The maths, without the spreadsheet
Everything here runs in your browser. No account, no logging, nothing leaves the page — these are the four sums worth doing before a position, and none of them needs a server.
Prediction market odds converter →
Formula, worked example and assumptions
No-vig probability calculator →
Formula, worked example and assumptions
Kelly criterion calculator for prediction markets →
Formula, worked example and assumptions
Prediction market payout and expected value calculator →
Formula, worked example and assumptions
Odds converter
A prediction-market price in cents is already an implied probability. This is the table you need when a sportsbook quotes the same event in American odds and you want to know which side is cheaper.
At 42¢ you pay $0.42 for a contract that settles at $1. The fee-free break-even probability is 42.0%; costs raise that threshold.
No-vig fair odds
Normalize two complementary outcome prices to a total of 100¢. This proportional estimate is not the true probability, and an order-book spread is not a platform fee.
The two quoted prices exceed 100¢ by 3.0¢. This is not a fee estimate.
Kelly stake
How much of a bankroll to put on a position, given the price and your own probability. Full Kelly maximises expected logarithmic growth under its assumptions. Fractional Kelly scales down that model output; neither validates your probability estimate.
Positive edge at your own numbers. Kelly assumes those numbers are right, which is the whole risk.
Payout and expected value
What the position pays if it resolves your way, what it costs if it does not, and what it is worth on average at your own probability.
Expected value is per-trade, before fees and before the spread you actually get filled at.
Not advice. These are arithmetic tools. They take your own probability as an input and cannot tell you whether it is right — which is the part that actually decides whether a position makes money.