Lay Betting Maths: Work Out Your Liability Before You Lay
Laying turns you into the bookmaker for one outcome, and the risk is bigger than you might expect.
Laying a bet means betting that an outcome will not happen. What you risk is called liability, not your stake.
On an exchange such as Betfair, Betdaq, Smarkets or Matchbook, another user backs an outcome and your lay is matched against it at the odds you both accept, so you get paid the way a bookmaker would be. If the outcome happens anyway, you pay out, and that payment is your liability: the lay stake times the odds minus one, which can run far higher than the stake itself. If it doesn't happen, you keep the stake, minus whatever commission the exchange charges on that net win. Work out the liability before you lay anything. That is the number that has to sit ready in your account, not the profit you are hoping for.
What changes when you lay instead of back
A normal bet backs an outcome: you pick a winner and you get paid if it comes in. Laying flips that around. You are betting that something will not happen, and the exchange matches your lay against another user's back bet at the same odds, which is what people mean by back and lay betting. Once matched, you are effectively standing where the bookmaker usually stands for that one selection (not always instantly, and not always in full, since matching depends on someone taking the other side).
That switch changes what risk actually means. On a back bet you can only lose your stake. On a lay bet, the amount you can lose is your liability: the lay stake multiplied by the odds minus one. Lay a 20 stake at odds of 10.00 and your liability is 180 (nine times the number you typed into the stake box), which is the whole point of checking it before you commit, not after. Worst case first, always. Liability first, not profit. That ordering matters more than any single number on this page.
We would treat any guide that leads with profit before liability as incomplete, because the liability figure is the one that can actually empty an account, not a minor footnote some pages treat it as. You can read the wider mechanics of matching, unmatched money and the rest on our guide to how a betting exchange works, which this page assumes you have already skimmed.
How much can you lose if the bet wins?
Liability is simple arithmetic once it is written down: lay stake times the odds minus one. The table below uses a lay stake of 100 across a spread of odds you might actually see on a match or a race, so you can compare the shape of the risk before you go anywhere near the calculator.
| Decimal odds | Liability if it wins |
|---|---|
| 1.50 | 50.00 |
| 2.00 | 100.00 |
| 3.00 | 200.00 |
| 5.00 | 400.00 |
| 10.00 | 900.00 |
Liability equals lay stake times (odds minus one), calculated from the stake and odds in each row, not a quoted market figure. At odds of 10.00 the liability is nine times the stake, and the multiplier only grows from there at longer prices: that is the number that has to be sitting in your account before you place the bet.
Not every lay is reactive. Some bettors lay a position on purpose, to hedge a bet already placed or as one leg of an arbitrage position built across two different prices. Either way, the liability above is what you are committing to, whatever the reason for the bet.
How the commission rate differs by exchange
Commission is the piece that turns a fair-looking price into something narrower once you actually collect. It is not charged on your stake. It is taken only from your net winnings in that market, and only if you win. Rates vary by exchange and by territory, so treat any single number below as a starting point, not automatically your own rate.
- Betdaq states on its own homepage that it charges 2 percent of net winnings for customers in the UK, Ireland, Gibraltar and Jersey, and 5 percent everywhere else.
- Smarkets' own commission page lists a standard rate of 2 percent, with a reduced 1 percent "Pro" rate for high-volume accounts and a higher 3 percent "Select" rate for the most consistently profitable ones.
- Betfair's Australian help pages publish a base rate of 6 percent on sport, plus an "Expert Fee" of 20 to 40 percent on profit above set thresholds for its most active winners. Betfair's own base rate in the United Kingdom is widely quoted as 5 percent, but Betfair's UK help pages could not be read directly for this page, so treat that number as unconfirmed and check the rate shown on your own account before you rely on it.
- Matchbook's own commission terms could not be read directly for this page either. Guides quote 2 percent for UK, Irish, Manx and Channel Island customers and 4 percent elsewhere, but that is unconfirmed at the source, not a published fact, so we would not print it as one.
