Slippage in trading: what it is and how to calculate it

The definition, precisely

Slippage is the difference between the price your strategy intended and the price the order actually received. For a buy, adverse slippage means filling higher than intended; for a sell, lower.

It is measured in price units and converted to money by the same arithmetic as any other price move: slippage in points, multiplied by the value of a point per unit of position, multiplied by position size.

Three things it is commonly confused with. The **spread** is the known, repeatable gap between bid and ask that you always cross — it is a cost, but it is not slippage, because it is the same every time. **Commission** is the broker's explicit fee, also known in advance. **Slippage** is the residual: the unknown part, the part that varies, and the part you cannot quote in advance.

How to calculate it from your own trades

You need two numbers per trade: the price your rule said to trade at, and the price you actually got. Most platform reports give you the fill and the time; the intended price you have to record yourself at signal time.

The procedure: log the expected price when the signal fires, log the fill when it happens, and compute the signed difference in the adverse direction. On a buy, slippage = fill − expected, and only positive values are adverse. On a sell, slippage = expected − fill.

Then convert to money terms per trade and average. Two averages are more useful than one: the mean, and the median. If the median is near zero and the mean is large, slippage is not a constant tax — it is a rare event, concentrated in a few trades, which tells you to look at when those trades happened rather than at your average.

Finally, normalise by volatility. Slippage in points is not comparable across regimes or instruments. Slippage as a fraction of the average true range at the time is, and it is the number that tells you whether your execution is degrading or the market simply got faster.

What causes it

**Speed of the market.** Prices move between the moment your order leaves and the moment it is filled. In a fast market that gap is large; in a quiet one it is nearly nothing. This is the dominant factor and it is the one you cannot change.

**Order type.** A market order accepts whatever price is available, by definition. A limit order does not slip in the adverse direction — it simply may not fill, which is a different problem with the same root. A stop order is a market order that activates on a trigger, so it inherits market-order slippage exactly when volatility is picking up, which is why stops slip more than entries.

**Liquidity at your size.** Slippage is partly a function of how much you are asking for relative to what is available. A retail position on a major instrument barely moves the book; a large one walks it. The important consequence for a backtest is that the same strategy has different costs at different account sizes, and a backtest calibrated at retail size is not valid at institutional size.

**Specific moments.** The daily rollover, the seconds around a scheduled data release, the Sunday open, and the moments when a large stop cluster is triggered. These are not random — they are predictable, which means they are partly avoidable.

**Broker execution model.** Whether your broker internalises flow, routes to liquidity providers, or uses a dealing desk changes the distribution of fills. Two brokers on the same instrument at the same moment can give you materially different slippage.

How much is normal?

There is no universal figure, and anyone quoting a single number is probably quoting their own instrument and conditions. What matters is the ratio: slippage relative to the distance between your entry and your stop.

If slippage is a tiny fraction of your stop distance, it is a rounding error on your risk. If it is a meaningful share of it — a tenth, a fifth — then your realised risk is systematically larger than your planned risk, and every position you size by stop distance is understating the loss it can produce.

The same logic applies relative to your target. On a strategy with tight targets, slippage on entry and exit can consume a large share of the gross edge. That is why scalping strategies are so sensitive to execution and why the same rules can be profitable for one participant and unprofitable for another on the same instrument.

What you can actually do about it

You cannot eliminate slippage, because it is the price of demanding immediacy in a market that is moving. But several things genuinely reduce it.

Use limit orders where your strategy permits. A limit order converts slippage risk into non-fill risk, which for some strategies is strictly better: you miss trades rather than pay more for them. For entries that are not time-critical, this is often the single biggest available improvement.

Avoid the predictable moments if your strategy can tolerate it. Not opening new positions in the seconds around a major scheduled release, and not placing market orders at the daily rollover, removes the worst tail of the distribution without changing the logic of the strategy.

Choose your instrument deliberately. Slippage is a property of liquidity, and the difference between a major currency pair and a thin cross is far larger than the difference between two brokers on the same major.

Model it pessimistically in the backtest. The point of modelling slippage is not to predict it but to make sure the strategy does not depend on it being small. A strategy that survives realistic slippage is a different proposition from one that was only tested without it.

What measurement does not solve

A measured average is backward-looking. Your slippage next month depends on the market conditions next month, not on the average you computed from last quarter.

Small samples mislead in both directions. A handful of trades in calm conditions will suggest slippage is negligible; a few around a volatile event will suggest it is catastrophic. Neither is an estimate of the distribution until you have enough trades to see the tail.

And reducing slippage does not create an edge. It reduces a cost. A strategy with no edge loses more slowly with better execution, which is worth something but is not the same thing as being profitable.