Back / Lay Calculator
Back at a sportsbook, lay on an exchange. This works out the lay stake, the liability behind it — which is bigger than people expect — and what commission leaves you with.
Laying, and the number that catches people out
When you lay, you are the sportsbook: you accept someone else’s bet. Your lay stake is what you stand to win. Your liability is what you stand to lose, and it is the lay stake multiplied by the decimal price minus one. Lay $50 at +300 (4.00) and you are risking $150, not $50. Exchanges show this clearly, but it still surprises people the first time, and it is the number this calculator puts front and center.
Why commission is not a detail
Exchanges charge a percentage of net winnings on the winning side. It sounds small and it is decisive: back and lay the same selection at exactly the same price and, with zero commission, you break even — with 5% commission you lose. That is why the gap between the back price and the lay price has to be wide enough to cover the commission before the position is worth anything at all.
This is not risk-free
Back-and-lay positions are often sold as risk-free money. They are not. Prices move while you are placing the second leg and you can end up unmatched on one side; exchange liquidity can be thin enough that your lay does not fill at the price you saw; sportsbooks void bets and limit accounts; and a stake-size error costs more than the position was ever going to make. The arithmetic is certain. The execution is not.
The formula
lay stake = (Oₑ × S) / (Oₗ − c)
liability = lay stake × (Oₗ − 1)
if the back wins → S × (Oₑ − 1) − liability
if the lay wins → lay stake × (1 − c) − S
where A is an American price and O its decimal equivalent, S is the back stake, Oₑ the back odds, Oₗ the lay odds, and c the commission as a fraction.
Worked example
Back $100 at +300 and lay at +310 with 5% commission. In decimal those are 4.00 and 4.10, so the lay stake is (4.00 × 100) ÷ (4.10 − 0.05) = $98.77, carrying a liability of $306.17. Both outcomes land within a cent of each other — and because the lay price is longer than the back price, the position is slightly negative once commission is taken.
Exchange prices are one reference. There are 14 others.
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FAQ
What is laying a bet?
Laying is betting that something will NOT happen — taking the sportsbook’s side. You win the lay stake if it loses, and you lose the liability if it wins.
What is liability?
What a lay bet actually risks: the lay stake multiplied by the decimal odds minus one. Laying $50 at +300 — 4.00 in decimal — risks $150.
How does commission work?
An exchange that charges commission takes a percentage of net winnings on the side that wins. It applies to the winnings, not the stake, and it is enough to turn a level position into a losing one.
Is back and lay risk-free?
No. The arithmetic is certain but the execution is not — prices move between legs, exchange liquidity can be thin, lays go unmatched, and sportsbooks void bets and limit accounts.