What is a back and lay bet?

Backing means betting on something to happen (at a bookmaker). Laying means betting on it NOT to happen (at a betting exchange like Betfair). Doing both on the same outcome covers every result, and if the prices are right you profit whatever happens.

How do I calculate the lay stake?

Lay stake = (back stake x back odds) / (lay odds - commission), with commission as a decimal. For example, a EUR100 back bet at 4.00 laid at 3.50 with 2% commission needs a lay stake of (100 x 4.00) / (3.50 - 0.02) = EUR114.94.

What is liability in a lay bet?

Liability is the amount the exchange holds from your balance to cover the bet if the outcome you laid actually happens. Liability = lay stake x (lay odds - 1). It is the real money you risk on the lay side.

How does exchange commission affect the lay stake?

Exchanges charge commission (typically 2-5%) on your net winnings, so a winning lay pays slightly less than the full lay stake. The calculator subtracts commission from the lay odds when sizing the stake, so both outcomes still return the same profit.

Is back and lay the same as arbitrage?

Yes — it is arbitrage between a bookmaker and an exchange. Instead of backing every outcome at different bookmakers, you back one side and lay the same side. If V (1/back odds + 1/lay-equivalent odds) is below 1, it is a surebet.

What lay odds do I need to make a profit?

As a rule, the lay odds must be low enough relative to the back odds to leave a gap after commission. The calculator shows V instantly: below 1 means guaranteed profit, above 1 means you would lose money (a qualifying loss).

Back Lay Calculator | Free Lay Stake & Liability Calculator

V =

Back an outcome at a bookmaker, lay the same outcome at an exchange — the lay stake, liability and profits update in real time. V below 1 means a guaranteed profit whatever happens.

Profitability = 0%

Back — bookmaker

Odds

Commission (%)

Stake

Odds w/ comm—
Profit if it wins—

Lay — exchange

Odds

Commission (%)

Lay stake (backer’s bet)

Odds w/ comm—
Liability—
Profit if it loses—

What is a back lay calculator?

A back lay calculator works out exactly how much to lay at a betting exchange to cover a back bet you placed at a bookmaker, so that your profit is the same whichever way the event goes. It handles the two things people get wrong by hand: the exchange commission and the liability the exchange holds.

It is the tool behind matched betting and bookmaker-vs-exchange arbitrage. Back one outcome, lay the same outcome, and if the prices leave a gap you lock in profit before the event starts.

How to use the calculator

Enter your back bet

The bookmaker odds, any commission, and the stake you are placing.

Enter the lay side

The exchange lay odds and your commission (usually 2–5%).

Read your lay stake

The exact lay stake to place, plus the liability the exchange holds.

Check the profit

Your profit if it wins and if it loses — equal by design.

How is the lay stake calculated?

Two formulas do all the work. With commission written as a decimal (2% = 0.02):

Lay stake = (back stake × back odds) ÷ (lay odds − commission)
Liability = lay stake × (lay odds − 1)

Subtracting the commission from the lay odds is the key step — it accounts for the exchange taking a cut of your winnings, so both results still pay the same. If your bookmaker also charges commission, the back odds become 1 + (back odds − 1) × (1 − commission).

What is liability, and why does it matter?

When you lay a bet you are acting like the bookmaker: if the outcome you laid does happen, you pay out. The exchange therefore ring-fences that potential payout — the liability — from your balance. It is always larger than your lay stake, so it is the number that decides how much money you actually need in your exchange account. The calculator shows it next to your lay stake, and adds it to your back stake to give the total stake you have at risk.

Worked example

You back an outcome at 4.00 with €100 at a bookmaker (no commission), and lay the same outcome at 3.50 on an exchange charging 2%:

Lay stake = (100 × 4.00) ÷ (3.50 − 0.02) = €114.94
Liability = 114.94 × (3.50 − 1) = €287.36
If it wins: back returns €300 profit − €287.36 liability = +€12.64
If it loses: lay wins 114.94 − 2% = €112.64 − €100 stake = +€12.64

Same profit either way, on a total stake of €387.36 — a 3.26% guaranteed return.

When is a back/lay bet a surebet?

The same rule as any arbitrage bet applies. Convert the lay side into its back-equivalent odds — (lay odds − commission) ÷ (lay odds − 1) — then add the inverses:

V = 1/back odds + 1/lay-equivalent odds

If V is below 1, you have a guaranteed profit; the further below, the bigger the margin. Above 1 and you are taking a qualifying loss — which is fine when you are unlocking a free bet, but not otherwise. The maths is explained in full in our arbitrage betting formula guide.

Odds shown in fractional or American format? Convert them first with the odds converter.

Learn more about arbitrage betting

New to this? Read our full guide on what is arbitrage betting, or see the arbitrage betting formula explained step by step with worked examples. Odds in a different format? Use our odds converter to switch between decimal, fractional, American and implied probability.

Frequently asked questions

What is a back and lay bet?: Backing means betting on something to happen (at a bookmaker). Laying means betting on it NOT to happen (at a betting exchange like Betfair). Doing both on the same outcome covers every result, and if the prices are right you profit whatever happens.
How do I calculate the lay stake?: Lay stake = (back stake x back odds) / (lay odds - commission), with commission as a decimal. For example, a EUR100 back bet at 4.00 laid at 3.50 with 2% commission needs a lay stake of (100 x 4.00) / (3.50 - 0.02) = EUR114.94.
What is liability in a lay bet?: Liability is the amount the exchange holds from your balance to cover the bet if the outcome you laid actually happens. Liability = lay stake x (lay odds - 1). It is the real money you risk on the lay side.
How does exchange commission affect the lay stake?: Exchanges charge commission (typically 2-5%) on your net winnings, so a winning lay pays slightly less than the full lay stake. The calculator subtracts commission from the lay odds when sizing the stake, so both outcomes still return the same profit.
Is back and lay the same as arbitrage?: Yes — it is arbitrage between a bookmaker and an exchange. Instead of backing every outcome at different bookmakers, you back one side and lay the same side. If V (1/back odds + 1/lay-equivalent odds) is below 1, it is a surebet.
What lay odds do I need to make a profit?: As a rule, the lay odds must be low enough relative to the back odds to leave a gap after commission. The calculator shows V instantly: below 1 means guaranteed profit, above 1 means you would lose money (a qualifying loss).