That is also why the calculator further down starts its commission field at zero, with no exchange's rate preselected, rather than picking a number for you: see the note above about Matchbook. Eligibility is a separate question from commission (check whether you can even open an account with a given exchange before you worry about its rate). For the complete side-by-side comparison across exchanges, see how these exchanges compare on commission and access. A bookmaker's margin works differently again, baked into both sides of the price rather than taken from a winning bet afterwards, so what a bookmaker's price costs you is not directly comparable to an exchange's commission, even though both eat into what you actually collect.
The gap where neither side has an edge
Take commission out of the picture and the back price and the lay price agree exactly: odds of 2.00 imply a fifty percent chance, with no gap between what a backer needs and what a layer needs. Add commission back in and that single line splits into two. A backer only has an edge once the true chance sits above one line; a layer only has an edge once it sits below a second, lower line. Between them sits a band where the true probability can land and neither side is being paid enough for the risk it is taking, a dead zone that widens at short odds and narrows at long ones.
Break-even probability for backing vs laying, at 5% commission
Published formula, calculatedView the full break-even table, including the no-commission line
| Decimal odds | No commission | Backer needs above (5%) | Layer needs below (5%) | Dead zone width |
|---|---|---|---|---|
| 1.50 | 66.7% | 67.8% | 65.5% | 2.3 pts |
| 2.00 | 50.0% | 51.3% | 48.7% | 2.6 pts |
| 3.00 | 33.3% | 34.5% | 32.2% | 2.3 pts |
| 5.00 | 20.0% | 20.8% | 19.2% | 1.6 pt |
| 10.00 | 10.0% | 10.5% | 9.5% | 0.9 pt |
None of the exchange calculators we checked while researching this page draw that band; most stop at your profit on a single bet, which is a fair thing to calculate but a different question from where the edge actually sits. One honest limit of the model above: it assumes commission is taken on the net result of a single isolated bet. In practice, Betfair and comparable exchanges charge commission on your net result across the whole market, all your matched bets on that selection combined, which is a simplification shared by every calculator we looked at, not a shortcut unique to this page.
Run your own numbers before you lay
The calculator below takes your own odds, lay stake and commission rate, and returns the liability, the payout after commission if the bet loses, and both break-even lines from the section above. The commission field starts at zero on purpose, with no exchange's rate preselected. At least one calculator we found while researching this page prints a Matchbook rate as though it were confirmed, and Matchbook's own commission terms are not readable from its help pages. A guessed rate is worse than an honest gap, so that field stays blank for you to fill in.
The two break-even readouts use the same formula as the chart above, at whatever commission rate you enter (yes, even at short odds like 1.50, where the dead zone is at its widest). This is not a probability estimate for any actual match; it only returns the arithmetic once you supply a number for how likely you think the outcome really is.
Frequently asked questions
What does it mean to lay a bet on an exchange?
You are betting that a specific outcome will not happen, taking the side a bookmaker normally takes for that selection. Another user's back bet is matched against your lay at the odds you both accept.
How much could I lose on a lay bet?
Your liability, not your stake: the lay stake multiplied by the odds minus one. See the table above for a 100 stake across five common odds. The same multiplier applies whatever stake you actually use.
Is commission charged on my stake or only on what I win?
Only on your net win in that market, and only if you win it. A losing lay costs you the stake you agreed to pay, nothing more, and no commission is taken from a loss.
Where is the point where neither backing nor laying is worth it?
Between the two break-even lines shown above. Above the higher line, backing has an edge; below the lower line, laying does. In between, commission has taken the edge from both sides.
Why was my lay bet only partly matched?
An exchange needs a backer on the other side, at your price, for your full amount, and that liquidity is not always available, especially away from the biggest markets and events. Unmatched money simply sits open until someone takes it or you cancel it.
Is laying the same as acting like a bookmaker?
Close, for that one bet: you set a price, someone takes it, and you carry the liability if it wins. It stops there. You are not licensed as a bookmaker, and the exchange handles matching, settlement and identity checks. For a related way of taking a position on an outcome, see how prediction markets price an outcome directly instead of through a back and lay pair.