Arbitrage calculator
Updated
Two venues quoting the same event do not always agree. When Yes on one and No on the other can both be bought for less than the $1 the pair is certain to pay, the difference is an arbitrage. Enter the two prices and a stake, and this splits it so both outcomes return the same.
Arbitrage calculator
- Both sides cost
- 97.0¢
- less than the $1 they pay
- Split the stake
- $49.48
- and $50.52 on the other
- Returns, either way
- $103.09
- +3.09% on the stake
This is the arithmetic, not a trade. Fees, the size actually available at each quote, and one leg filling while the other moves all come out of that margin — and on a thin book they can take all of it.
Why the margin is usually smaller than it looks
The quote is not the fill. The price you see is the best offer, and it may be a handful of contracts deep. Putting real size through it means walking up the book, and the average price you actually pay is worse than the one this calculator used.
Both legs have to land. Filling the first leg tells the market something, and the second price can move before you reach it. A half-filled arbitrage is an ordinary directional position you did not intend to take.
Fees come out of the margin, not the profit. A 1.5% edge and a 1% round trip is a 0.5% trade before anything goes wrong, and each venue charges on its own schedule.
Capital sits on both sides until it settles. The return is certain but not quick, and the money is committed at two venues for the life of the event.
Questions
- What is an arbitrage bet?
- Buying both sides of the same event at two venues whose prices disagree enough that the pair costs less than the $1 it is certain to pay. The stake is split so that both outcomes return the same amount, and the difference between what you paid and what comes back is the margin.
- How do you calculate an arbitrage split?
- Add the two prices. If they sum to less than $1 there is an arbitrage, and the return on any total stake is that stake divided by the sum. Each side gets the share of the stake matching its own price — 48¢ and 49¢ on a $100 stake means $49.48 and $50.52, returning $103.09 either way.
- Why does arbitrage disappear before you can take it?
- Because the quoted price is not the price you get for size. The best offer may be a few contracts deep, fees come out of the margin, and filling one leg moves the market you were about to hit on the other. A 2% edge on paper regularly nets nothing once all three have had their turn.
- Do Polymarket and Kalshi prices differ?
- Routinely. They are separate books with separate participants, and Papermarkets lists both prices on the same event page where an event is carried by each. Most of the gaps are smaller than the cost of crossing them, which is what keeps them there.
- Is arbitrage legal?
- Taking a price that two venues disagree about is not itself wrongdoing, but every venue has its own terms about who may trade and how, and some restrict accounts that do it systematically. Read the terms of the venues you are using. Nothing here is legal or financial advice.
See where the two books actually disagree
Papermarkets lists Polymarket’s and Kalshi’s prices side by side on every event both venues carry, so you can watch real gaps rather than invent numbers for this calculator. Browse the live markets, see how much is open across both books right now, or strip the margin out of a single two-way line with the implied probability calculator.