Vig & Implied Probability Calculator
Enter the YES and NO prices from any prediction market. See the total house edge (vig), vig-adjusted implied probabilities for both sides, and the break-even probability you need to trade profitably.
Vig formula: overround = (YES¢ + NO¢) / 100. Vig-adjusted probability: p_yes = YES¢ / (YES¢ + NO¢). A market is fair when overround = 100¢ exactly.
Vig & Implied Probability FAQ
- What is vig in prediction markets?
- Vig (short for vigorish) is the house edge built into a market's prices. When a YES share costs 45¢ and a NO share costs 58¢, buying both sides costs 103¢ for a guaranteed $1.00 payout — the extra 3¢ is the vig. Polymarket and Kalshi embed vig in their bid-ask spreads and fees, so understanding it is essential to finding real positive-EV trades.
- What is implied probability?
- Implied probability is the win probability that a market price implies if the market is fair. A YES share at 45¢ implies the event has a 45% chance of happening. But once you adjust for the vig, the true implied probability is slightly different — vig-adjusted probability = price / (YES + NO prices combined).
- What is a good vig on Polymarket and Kalshi?
- For liquid Polymarket markets, total overround is typically 1–3% on major political and crypto markets. Kalshi tends to run 2–5% on most markets. Sports markets can have 5–8% overround. Markets with vig above 6–7% are expensive to trade and require a large edge to be profitable.
- What does vig-adjusted probability mean?
- Raw implied probability takes each side at face value: a 45¢ YES share = 45% probability. Vig-adjusted probability removes the house cut to estimate the market's true consensus. If YES trades at 45¢ and NO trades at 58¢, vig-adjusted YES probability = 45 / (45 + 58) = 43.7%, and vig-adjusted NO = 56.3%. This is a better estimate of what the market consensus actually is.
- How do I find arbitrage with this calculator?
- Enter YES and NO prices. If the overround is below 100, meaning prices sum to less than $1.00, the calculator flags a theoretical price gap. This is rare on single platforms but more common across two platforms when the same event is priced differently. Fees, liquidity, and resolution risk still matter.
Worked example
A Polymarket market shows YES at 45¢ and NO at 58¢.
Overround: (45 + 58) / 100 = 1.03 — the market has 3% vig.
Vig-adjusted YES probability: 45 / (45 + 58) = 43.7%. The market's true consensus says YES is a 43.7% shot, not 45%.
To buy YES profitably, your true probability estimate must exceed 45% (the break-even). At 43.7% vig-adjusted, that means you need at least 1.3 cents of edge above the market consensus to profit after vig.