Four different quantities
| Quantity | What it records | What it does not establish |
|---|---|---|
| Matched volume | Amount already matched under the platform definition | Amount still available at the displayed price |
| Unmatched request | Amount waiting for an opposing request | A guaranteed future match |
| Available depth | Amount currently offered across price levels | The price that will remain available later |
| Spread | Gap between the best available back and lay prices | The complete cost of execution |
Betfair's exchange rules explain that requests are matched against opposing requests, can be cross-matched, and use price-time priority when more than one request is available at the same price. These are platform-specific rules, so another venue needs its own contract.
Partial matching
Suppose a reader requests a GBP 100 back bet with a minimum acceptable price of 1.98. The available opposing depth is illustrative:
| Price | Size matched | Gross return if the selection wins |
|---|---|---|
| 2.00 | GBP 30 | GBP 60.00 |
| 1.99 | GBP 40 | GBP 79.60 |
| 1.98 | GBP 30 | GBP 59.40 |
Total matched stake = 30 + 40 + 30 = 100
Total gross return = 60 + 79.60 + 59.40 = 199
Stake-weighted decimal price = 199 / 100 = 1.99
Net profit before commission = 199 - 100 = 99
The accepted position should store each fill or the verified weighted price. A screenshot of the best price before submission does not prove that the full stake was matched there.
If only part of the request matches
If GBP 30 of a GBP 100 request matches and GBP 70 remains unmatched, only GBP 30 has an open betting position. The remainder can wait, be cancelled, or match later according to the platform instructions. Betfair's getting-started guide provides one current operator explanation of backing, laying, and matching.
Record requested stake and accepted stake separately. Performance calculated from requested prices and stakes can describe a position that never existed.
Slippage and effective price
Define slippage against a timestamped benchmark:
Slippage in decimal price = benchmark price - weighted accepted price
For an illustrative benchmark of 2.00 and accepted price of 1.99:
Slippage = 2.00 - 1.99 = 0.01
Also report the effect on expected return for the accepted stake. A one-cent decimal-price difference on GBP 100 changes the potential gross return by GBP 1. It does not reveal whether the probability estimate was correct.
Commission and liability
Exchange commission can depend on net winnings under the venue's rules. Betfair's current commission guide is one product example. Store the applicable rate and actual deduction rather than applying one universal percentage.
For a lay bet, liquidity and bankroll exposure must be read alongside liability. At lay odds 3.50 for GBP 20:
Lay liability = (3.50 - 1) * 20 = 50
The nominal GBP 20 lay stake is not the maximum loss. Continue to staking different bet types for exposure normalisation.
Liquidity is not a probability verdict
More matched money can make a quoted price easier to execute, but volume alone does not validate the price as a calibrated probability. A market can be liquid and wrong, or thin and right. Probability assessment, execution quality, and settlement are separate evidence layers.
Audit checklist
- Save the market and selection IDs.
- Record the full visible depth, not only the best quote.
- Store requested, matched, cancelled, and unmatched amounts.
- Calculate the stake-weighted accepted price.
- Include commission and lay liability where applicable.
- Reconcile the final position after partial fills and cash-outs.
Continue learning
- Next guide: Both Teams to Score Betting
- Related guide: Correct Score Betting
Assumptions and limitations
The order-book values are illustrative and do not describe a live market. Exchange interfaces, matching algorithms, cross-matching, minimum increments, commission, and in-play delay rules differ by venue and can change.
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