Lay Bet Calculator — Exchange Liability & Profit Tool

The lay bet calculator is designed for anyone using a betting exchange such as Betfair, Smarkets, or Betdaq. When you lay a bet, you are acting as the bookmaker — you accept a backer's wager and promise to pay out if their selection wins. If it loses, you pocket the stake (minus commission).

Enter three values to get a full breakdown:

1. Lay Stake — the amount the backer is staking (the amount you are accepting). This is not what you put up; the backer puts up the lay stake, and you put up the liability.
2. Lay Odds — the price at which you are offering the bet, in the odds format you picked (exchanges quote decimal, e.g. 3.0 = +200 = 2/1). You can see available lay odds in the exchange's betting interface, usually displayed in the pink/red column.
3. Exchange Commission — the percentage the exchange charges on your winning lays. Betfair's standard rate is 5%; Smarkets charges 2%; Betdaq typically charges 2–5%. Commission only applies when you win.

The calculator shows your lay liability (the funds you need in your account), your net profit if the selection loses, and your loss if it wins.

The core lay bet calculations are simpler than they appear at first:

1. Define variables:
   • S = lay stake (the backer's wager you are accepting)
   • O = lay odds (decimal)
   • C = commission rate (as a percentage, e.g. 5 for 5%)
2. Lay Liability:
   Liability = S × (O − 1)
   (This is what you stand to lose if the selection wins. You pay out the backer's profit: S × (O − 1).)
3. Profit if selection loses (you win):
   Profit = S × (1 − C / 100)
   (You collect the backer's stake, then the exchange deducts commission from your winnings.)
4. Loss if selection wins (you lose):
   Loss = −Liability = −S × (O − 1)
   (No commission applies when you lose a lay bet.)
5. ROI on liability:
   ROI = (Profit if loses / Liability) × 100
   (Your return relative to the capital you have tied up.)

Implied probabilities: At lay odds O, the implied probability that the selection wins is 1/O × 100%. The probability from the layer's perspective (that it loses) is (1 − 1/O) × 100%. Note these two figures always sum to exactly 100%.

Example 1 — Laying an Even-Money Selection at 2.0

You lay £10 on a football team to win at odds of 2.0 on Betfair (5% commission).

• Lay liability: £10 × (2.0 − 1) = £10.00
• If selection loses: £10 × (1 − 0.05) = +£9.50 profit
• If selection wins: −£10.00
• ROI on liability: 9.50 / 10 × 100 = 95%

At even money, your liability equals your lay stake — the simplest case. You're essentially betting that the team will not win, collecting £9.50 net if you're right.

Example 2 — Laying a Longshot at 6.0

You lay £10 on a horse at odds of 6.0 (5/1) on Betfair.

• Lay liability: £10 × (6.0 − 1) = £50.00
• If horse loses: £10 × 0.95 = +£9.50 profit
• If horse wins: −£50.00
• ROI on liability: 9.50 / 50 × 100 = 19%

This illustrates the asymmetric risk of laying at high odds. Your potential profit is capped at the lay stake (minus commission), but your potential loss is five times the lay stake. This is why experienced lay bettors typically prefer shorter-priced selections.

Example 3 — Lay Bet for Matched Betting

You have a £20 free bet at a bookmaker and want to extract maximum value via matched betting. You back a selection at 4.0 with the free bet and lay the same selection on Betfair at 4.1. Because a free bet's stake is not returned, the balanced lay stake is £20 × (4.0 − 1) / (4.1 − 0.05) ≈ £14.81.

• Lay liability: £14.81 × (4.1 − 1) = £45.91
• If selection loses: £14.81 × 0.95 ≈ +£14.07 (from lay)
• If selection wins: £60.00 (free bet win) − £45.91 (lay) ≈ +£14.09

Matched betting uses lay bets to neutralise the risk of the bookmaker bet, converting the free bet into near-cash profit regardless of the outcome.

Understanding Exchange Commission

Commission is the exchange's fee for providing the marketplace. It is deducted from your net winnings on markets where you have a positive balance. Key points:

• Betfair: Standard 5%, but premium charges may apply to highly profitable accounts
• Smarkets: Flat 2%, making it better value for frequent traders
• Betdaq: 2% standard, with loyalty discounts for volume
• Commission is never charged on losing lay bets
• Market-specific discounts and loyalty rates can reduce your effective commission