The Complete Guide to Prediction Market Odds Conversion
Prediction markets and sportsbooks use fundamentally different ways to express the same thing: how likely an event is to happen and how much you'll be paid if it does. Polymarket and Kalshi quote contracts in cents from 0 to 100, where the price equals the implied probability. American sportsbooks like FanDuel, DraftKings, and BetMGM use moneyline odds (-110, +200). European books quote decimal odds (1.91, 3.00). UK bookmakers still publish fractional odds (10/11, 2/1). To compare value across platforms, you need to translate fluently between all four formats.
Why prediction market prices are the cleanest format
A 65¢ Polymarket contract pays out exactly $1 if the event resolves YES and $0 if it resolves NO. The price is therefore the market's collective probability estimate: 65%. There's no vig built into the format itself — fees come from the trading platform, not from the price. This makes prediction market quotes the most transparent way to think about probability, which is why hedge funds, election forecasters, and macro traders increasingly cite Polymarket prices as their reference point. Sportsbook odds, by contrast, always include a built-in margin (the vig) that must be backed out before you can read the true probability.
American odds explained
American odds use positive and negative numbers anchored at $100. Negative numbers (e.g. -186) tell you how much you must wager to win $100 in profit. Positive numbers (e.g. +200) tell you how much profit you'd win on a $100 wager. Convert American to implied probability with these formulas: for negative odds, probability = |odds| / (|odds| + 100); for positive odds, probability = 100 / (odds + 100). A -186 line implies 65% probability — exactly the same as a 65¢ Polymarket contract on the same event.
Decimal odds explained
Decimal odds are the easiest format to do math with. The decimal number is your total return per $1 wagered, including your stake. Decimal 1.54 means a $1 bet returns $1.54 total ($0.54 profit). Implied probability is simply 1 divided by the decimal odds, so 1/1.54 ≈ 65%. Most professional bettors and quant traders work exclusively in decimal because parlay math reduces to multiplication and arbitrage math reduces to addition of inverses. If you're doing serious bankroll work — Kelly sizing, EV calculations, arb hunting — convert everything to decimal first.
Fractional odds explained
Fractional odds (10/11, 5/2, 1/4) display profit relative to stake. A 10/11 line means a $11 wager returns $10 profit plus your stake. They're standard in UK horse racing and some European books, and they're nearly extinct on US platforms. To convert fractional to decimal, divide and add 1: 10/11 = 0.909 + 1 = 1.909. To convert to implied probability, divide the denominator by the sum: 11 / (10 + 11) ≈ 52.4%. Fractional is the least useful format for active traders but you'll still see it on UK liquidity, especially in horse racing markets.
How to spot value across platforms
The whole point of converting odds is to compare prices across venues. If Polymarket prices an outcome at 42¢ (42% implied) but DraftKings offers +160 American odds (38.5% implied) on the same event, the sportsbook is offering better value because it implies a lower probability — meaning higher payout for the same bet. Conversely, if Kalshi shows 70¢ on a Fed rate hike while a sportsbook shows -300 (75% implied), the prediction market is the better buy. Build a habit of converting every price you see into implied probability — it instantly tells you which venue is giving you the best deal.
The vig and why it matters when comparing odds
Sportsbooks build a margin — the vig, juice, or overround — into every market by pricing both sides such that the implied probabilities sum to more than 100%. A typical -110/-110 NFL spread implies 52.4% on each side, totaling 104.8%; the 4.8% overround is the bookmaker's edge. Prediction markets like Polymarket are designed so YES and NO sum to 100%, with fees charged separately and transparently. When you convert a sportsbook price to compare against a prediction market, remember that the sportsbook number already includes vig. Sharp traders mentally back out 2-3% per side to get the bookmaker's "true" implied probability before comparing.
Quick mental conversion shortcuts
Memorize a few anchor points and you'll never need a calculator for rough comparisons. 50¢ = +100 = 2.00 decimal = 1/1 = 50%. 67¢ = -200 = 1.50 decimal = 1/2 = 67%. 33¢ = +200 = 3.00 decimal = 2/1 = 33%. 25¢ = +300 = 4.00 decimal = 3/1 = 25%. 80¢ = -400 = 1.25 decimal = 1/4 = 80%. With those five anchors in your head, you can interpolate any price you see on Polymarket, Kalshi, or a sportsbook within seconds and immediately know whether it represents value relative to your own probability estimate.