What Is Slippage and How It Affects Automated Strategies

Slippage is the difference between the price a trade was requested at and the price it actually executed at. It happens because there's always some delay — however small — between an order being sent and a broker's server filling it, and in that window, the market price can move. During calm conditions with a liquid pair, slippage is often negligible. During high-impact news releases, thin holiday sessions, or fast-moving breakouts, it can be significant. Slippage isn't inherently one-directional — it can work in a trader's favor (positive slippage, a better fill than requested) just as easily as against it. But automated strategies that trade frequently, or specifically target volatile breakout moments, feel the effect more than an occasional discretionary trader, simply because of the volume of orders exposed to it. A backtest that assumes perfect fills at the exact requested price is, by definition, overstating the strategy's real-world performance — every live trading system needs to budget for some average slippage, and the strategies most sensitive to it are typically high-frequency scalping systems and news-based breakout systems, where the whole edge depends on getting in within a very narrow price window. Practical mitigation includes trading pairs with naturally tight spreads and deep liquidity, avoiding scheduled high-impact news windows unless the strategy is specifically designed for them, using a broker and execution infrastructure with low latency to the server, and — for the strategy itself — setting a maximum acceptable slippage tolerance so an order is rejected rather than filled at a price far worse than intended. Ignoring slippage in strategy design is one of the most common reasons a beautiful backtest and a disappointing live account diverge.