Betting Exchanges like Betfair, Smarkets, and Matchbook let you act as the bookmaker. You can either Back a selection (bet that it will win) or Lay it (bet that it will lose). Combining a back and lay bet on the same selection at different odds creates a hedge — and if you size the lay stake correctly, you can lock in a guaranteed profit regardless of the outcome. This is called a “green book”.
This calculator handles two scenarios: Cross Exchange (back at a bookmaker, lay on an exchange — common for matched betting) and Same Exchange (both bets on the same platform). Each requires a different formula because of how commission applies.
Back/Lay Hedging Calculator
Calculate the lay stake, liability, and P/L for a back/lay hedge.
Back Bet
Lay Bet
What is Back/Lay Betting?
Traditional sports betting is one-directional: you bet that something will happen, and you win if it does. Betting exchanges add a second direction.
- Back bet: You bet that a selection will win. If it wins, you keep your winnings (back stake × (odds − 1)); if it loses, you lose your stake.
- Lay bet: You bet that a selection will not win. If it loses, you keep the lay stake (the layer’s stake — the money the backer puts up to wager against you). If it wins, you pay out the liability — the lay stake × (lay odds − 1).
By placing both a back and a lay on the same selection at different prices, you create a position that pays out on both outcomes. The trick is sizing the lay stake correctly so that the profit is equal whether the selection wins or loses.
The Lay Stake Formula
The math depends on whether commission applies to one side of the trade or both. Both formulas produce a “green book” — equal profit on both outcomes.
Cross Exchange Formula
When you back at a bookmaker (no commission) and lay on an exchange (commission on lay winnings only):
Lay Stake = (Back Stake × Back Odds) / (Lay Odds − Commission)
This is the standard formula for matched betting and most exchange trading scenarios. Commission is expressed as a decimal (0.05 for 5%).
Same Exchange Formula
When both bets are on the same exchange and commission applies to both sides:
Lay Stake = Back Stake × (Back Odds × (1 − c) + c) / (Lay Odds − c)
This produces a slightly smaller lay stake because the back side now pays commission on its winnings as well, so less is needed on the lay side to balance.
Liability — The Money You Must Hold
Whenever you lay a bet, the exchange reserves your liability — the maximum amount you’d pay out if the selection wins:
Liability = Lay Stake × (Lay Odds − 1)
For a $100 lay at odds of 3.00, the liability is $200. Always check that your exchange account holds at least this much before placing the bet — otherwise the lay won’t be matched.
Worked Example 1: Pre-Race Trading
This is the textbook trading scenario: you back a horse at a generous price expecting the market to firm up, then lay it at lower odds once the price moves.
You back a horse at 4.00 with a $100 stake on Betfair (5% commission). The market firms up over the morning and the horse becomes a favorite. Lay odds drop to 3.00. You decide to trade out.
Same Exchange formula: Lay Stake = 100 × (4.00 × 0.95 + 0.05) / (3.00 − 0.05) = $130.51
- If the horse wins: $300 back winnings × 0.95 commission = $285. Minus $261.02 lay liability = +$23.98 profit
- If the horse loses: −$100 back stake + $130.51 lay winnings × 0.95 = +$23.98. +$23.98 profit
Both outcomes pay $23.98 — a perfect green book.
Worked Example 2: Matched Betting Qualifying Bet
You’re claiming a $50 free bet bonus. The bonus requires a $50 qualifying bet at minimum odds of 2.00. To extract the bonus with minimal loss, you back at the bookmaker and lay on Betfair to neutralize the qualifying bet.
You back $50 at 2.10 at the bookmaker (no commission) and lay at 2.15 on Betfair (5% commission).
Cross Exchange formula: Lay Stake = (50 × 2.10) / (2.15 − 0.05) = $50.00
- If the selection wins: +$55 back winnings − $57.50 lay liability = −$2.50 net loss
- If the selection loses: −$50 back stake + $47.50 lay winnings (after commission) = −$2.50 net loss
You’ve sacrificed $2.50 to “qualify” for the $50 free bet — leaving the bonus extraction profit to be realized in the next leg of the matched betting cycle.
Worked Example 3: Failed Trade (Cutting Losses)
Sometimes a trade goes against you — the market moves the wrong way and you need to decide whether to hold or hedge to limit damage.
You back a team at 3.00 with $100 expecting odds to drop. Instead, the team’s star player gets injured and lay odds drift to 4.00.
Cross Exchange formula: Lay Stake = (100 × 3.00) / (4.00 − 0.02) = $75.38
- If team wins: +$200 back winnings − $226.13 lay liability = −$26.13 loss
- If team loses: −$100 back stake + $73.87 lay winnings (after 2% commission) = −$26.13 loss
Both outcomes lock in a $26.13 loss. Sometimes this is the right move: instead of risking the full $100, you accept a smaller guaranteed loss. Whether to hedge depends on your read of the market — if you still believe the team will win, holding the position has higher expected value.
