TL;DRThe difference between the price you expected to get filled at and the price you actually received. Slippage is a hidden cost of trading that increases in thin markets, fast markets, and around news events.
Slippage is the difference between your expected fill price and your actual fill price. If you place a buy market order expecting to get filled at 5,200.25 and your fill comes back at 5,200.75, you experienced 2 ticks (0.50 points) of slippage on ES, which is $25 per contract.
Slippage can be positive or negative. Negative slippage means you got a worse price than expected. Positive slippage means you got a better price, which does happen occasionally when the market moves in your favor between order submission and execution.
Three factors drive slippage. First, speed of price movement. In a fast-moving market, the price can change between when you click the button and when your order reaches the exchange. This is measured in milliseconds, but volatile markets can move several ticks in that time.
Second, available liquidity. If there are only 50 contracts offered at the best ask and you're buying 100, the first 50 fill at the ask and the remaining 50 fill at the next available price. The average fill is worse than the displayed price.
Third, order type. Market orders are most susceptible to slippage. Limit orders eliminate slippage by design (you specify the worst acceptable price). Stop orders are vulnerable because they convert to market orders when triggered.
Slippage peaks around major economic releases: Non-Farm Payrolls, CPI, FOMC decisions, and GDP reports. ES can move 20-40 points in seconds after these releases, and stop orders triggered during this window can fill many ticks away from the trigger price.
Overnight and pre-market sessions carry higher slippage risk because fewer participants mean wider spreads and less depth at each price level. A market order that gets 1 tick of slippage during RTH might get 3-5 ticks during Globex.
Limit-up/limit-down events and trading halts create extreme slippage when markets reopen, because the equilibrium price has shifted significantly while no trading occurred.
Trade during high-liquidity periods. For equity index futures, this means regular trading hours (9:30 AM to 4:00 PM ET), especially the first and last hour of the session.
Use limit orders for entries and profit targets. This eliminates slippage entirely on those orders, at the cost of potentially missing fills.
Account for slippage in your risk calculations. If your stop is 10 points away, assume the actual fill might be 10.5 or 11 points away. This small buffer prevents your actual losses from exceeding your planned risk.
Avoid placing stops right at major news events. Either flatten before the release or widen your stops to account for the volatility.
Normal slippage on a stop
You're long ES with a sell stop at 5,190. The market trades down through your stop level.
Your stop triggers at 5,190 and becomes a market order. The best bid at that instant is 5,189.75. You're filled at 5,189.75, which is one tick of slippage ($12.50). This is normal and expected in liquid markets.
News event slippage
You're short NQ with a buy stop at 18,500. FOMC announces an unexpected rate cut. NQ spikes from 18,480 to 18,540 in 2 seconds.
Your stop triggers at 18,500 but the best offer is 18,528 by the time your order executes. You're filled 28 points ($560) above your stop. Your planned $500 loss becomes a $1,060 loss. This is why traders flatten or widen stops before major news.
Assuming your stop will fill at exactly the stop price
Stops become market orders when triggered. Build 1-3 ticks of slippage into every risk calculation. On news events, assume more.
Trading large position sizes in thin markets
If you're trading 5+ contracts in a product with 200 contracts on the bid, your order alone can move the price. Trade smaller or scale into positions during low-liquidity periods.
Backtesting without accounting for slippage
A strategy that looks profitable with perfect fills may be a loser after slippage. Subtract 1 tick per side ($25 round trip on ES) from every trade in your backtest.