Estimate the price you can actually execute
A market can display an attractive probability while offering very little size at that price. Once an immediate order consumes the first quote, the remaining quantity may execute at worse levels.
Slippage is the difference between an expected or realized average fill and a clearly stated reference price. It is size-dependent and may matter more than an advertised or explicit trading fee.
This guide separates spread, depth, and slippage so you can estimate realistic entry and exit costs. Examples are hypothetical rather than live quotes.
Order books change quickly. A page may show a midpoint, last trade, or small top-of-book quote rather than a price available for your full order.
Why slippage matters
Displayed price is not executable size. A forecast has value only relative to the average price you can receive for the quantity you want. A small quote at 56 cents does not mean 1,000 shares are available there; orders that are large relative to depth are more likely to consume worse levels or remain partly unfilled.
Exit cost can erase apparent profit. A position marked above cost may still return much less when sold into thin bids. Entry and exit assumptions belong in the same pre-trade scenario; the analytics tools directory can help inspect liquidity, but it cannot replace an order preview or size-aware calculation.
The mechanics of execution cost
1. Identify the reference price
State the reference before measuring slippage. Common choices include the midpoint at decision time, the best ask before an immediate buy, or a documented interface estimate.
A midpoint reference includes the cost of crossing the spread plus any additional deterioration through depth or book movement. With the best ask as reference, deterioration beyond that ask mainly reflects depth consumed and movement in the book.
Record the timestamp and source. Best for: making expected and realized average fills comparable instead of mixing midpoint, last trade, and top-of-book references.
2. Separate bid-ask spread from slippage
The spread is the gap between the best displayed bid and ask. An order seeking immediate execution usually trades against available quotes on the other side of the book, so spread crossing may exist even if all shares fill at one level.
Slippage is deterioration between the chosen reference and the expected or realized average fill. Reality check: a midpoint comparison answers a broader execution-cost question than a best-ask comparison, so the reference must appear beside the number.
| Component | What it reflects | How to observe it |
|---|---|---|
| Spread | Cost of immediate liquidity | Best bid versus best ask |
| Depth | Quantity available by price | Cumulative order-book levels |
| Slippage | Average fill versus stated reference | Expected or realized fills |
| Delay risk | Book movement before completion | Timestamps and partial fills |
3. Calculate a size-weighted average fill
Walk available levels in price order. Multiply quantity at each level by its price until planned size is covered, then divide total cost by filled quantity.
For a buy, slippage is the expected average fill minus the chosen reference. Report it in cents per share and, when useful, as a percentage of the reference.
If visible depth does not cover the order, stop the estimate at visible liquidity and label the remainder uncertain. What to look for: a weighted average based on all levels, not the first or most attractive fill.
View tool detailsA current directory entry that may help inspect market data. Confirm whether prices are live, delayed, midpoint, last trade, or executable quotes before using them in a slippage estimate.4. Include partial fills and book movement
A price-bounded order can control the worst accepted price but cannot guarantee completion. A partial fill creates a new decision: wait, cancel, or revise the boundary.
During the delay, orders may appear or disappear. Record filled quantity, average price, unfilled quantity, and time in market.
Limitation: a static snapshot cannot predict cancellations, incoming liquidity, or queue position, so expected fill remains a scenario rather than a promise.
5. Estimate exit cost before entry
Entry liquidity does not guarantee exit liquidity. Build an orderly exit near current depth, a stressed exit after the spread widens, and a hold-to-resolution scenario.
Include the possibility that evidence changes while buyers retreat. Best for: deciding whether smaller size or a longer holding assumption is required before taking the position.
6. Compare total execution cost
Combine explicit fees, spread crossing, estimated slippage, and relevant funding or transfer cost. Keep each component separate so assumptions can be updated.
The Polymarket fee guide covers explicit charges, while this calculation focuses on the order book's contribution. What to look for: whether expected edge remains after both entry and exit costs.
How to evaluate slippage risk
Track estimated and realized slippage by market, order size, time, and execution instruction. The aim is to learn where your assumptions fail, not to publish a universal ranking.
Compare similar orders and preserve the pre-trade book snapshot when possible. If estimates remain optimistic, add a buffer or reduce size.
A useful stress test cuts top-level depth in half and widens the spread. Another asks whether the thesis still offers enough edge after entry and exit costs.
If a small change in book assumptions removes the advantage, passing on the trade is a valid editorial conclusion. A comparison of analytics interfaces can support further evaluation, but direct tests must match your market and size.
Limits and risks
Order-book uncertainty. Visible depth can be cancelled before execution, while hidden or incoming liquidity can improve a fill. A snapshot is an estimate, not a promise.
Data feeds can lag, aggregate levels differently, or omit transactions. Immediate-execution instructions expose you to uncertain prices, while price-bounded instructions expose you to missed or partial execution.
Thin-market and interpretation risk. Abrupt gaps and tiny last trades can create a misleading probability or mark. Confirm market wording and the official resolution source before treating execution analysis as a complete trading case.
Protect account credentials and remember that estimating slippage does not determine whether the underlying forecast is correct.
Getting Started
- Record the best bid, best ask, midpoint, and timestamp.
- Choose the exact order size and reference price.
- Walk visible levels to estimate a weighted average fill.
- Add explicit fees and a conservative exit scenario.
- Set a price boundary or reduce size if the edge becomes too small.
- After execution, compare actual fills with the saved estimate.
Practice with a hypothetical order before committing capital. Then use the same worksheet for every meaningful trade so thin markets become easier to recognize.
FAQ
Is slippage the same as the bid-ask spread?
No. The spread is the best-bid and best-ask gap. Slippage compares an average fill with a stated reference; a midpoint reference can include spread crossing.
Do price-bounded orders eliminate slippage?
They cap the price accepted, but do not guarantee a fill. Partial execution may leave less exposure than planned.
Why can displayed profit differ from exit profit?
A display may use midpoint or last trade, while a sale executes against bids and may consume several lower levels.
How can I reduce slippage?
Use smaller size, inspect cumulative depth, set a reasoned boundary, avoid chasing rapid moves, and test edge after entry and exit costs.