Same Exchange vs Cross Exchange Commission
The key difference between these two modes is who collects the commission. Choose the right mode in the calculator based on where your bets are placed.
| Scenario | Mode | Commission Applies To |
|---|---|---|
| Both bets on Betfair | Same Exchange | Winnings on both back and lay sides |
| Back at William Hill, lay on Betfair | Cross Exchange | Lay winnings only (Betfair commission) |
| Back on Smarkets, lay on Betfair | Same Exchange (with caveat) | Each exchange’s commission applies separately to its side — most calculators approximate this as Same Exchange with the higher commission |
| Matched betting qualifying bet | Cross Exchange | Lay commission only (bookmaker takes no commission) |
| Free bet extraction (SNR) | Cross Exchange (modified formula) | This calculator does not handle SNR free bets — use a dedicated matched betting calculator |
The Cross Exchange formula tends to produce a slightly larger lay stake because there’s no commission “discount” on the back side. This translates to a larger liability — make sure your exchange account holds enough.
Liquidity and Slippage Considerations
The math assumes you can execute both bets at the displayed prices. In reality, exchange markets have liquidity — the volume of money available at each price level. If you try to lay $1,000 at 3.00 and only $200 is available, your bet will be partially matched at 3.00 and the rest will queue (potentially never matching) or you’ll need to accept worse odds.
Three practical tips:
- Check market depth before placing the back bet, not just after. If liquidity at your target lay price is thin, the trade may fail to execute.
- Round lay stakes up to ensure full coverage. If the calculator says lay $134.23, lay $135 — small over-laying is usually worth more than being under-laid.
- Avoid trading low-liquidity markets (rare horse races, obscure soccer leagues). Major Betfair events typically have £10,000+ in available volume per price level near match time.
Common Mistakes
- Forgetting commission. A $50 lay win at 5% commission pays $47.50, not $50. Skipping commission in mental math leads to under-laying and an unequal book.
- Using the wrong commission mode. If you back at a bookmaker but use the Same Exchange formula, you’ll over-lay and reduce your green-book profit. Always select the correct mode.
- Ignoring liability. Lay liability scales with both lay stake and lay odds. A $100 lay at odds of 10.00 has a $900 liability — far more than most casual exchange accounts hold.
- Hedging too late or too early. Wait too long and lay odds may have crossed over your back odds, locking in a loss. Hedge at the first small move and you sacrifice the larger profit available from further price movement. Trading is partly about reading momentum.
Frequently Asked Questions (FAQ)
What is “Liability” in lay betting?
Liability is the maximum amount you’d pay out if your lay bet loses (i.e. the selection you laid actually wins). The formula is Lay Stake × (Lay Odds − 1). For a $100 lay at odds of 3.00, your liability is $200. The exchange will hold this amount in reserve from your account balance until the bet is settled, so make sure you have enough cleared funds before placing the bet.
Why is the lay stake larger than my back stake?
When the lay odds are lower than the back odds, the lay stake exceeds the back stake because lay odds are inverted. To match a $100 back at 4.00 (potential winnings of $300) with a lay at 3.00, you need to lay enough to cover that $300 winnings — which translates to roughly $134-$135 depending on commission.
What’s the difference between back/lay trading and matched betting?
Trading aims to profit from price movements on the exchange itself — back high, lay low, or vice versa. Matched betting is specifically about extracting bookmaker bonuses by neutralizing qualifying bets and free bets through corresponding lays on an exchange. Both use the same back/lay math, but trading is about market direction while matched betting is about bonus arbitrage.
What commission rate should I use for Betfair?
Betfair’s commission ranges from 2% to 5% depending on your account history (Premium Charge can push it higher for very profitable accounts, but this affects fewer than 1% of users). Most active traders pay 5% as standard. Some markets have lower rates (Italian customer base 6.5%, Australian 6%). Check your account’s “My Account” page for the exact current rate.
Can I use this calculator for in-play (live) trading?
Yes — the math is identical for pre-event and in-play markets. The challenges in-play are about execution: market lag (delays between live action and odds updates), partial matching as liquidity dries up, and the higher volatility of in-play prices. The calculator gives you the correct lay stake; whether you can actually get it matched is a market reality question.
Why does my actual profit differ from what the calculator shows?
Three common reasons: (1) you didn’t get fully matched at the calculated lay stake (partial match); (2) the commission you actually paid differs from what you entered; (3) you used decimal odds with rounding errors. Exchange odds tick in increments (1.50, 1.51, 1.52 — not continuous), so a calculated lay of $134.23 must round to a placeable amount.
Does this calculator work for SNR (Stake Not Returned) free bets?
No. SNR free bets — where you keep only the winnings (not the original stake) — require a different formula because the back-side payout is reduced. Use a dedicated matched betting calculator for free bet extraction. This calculator handles standard cash back/lay trading only.
Is exchange trading profitable long-term?
For skilled traders with disciplined money management, yes — but it’s a competitive market where bots and professional traders compete on milliseconds of execution speed. Casual traders typically break even after commission once novelty wears off. The most consistent profits come from matched betting on bookmaker bonuses rather than pure trading.