Free tools
Prediction Market Calculators
Six calculators for trading Kalshi and Polymarket — all free, all client-side. Nothing you type here leaves your browser. Start with the odds converter below, or jump straight to expected value, Kelly sizing, no-vig fair odds, fees, or parlays.
Odds Converter
Type into any field — price in cents, implied probability, American odds, or decimal odds — and the rest update instantly.
Every prediction-market price is odds in disguise. A 40¢ YES contract implies a 40% chance and pays out 2.50x your stake if it resolves YES — the same market priced as American odds is +150, or 3/2 as a fraction.
Expected Value & Edge
Compare the market's price to your own estimate of the true probability.
Kelly Stake
Sizes a position from your edge and bankroll. Quarter-Kelly is the conservative default — full Kelly is shown for reference.
No-Vig Fair Odds
Strip the vig out of both sides of a two-sided market to see the true, no-vig probability.
The book is charging 5¢ of vig across both sides — remove it and YES is fairly priced at 52.38%.
Fee Calculator
Kalshi charges a per-contract trading fee that scales with how close the price is to 50¢. Polymarket charges $0 on binary markets.
Kalshi trading fee: ceil(0.07 × P × (1−P) × 100)/100 per contract, where P is price in dollars.
Parlay Calculator
Combine 2–4 legs into one parlay. Leave a leg blank to exclude it — at least 2 legs are required.
Risk $100 to win $138.10.
These calculators are informational tools, not financial advice. MarketMaster does not place trades, guarantee outcomes, or account for fees, slippage, or liquidity beyond what's modeled above — verify numbers on the venue before you trade.