Slippage: why an order fills worse than expected

Slippage — an order filling at a price different from the expected one — is a surprise that spoils the result at the most inconvenient moment. Especially in volatility and on stops. Understanding the nature of slippage matters, because it's a real cost that a backtest often underestimates. Let's look at where it comes from and how to reduce its impact.

What slippage is

Slippage is the difference between the price at which you expected to fill an order and the price at which it actually filled. You hit entry at one price, but the trade opens a bit worse; your stop is set at one level, but it triggers lower. The reason is that price moves, and there may not be enough liquidity at the needed level, so the order fills at the nearest available price. Slippage can also go in your favor, but it's more often felt as worse execution.

When slippage is dangerous

Slippage intensifies where you least want it: in moments of high volatility and low liquidity. On the release of important news, price moves in jumps, liquidity is sparse, and orders (especially stops) fill notably worse. The same happens at the open after a weekend (a gap) and in the thin overnight market. It's on stops that slippage is especially painful: counting on a 20-point loss, on a news release with slippage you can get 30-40. This turns controlled risk into more than was planned.

Why the backtest underestimates it

The danger of slippage is that tests and calculations usually assume perfect execution at the given price. In the history, a stop 'triggers' exactly at its level, and entry is precisely to plan. In reality, slippage adds costs that weren't in the test. A strategy profitable on paper can turn out losing in real execution, especially if it trades on news or in a thin market. That's why a realistic backtest builds in slippage rather than assuming perfect execution.

How to reduce the impact

You can't fully remove slippage, but its impact can be reduced. Avoid entries — and especially being in a position without a buffer — in moments of peak volatility (the release of key news), unless the strategy is adapted for it. Trade liquid instruments (majors) in active sessions, where there's more liquidity and less slippage. Bear in mind that market orders fill at the available price (slippage possible), while limit orders fill at the set price or better (but may not fill). Build slippage into your risk calculation and into testing as a real cost.

The practical takeaway

Slippage is an order filling at a price worse than expected because price moves and there's not enough liquidity at the needed level. It's dangerous in moments of high volatility and low liquidity (news, gaps, the thin overnight market) and especially painful on stops, where it turns a planned loss into a larger one. A backtest usually underestimates slippage by assuming perfect execution, so a strategy profitable on paper can lose in reality. Reduce the impact: avoid peak volatility if the strategy isn't adapted for it, trade liquid instruments in active sessions, understand the difference between market and limit orders, and build slippage into calculations and tests. Understanding slippage as a real, underestimated cost helps you avoid building your trading on the illusion of perfect execution and soberly assess risk, especially around news.

This material is for educational purposes and is not individual investment advice.