Slippage: The Cost Between the Quote and the Fill

Slippage is the difference between the price you were shown and the average price you actually got. It is not a fee, it appears on no invoice, and on a large order it can cost more than the commission. Every market order carries it, and most users only notice it after a trade that felt wrong.

The reason is structural: a quote describes the top of the book, and your order may need more than the top of the book. Slippage is what you pay for the depth behind that first level. This piece sets out where it comes from, computes it on a real book, and shows how to measure your own rather than guessing.

• Slippage scales with size, not with confidence. Doubling an order does not double the average price impact, it usually more than doubles it, because each additional unit reaches deeper into worse prices.
• It is invisible in the UI until after the fill. The confirmation screen often shows the best available price, not the average you will receive.
• It is a cost you can quote in advance. The book is public. Walking it by hand takes a minute and gives a number, not a feeling.
• Thin books make it worse at exactly the wrong time. Depth tends to vanish during volatility, so the moments when you most want to trade are the moments when slippage is highest.

What slippage actually measures

Slippage is the gap between a reference price and your execution price, expressed in currency or in basis points. The reference is usually the mid price or the last trade at the moment you submitted, and the execution price is the volume-weighted average of everything you filled against.

Both halves of that sentence matter. If you pick a different reference, you get a different number, and venues are not always explicit about which one they use in their reports. When you compare your own figures across venues, fix the reference first or the comparison is meaningless.

The quote is not a promise

A quote is a snapshot of resting orders at one instant. By the time your order arrives, some of those orders may be gone, some may have been replaced, and new ones may have appeared. On a fast market the book you clicked is already history.

Three costs that look like slippage

Not everything that widens the gap between expectation and fill is slippage. Separating them matters, because each has a different fix.

• Spread. The gap between best bid and best offer. You cross it the moment you send a market order, before any depth is consumed.
• Depth cost. The deterioration in price as your order consumes successive levels. This is slippage in the strict sense.
• Fees and funding. Charged separately, quoted separately, and often larger than either of the above on small orders.

Slippage in a thin book: a worked example

Suppose you want to buy 10 units and the sell side of the book looks like this. Prices and sizes are illustrative, chosen to make the arithmetic visible.

Ten units cannot be filled at level one, which holds only 3.90. The order takes all of level one, all of level two, and then 1.90 of level three. That last partial fill costs 1.90 × 2,012 = 3,822.80.

Total spent is 7,800.00 + 8,421.00 + 3,822.80 = 20,043.80 for 10 units, an average of 2,004.38. The best price on screen was 2,000. Slippage is 4.38 per unit, or 43.80 in total, which is 22 basis points on the notional.

Now add a taker fee. At an illustrative 0.10% of notional, that is 20.04 more. Total cost of the trade is 63.84, or about 32 basis points, and roughly two thirds of it is slippage rather than commission. On a book half this deep, level two would be reached sooner and the same order would pay visibly more.

Slippage is not a malfunction. It is the price of demanding liquidity that is not there, and the book tells you the price before you commit.

Nothing in that calculation requires a forecast. The cost is determined by the book as it stands, which means you can price your own trade before sending it.

How to measure your own slippage

Measurement is a habit rather than a tool. Six steps, repeated for a month, will tell you more about your execution than any dashboard summary.

1. Record the mid price and the timestamp at the moment you submit, not when the fill notification arrives.
2. Record the average fill price from the trade history rather than the confirmation screen.
3. Subtract to get slippage per unit, then multiply by filled size for the currency cost.
4. Divide by notional and express the result in basis points so trades of different sizes are comparable.
5. Group the results by order size band and by time of day to see whether the cost is structural or situational.
6. Compare against a limit-order alternative: what you would have paid by working the order at a fixed price, including the times it did not fill.

The last step is the one people skip. A limit order that never fills has a slippage cost of zero and an opportunity cost that may be larger. Execution quality is measured across a series of trades, not on the one that felt bad.

Reducing slippage without pretending it disappears

You cannot remove depth cost, but you can stop paying for it unnecessarily. Most of the reduction comes from three choices made before the order is sent.

Choose the order type deliberately

Market orders pay whatever the book asks. Limit orders cap the price and accept the risk of partial or no fill. For anything larger than the visible best level, a limit order priced a little through the market often fills with less damage than a market order, because it does not sweep every level indiscriminately.

Match the venue to the size

Depth differs enormously between venues for the same pair. A size that is trivial on one book is a market-moving order on another. Check the cumulative size within a given percentage of the mid before choosing where to send, and prefer the venue where your size sits inside a tight band.

Timing matters too. Depth commonly thins around scheduled events and in quiet hours, and returns when volume does. Trading into a thin book is a choice even when the reason feels urgent.