What Is Slippage in Trading and Why It Matters

When you place an order, you have a price in mind. Slippage is the gap between that expected price and the price your order actually fills at. It can occur in either direction, but for active strategies it works against you more often than not, because you tend to trade when others want to trade the same way. Slippage is a real, recurring cost — and ignoring it is one of the most common reasons backtests do not match live results.

What causes slippage

Market movement. Prices move continuously. Between the moment you decide to trade and the moment your order reaches the market, the price can change, especially in fast conditions.

The bid-ask spread. There is always a gap between the best buying and selling prices. A market order to buy typically fills at the higher ask, not the price you saw quoted as the last trade.

Liquidity. If you trade more than is available at the best price, the rest of your order fills at worse prices. Thinly traded markets and large orders make this worse.

Volatility and news. Around major news or in volatile markets, prices can gap, and fills can land far from expectations.

Why slippage matters so much

Slippage is small per trade but relentless. A strategy that trades frequently pays it again and again, and those small gaps compound into a large drag. Many strategies that look profitable in a backtest assume fills at idealized prices; once realistic slippage is applied, the edge can disappear entirely. This is especially true for high-frequency and scalping strategies, where the expected profit per trade is tiny and easily swallowed by execution costs. The faster and more often a strategy trades, the more slippage decides its fate.

How to model slippage honestly

The honest approach is to assume slippage exists and build it into every test. There are a few practical ways:

Fixed estimate. Subtract a set amount per trade based on the typical spread of the instrument you trade. Simple and conservative.

Spread-based. Assume market orders fill at the far side of the spread rather than the mid price, which is closer to reality.

Scenario testing. Re-run a strategy with higher slippage assumptions to see how sensitive it is. A strategy that only survives at zero slippage is not a viable strategy.

The point is not to find the one true number — slippage varies — but to avoid the fantasy of frictionless trading. A backtest that ignores slippage is answering a question you will never actually face.

The takeaway

Slippage is where optimistic strategies meet reality. If you want a backtest you can trust, model it deliberately, test how sensitive your strategy is to it, and be especially skeptical of fast-trading strategies whose edge is thin. A strategy that still works after honest slippage and costs is worth paper trading. One that needs perfect fills was never real.

TRION takes friction seriously: when you backtest a strategy on real stored data, you can model realistic costs including slippage, so the results reflect what execution actually does to an edge — with "N/A" shown rather than an invented number when a figure cannot be computed.

It is paper-only and simulation-only: no broker, no real orders, no profit promise. AI assists, TRION validates, risk protects, humans decide.