Arbitrage Calculator
Find the exact stakes to lock in a risk-free profit across two books or a book and a prediction market.
The Two Prices
Opposite outcomes from two different venues.
-110 · 1.91 · 10/11 · 52.4%
+120 · 2.20 · 6/5 · 45.5%
✅ Arbitrage Found
Combined implied probability: 97.81% — below 100%, guaranteed profit.
Optimal stake split:
Bet on Side 1:$53.53
Bet on Side 2:$46.47
Guaranteed Profit$2.24
ROI2.24%
How to use this tool
- Enter your total investment.
- Enter the price for each opposite outcome — switch formats per side, e.g. American on a book and Probability/Prediction-Market cents on Kalshi.
- If the combined implied probability is below 100%, place both stakes as shown to lock in the profit.
The formula
stakeᵢ = total × (impliedᵢ / Σ implied) · arb exists when Σ implied < 100%- impliedᵢ = 1 / decimalᵢ
- Σ implied = sum of both sides' implied probabilities
Worked example
Side 1 −110 (52.38%) and Side 2 +120 (45.45%) sum to 97.83%. On $100 you stake $53.54 / $46.46, both returning ≈ $102.21 — a $2.21 (2.21%) sure profit.
Book vs prediction market: prediction-market contracts carry little to no vig, so a book offering a soft line against a Kalshi/Polymarket price is one of the most reliable sources of true arbitrage. Enter the contract price via the Probability format.