Arbitrage calculator

Enter the best price available on each outcome and a total stake. Legs can be a bookmaker back, an exchange back or lay with commission, or a YES or NO contract on a prediction market — each is converted to the price of an equivalent back bet, so they can be combined freely. The calculator splits the stake so that every outcome returns the same amount, and tells you whether that return beats what you put in.

prediction-market legs are always read as a price

Leg 1Back price

Leg 2Back price

Total stakeBack stakes plus lay liabilities — the capital actually committed.

Arbitrage — guaranteed profit

+37.35

3.73% of stake, whichever outcome wins. The prices imply 96.40% in total — less than a whole event.

Leg	Effective	Stake	Returns	Profit
Back 2.10	2.100	493.98	1,037.35	+37.35
Back 2.05	2.050	506.02	1,037.35	+37.35

Stakes are split in proportion to each effective price's implied probability, which is what makes every return identical. A lay leg shows the stake to offer and, beneath it, the liability — the liability is the money committed, so that is what counts towards the total.

Common questions

When does an arbitrage exist?: When the implied probabilities of every outcome add up to LESS than 100%. The prices then describe less than a whole event, so buying all of it costs less than it pays out. A total above 100% is the ordinary case — the excess is the bookmaker's margin, and no split of stakes can turn that into a profit.
How do you calculate arbitrage stakes?: Each stake is proportional to its outcome's implied probability: the total stake multiplied by one divided by that price, divided by the sum of one-over-each-price. That is exactly what makes every return identical.
How does a lay bet fit into an arbitrage?: Laying an outcome is backing everything else, so it is priced as one. A lay at decimal odds L with commission c is worth the same as a back at 1 + (1 − c) / (L − 1): you risk L − 1 to win 1, less the exchange's cut. Back a team at 2.10 with a bookmaker and lay it at 2.05 on an exchange paying 2% commission and the two prices come to 99.34% — an arbitrage worth 0.66%.
What stake does the calculator show for a lay bet?: Both numbers. A lay is entered as the backer's stake but risks the liability, which is that stake multiplied by (L − 1), and the liability is the money actually committed. The table shows the stake to offer with the liability beneath it, and the liability is what counts towards your total.
Can you arbitrage prediction markets like Kalshi or Polymarket?: Yes, and both sides go in together. A contract that settles at 1 for a price of p is simply decimal odds of 1 / p, so YES at 0.62 and NO at 0.36 add up to 98% and pay 2.04% whichever way it lands. Prediction markets are not no-vig — the two sides usually sum above 100% just as a bookmaker's do, and the fee field is there because Kalshi charges per contract on a price-dependent formula rather than on profit.
Why do arbitrages disappear so quickly?: Prices move, and the side you were counting on shortens as soon as money arrives on it. Arbitrages are usually small, brief, and limited by how much a book will accept.

Arbitrage calculator

prediction-market legs are always read as a price

Leg 1Back price

Leg 2Back price

Total stakeBack stakes plus lay liabilities — the capital actually committed.

Arbitrage — guaranteed profit

+37.35

3.73% of stake, whichever outcome wins. The prices imply 96.40% in total — less than a whole event.

Leg	Effective	Stake	Returns	Profit
Back 2.10	2.100	493.98	1,037.35	+37.35
Back 2.05	2.050	506.02	1,037.35	+37.35

Common questions

When does an arbitrage exist?

When the implied probabilities of every outcome add up to LESS than 100%. The prices then describe less than a whole event, so buying all of it costs less than it pays out. A total above 100% is the ordinary case — the excess is the bookmaker's margin, and no split of stakes can turn that into a profit.

How do you calculate arbitrage stakes?

Each stake is proportional to its outcome's implied probability: the total stake multiplied by one divided by that price, divided by the sum of one-over-each-price. That is exactly what makes every return identical.

How does a lay bet fit into an arbitrage?

Laying an outcome is backing everything else, so it is priced as one. A lay at decimal odds L with commission c is worth the same as a back at 1 + (1 − c) / (L − 1): you risk L − 1 to win 1, less the exchange's cut. Back a team at 2.10 with a bookmaker and lay it at 2.05 on an exchange paying 2% commission and the two prices come to 99.34% — an arbitrage worth 0.66%.

What stake does the calculator show for a lay bet?

Both numbers. A lay is entered as the backer's stake but risks the liability, which is that stake multiplied by (L − 1), and the liability is the money actually committed. The table shows the stake to offer with the liability beneath it, and the liability is what counts towards your total.

Can you arbitrage prediction markets like Kalshi or Polymarket?

Yes, and both sides go in together. A contract that settles at 1 for a price of p is simply decimal odds of 1 / p, so YES at 0.62 and NO at 0.36 add up to 98% and pay 2.04% whichever way it lands. Prediction markets are not no-vig — the two sides usually sum above 100% just as a bookmaker's do, and the fee field is there because Kalshi charges per contract on a price-dependent formula rather than on profit.

Why do arbitrages disappear so quickly?

Prices move, and the side you were counting on shortens as soon as money arrives on it. Arbitrages are usually small, brief, and limited by how much a book will accept.