strategy instead of the execution friction eating it alive.

CONCEPTSlippage is a fixed tax on every trade — model it as a cost, not a surprise.
WARNINGIgnoring slippage in backtests inflates win rates and destroys live performance expectations.
KEY IDEAReduce position size until your risk model absorbs realistic slippage without breaching 1R.

Proper slippage modelling follows a mechanical rule: measure average entry slippage and average exit slippage over 30 live trades. Add them. That sum becomes a hard deduction from your theoretical R. If your stop is $500 and combined slippage averages $60, your real 1R is $560. Every position size calculation must use the slippage-adjusted figure — not the clean backtested number.

Modelled vs Real 1R After Slippage$0$300$600$90050 trades100 trades200 tradesModelledReal (slippage)

The structural fix is a slippage buffer built directly into position sizing formulas. Traders often use the Kelly Criterion or fixed fractional sizing — both break down when slippage is excluded. Adding a conservative 0.15% round-trip slippage assumption to every trade, then recalculating maximum position size, keeps drawdowns within modelled bounds. Resources such as Investopedia's slippage definition, the Wikipedia entry on financial slippage, and Investopedia's position sizing guide provide the mechanical foundations every systematic trader should review before live deployment.

Slippage is not random noise — it is a measurable, modelable cost that either gets priced into the system or gets priced out of the account.

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.