Free prediction market odds calculator

Convert between probability, decimal, fractional, and American odds instantly. Calculate expected value, Kelly criterion optimal bet size, and break-even probability for any prediction market contract.

Understanding odds formats is essential for prediction market traders. A contract priced at $0.72 on PolyGram represents a 72% implied probability — but if you are coming from sports betting, you might think in terms of decimal odds (1.39), fractional odds (7/18), or American odds (-257). This calculator converts between all four formats instantly so you always know exactly what a price means. Beyond simple conversion, the calculator includes two powerful analytical tools. The expected value (EV) calculator shows whether a trade is profitable given your estimated true probability. If you believe an event has an 80% chance of occurring but the market prices it at 65%, the positive expected value tells you the trade has an edge. The Kelly criterion calculator goes further — it tells you exactly what fraction of your bankroll to risk on a positive-EV trade to maximize long-term growth without risking ruin. Named after Bell Labs researcher John Kelly, the Kelly formula balances the tension between betting too much (risking a drawdown you cannot recover from) and betting too little (leaving money on the table). Professional prediction market traders use Kelly sizing as a core part of their risk management toolkit. The break-even probability display rounds out the tool: for any given contract price, it shows the minimum true probability needed for the trade to be profitable after fees. All calculations happen client-side in your browser — no data is sent to any server.

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