What is a lay bet?
Lay bet — A lay bet is a wager AGAINST a selection — at a betting exchange you win if the selection does NOT happen, which is what makes matched betting work.
While a back bet stakes money on something happening, a lay bet stakes on the opposite — you win if the selection does not happen. You don't need a special account type to lay; the regulated UK exchanges (Smarkets, Betfair Exchange, Matchbook) all let any logged-in user place lay bets alongside conventional back bets.
The lay side is what makes matched betting work. Without an exchange to take the opposing position, the bookmaker free bet at the back-leg side couldn't be locked in for an extracted value. The bookmaker's promo, the exchange's lay market, and a calculator that sizes the two stakes correctly are the three ingredients in every matched bet — and the structural maths is laid out in detail in our matched betting beginner guide and the technique-focused backing and laying guide.
Lay odds are usually slightly higher than the back odds at the bookmaker. That gap (the spread between the two prices) is the small cost of running the matched-betting structure — typically £0.20–£0.50 on a £10 qualifying bet. Our standard calculator does the sizing automatically.
Worked example
You lay £6.45 on Man Utd at 3.10 odds at Smarkets, paired with the £10 back bet from the back-bet entry. If Man Utd don't win — they draw or lose — you keep the £6.45 lay stake (minus the exchange's commission, usually 2%). If they do win, the exchange takes £13.55 from your balance to pay out the back-bet winners.
Either way, the back leg at the bookmaker either pays you £30 or loses you £10, and the lay leg at the exchange either loses you £13.55 or wins you £6.45. Stack the two together and the net is around -£0.30 whichever side comes in — that's the qualifying loss.
Common mistake
Confusing the lay stake with the liability. The lay stake is what you keep if the lay wins (the bit the calculator usually shows largest). The liability is the much bigger number the exchange holds in escrow against your balance until the event settles. People deposit just enough to cover the lay stake and then can't actually place the bet because the liability is bigger. Always read both before clicking.