Back/lay arbitrage explained
Standard arbing needs two bookmakers to disagree. Back/lay arbing needs only one bookmaker — plus a betting exchange. It's the most frequent and most sustainable type of arb, because the exchange half of the bet never gets your account limited.
Back vs lay: the one idea you need
At a normal bookmaker you back — you bet that something will happen. On a betting exchange (Betfair, Smarkets, Matchbook) you can also lay — you bet that something won't happen. When you lay, you become the bookmaker: you accept someone else's bet.
So if you back Arsenal at a bookie and lay Arsenal on an exchange, you've covered both sides of "Arsenal win". If the bookie's back price is high enough relative to the exchange's lay price, you make money either way.
Two terms that trip people up
- Liability — when you lay, this is the amount you'd have to pay out if the selection wins. Lay Arsenal at 3.0 for a £50 stake and your liability is £50 × (3.0 − 1) = £100. The exchange ring-fences that from your balance.
- Commission — exchanges don't build a margin into the odds; they take a small cut (typically 2–5%) of your net winnings instead. It only applies when you win, and it's the number that decides whether a back/lay arb is actually profitable.
A worked example
Say you find:
- Back Player A at a bookmaker: 2.10, staking £100
- Lay Player A on the exchange: 2.00, commission 2%
To lock equal profit, you lay roughly £106 (the exact figure comes from the back odds, lay odds and commission). The result:
- If Player A wins: you collect from the bookmaker but pay your lay liability — net ≈ +£3.94.
- If Player A loses: you lose the back stake but keep the layer's stake, minus commission — net ≈ +£3.94.
Same profit either way — that's the lock. You can play with these exact numbers on our free calculator's Back/Lay tab: enter 2.10, 2.00 and 2% commission and you'll see the lay stake, liability and locked profit appear.
The rule of thumb: a back/lay arb exists when your back odds are higher than the lay odds after commission. The closer the lay price sits to the back price, the bigger your edge.
Why it's the most sustainable arb
Here's the part bookmakers don't advertise. Only one of your two bets sits with a traditional bookmaker — the back. The lay sits on an exchange, and exchanges don't limit winning customers because they earn their commission whether you win or lose. So your most-restricted account (the bookie) only carries half the action, and the half that does the heavy lifting (the exchange) is bulletproof. Spread your backs across several bookies and no single account takes much heat. More on that in how bookmakers spot arbers.
Practical tips
- Place the lay first. Exchange prices move faster and the available liquidity can disappear; lock the harder leg, then back at the bookie.
- Check the liquidity. The exchange needs enough money available at the lay price to match your stake. A great price with £12 available is no use for a £100 lay.
- Know your commission rate. 5% default on Betfair, lower on Smarkets/Matchbook. A point or two changes which arbs are worth it.
- Watch whole-number handicap and total lines — they can push or void, which breaks the "guaranteed" part. Stick to clean two-way markets when you're starting out.
Let the bot do the spotting
SureBetUK scans bookmaker back prices against live exchange lay prices and only alerts when there's genuine profit after commission. The free channel includes back/lay arbs daily.
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