What it means
Slippage is the gap between the price you expected to fill at (limit price, displayed quote at time of order, stop trigger price) and the price you actually got. Negative slippage works against you; positive slippage in your favor. Caused by latency between order submission and matching, fast-market book movement, or partial fills walking the book. Slippage tends to compound during news releases and at session opens.
Why it matters
Slippage is the single largest hidden cost in active trading. A strategy backtested with no-slippage assumptions can be unprofitable live once realistic slippage is modelled — especially for high-frequency strategies, stops triggered during news, and large size relative to book depth. Knowing your strategy's slippage profile is the difference between 'verified backtest' and 'actual edge'.
How to use it
Track every execution: expected price, actual price, time of day, market condition. Build a slippage histogram for your strategy by time of day and by event type. Avoid trading during the highest-slippage windows (1-2 minutes around scheduled news). For backtests, model conservative slippage: 1 pip on majors at news times, 0.3 pip during overlap, 0.5 pip during off-hours.
Stop-sell triggered at EUR/USD 1.0820 during NFP release. Actual fill 1.0817 — 3 pips of slippage. On a standard lot, $30 worse than expected. Across 100 stops per year with similar slippage profile: $3,000 leak the backtest didn't model.
Where slippage comes from
Three sources. (1) Latency: order submitted at price X, by the time it reaches the matching engine the market has moved. Distance traders are most exposed; co-located HFT firms experience near-zero latency slippage. (2) Book depth: large orders walk deeper levels at progressively worse prices. (3) Quote stalls: during fast markets, market makers pull or widen quotes; the visible inside spread becomes stale before your order arrives.
Slippage by broker model
ECN/STP brokers expose you to true book slippage but don't add markup — slippage is symmetric (sometimes positive, sometimes negative). Market-maker brokers internalize order flow and often apply asymmetric slippage: positive slippage rarely passed through, negative slippage standard. This is part of why ECN models are preferred for active strategies despite the explicit commission.
Frequently asked
How can I reduce slippage?
Use limit orders instead of market orders. Avoid trading the minute around scheduled news. Trade during overlap when book depth is greatest. For large size, scale in/out across multiple orders rather than single block fills.
Is slippage always bad?
No — slippage can be positive (better than expected fill) too. ECN brokers pass through both directions; market makers often suppress positive slippage. Symmetric slippage on average is acceptable; asymmetric slippage means hidden cost.
Do brokers profit from slippage?
On ECN models, no — the broker just routes the order to the venue with the best price. On market-maker models, the broker internalizes the trade and can profit from slippage spread. This is the structural reason ECN-style execution is preferred for cost-sensitive strategies.
Want a worked example or a deeper dive? Ask Rocky how this concept applies to your specific watchlist or trade idea.
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