From a back stake, both prices and your commission rate, this back lay calculator works out the lay stake that balances a bookmaker bet against an exchange lay, and what the pair leaves either way.
Because the lay covers the back bet, the pair costs only the gap between the two prices and the commission on a winning lay, not the whole stake. That cost is what betting promotions have to outweigh, and the method is matched betting.
How to use the back lay calculator
- What the back bet is: a cash bet, or a bonus bet whose stake is returned with a winner.
- Back stake: what goes on at the bookmaker, or the bonus bet's face value.
- Back odds: the bookmaker's decimal price, $1.01 to $10,001.
- Lay odds: the exchange price you can lay at, usually a little longer.
- Commission %: your account's rate on net winnings. An empty box is 0%, and none is filled in.
- Lay stake: empty for the balanced stake, or your own to underlay or overlay.
Four tiles give the lay stake, the liability, the worst of the two results and that worst result against the back stake.
Lay stake formula, and where commission goes
Commission is charged on net winnings in a market, so only a winning lay pays it, never a stake.
- liability = lay stake x (lay odds - 1)
- if the selection wins = back profit - liability
- if the selection loses = lay stake x (1 - commission rate) - back stake
- balanced lay stake = back stake x back odds / (lay odds - commission rate)
The commission box opens empty, so the tool shows 150 / 3.10 = $48.39 until you type a rate: 150 / 3.03 = $49.50 at an illustrative 7%.
A worked qualifying bet at $50 and $3.00
With illustrative prices and an illustrative 7% rate, a $50 cash bet at $3.00 laid at $3.10:
| Step | Working | Result |
|---|---|---|
| Balanced lay stake | 50 x 3.00 / (3.10 - 0.07) | $49.50 |
| Liability | 49.50 x (3.10 - 1) | $103.95 |
| Selection wins, bookmaker | 50 x (3.00 - 1) | $100.00 |
| Selection wins, total | 100.00 - 103.95 | -$3.95 |
| Selection loses, exchange | 49.50 x (1 - 0.07) | $46.04 |
| Selection loses, total | 46.04 - 50.00 | -$3.96 |
About $3.96 goes whichever way it lands, or 7.92% of the stake. That expected cost is a qualifying loss, and the closeness of the two prices is a match rating. The two sides sit a cent apart because a bet slip takes cents: 150 / 3.03 is about $49.505, so $49.50 goes on.
Risk: Betting involves risk. Until the lay is matched at the price shown, the back bet is unhedged and its whole stake is at risk, a price can move between the two bets, and there is no guarantee of profit. See responsible gambling for support.
What a stake returned bonus bet changes
A bonus bet whose stake comes back pays back stake x back odds and costs nothing to lose, so the balanced lay stake does not move.
On an illustrative $50 bonus bet at $4.00 laid at $4.20 with an illustrative 5% rate, the lay stake is 200 / 4.15 = $48.19. It keeps 200 - 48.19 x 3.20 = $45.79 if the selection wins and 48.19 x 0.95 = $45.78 if it loses, or 91.56% of the bonus bet. The same bet in cash costs 45.78 - 50 = -$4.22 if it loses.
A bonus bet whose stake is not returned pays back stake x (back odds - 1) instead, which the bonus bet converter handles. Under the National Consumer Protection Framework for online wagering, winnings from a bonus bet must be withdrawable with no turnover requirement (National Policy Statement, updated 3 May 2022). Each bookmaker's terms decide how the bet works.
Risk: Betting involves risk. A bonus bet that loses pays nothing, it is unhedged until your own lay is matched, and many bookmakers restrict or prohibit automated betting, third-party access and multiple accounts in their terms. A bookmaker can limit stakes, void bets or close an account, and that risk is yours.
Underlay and overlay
Below the balanced stake is an underlay, which keeps more if the selection wins and loses more if it does not; above it is an overlay, which does the reverse. In racing an overlay is a different thing: a price longer than the fair price. How far to shift is yours.
| Lay stake | Liability | If it wins | If it loses |
|---|---|---|---|
| $40.00 (underlay) | $84.00 | +$16.00 | -$12.80 |
| $49.50 (balanced) | $103.95 | -$3.95 | -$3.96 |
| $60.00 (overlay) | $126.00 | -$26.00 | +$5.80 |
| $0.00 (no lay) | $0.00 | +$100.00 | -$50.00 |
The lay bet calculator covers a lay on its own.
Risk: Betting involves risk. An underlay or an overlay leaves part of the back bet unhedged, so one result costs more: $40.00 laid loses $12.80 if the selection loses, against $3.96 balanced.
Refusals, part matches and moving prices
It refuses a back stake at or under zero, either stake over $1,000,000, a price outside $1.01 to $10,001, a rate below 0% or at 100% or more, and anything that is not a plain number. A half-typed box blanks the result rather than a wrong one.
It works from the prices typed in, so re-run it at the lay price you can get: a drift to $3.30 takes the balanced lay to 150 / 3.23 = $46.44 and the cost to 100 - 106.81 = -$6.81. If only $30 of the $49.50 is matched, type 30 as the lay stake and lay the rest before the event starts.
The Terminal shows prices from 40+ bookmakers across racing and sports, beside Betfair back and lay prices, so both numbers sit on one odds comparison board. A free account opens a limited view of the Terminal with live odds, and the Betfair back and lay columns start at Terminal View. B337 places no exchange bets, so the lay is yours. Bookmaker names are trade marks of their owners. B337 is not affiliated with them.
Risk: Betting involves risk. Prices on the Terminal are read on a repeating cycle, not tick by tick, so a price on screen can trail the bookmaker's own: confirm both in your own accounts before you bet.
Back and lay mistakes, and what each costs
With illustrative numbers, on the $50 at $3.00 example:
| Mistake | What it costs | The fix |
|---|---|---|
| Typing the liability into the lay stake box | $103.95 laid gives -$118.30 if it wins, against -$3.95 | Lay $49.50 and read $103.95 beside it |
| Laying the back stake, not the return | $50 laid gives -$5.00 if it wins and -$3.50 if it loses | Lay back stake x back odds, then allow for commission |
| Leaving commission at 0% on an account charging 7% | $48.39 goes on, not $49.50, and the losing side comes to -$5.00 | Type your own rate in the box |
| Laying a bonus bet whose stake is not returned | $49.50 laid where 100 / 3.03 = $33.00 balances, so -$3.95 if it wins against +$30.70 | Use the bonus bet converter |