Every price on a prediction market is a probability. If a Kalshi contract on “Fed cuts rates in December” trades at 67 cents, the market is telling you there is roughly a 67% implied chance of a cut. Reading these numbers correctly, and knowing when they are lying to you, is the single most valuable skill in prediction market trading. This guide walks through how professionals decode probabilities, adjust for fees and liquidity, and turn raw prices into actionable edges.
The Basic Math: Price Equals Probability
Binary prediction market contracts pay out $1 if the outcome resolves YES and $0 if it resolves NO. That structure makes the price a direct probability estimate. A YES contract at 42 cents implies a 42% chance the market believes the event will happen. A NO contract on the same event should trade near 58 cents, because YES and NO together must sum to roughly $1.
Here is the quick conversion table professionals memorize:
| Contract Price | Implied Probability | Fair Decimal Odds |
|---|---|---|
| $0.10 | 10% | 10.00 |
| $0.25 | 25% | 4.00 |
| $0.50 | 50% | 2.00 |
| $0.67 | 67% | 1.49 |
| $0.80 | 80% | 1.25 |
| $0.95 | 95% | 1.05 |
Decimal odds are simply 1 divided by the probability. Any time you see a sportsbook line, you can convert it back to an implied probability and compare directly against what Kalshi or Polymarket is showing. If a sportsbook has an event at 2.20 decimal (about 45%) and Polymarket has the same event trading at 38 cents, you have found a potential edge, before fees.
YES and NO Should Sum to $1 (But Often Do Not)
In a perfectly efficient market with no spread, YES + NO = $1.00. In reality, you almost always see a gap. That gap is the bid-ask spread, and it is a hidden cost of trading. If YES is offered at $0.68 and NO is offered at $0.34, the market has a 2 cent spread built in. Cross that spread twice (buy YES, later sell YES) and you have paid 4 cents of friction on a $1 contract, which is a 4% haircut on your position.
Pros always check the top of book before entering a position. Rules of thumb worth burning in:
- Spread under 1 cent: highly liquid, safe to trade at market
- Spread of 1 to 3 cents: acceptable, but use limit orders when possible
- Spread over 3 cents: illiquid, wait for depth or place a resting limit
- YES + NO under $0.98 or over $1.02: something is off, double check before you trade
Adjust for Fees Before You Call It an Edge
An implied probability is only “fair” once you subtract trading costs. Kalshi charges a per-trade fee that scales with contract price and quantity, and cashes out winning contracts at $1. Polymarket takes fees at deposit and withdrawal via USDC and charges gas on some actions. If a contract is priced at 60 cents and you think fair value is 62 cents, that 2 cent theoretical edge can be entirely eaten by fees on a small position.
A simple professional check: before placing a bet, calculate your break-even probability. If you buy YES at 60 cents and fees add roughly 1 cent of round-trip cost, you actually need the true probability to be above 61% to make money in expectation. Anything less and you are paying the house to feel smart.
Time Decay and How Probabilities Should Move
Unlike sports betting, prediction market probabilities move continuously as new information arrives and as time passes. A contract on “Bitcoin above $100k by year end” should slowly drift toward 0 or 1 as the deadline approaches and uncertainty resolves. Pros watch two things:
- Direction of drift. Is the market steadily moving one way with no news, or is it whipsawing on each headline? Steady drift often reflects informed traders accumulating a position.
- Volume behind the move. A 5 cent price jump on $500 of volume means little. The same move on $50,000 of volume is a strong signal.
A useful mental model: treat each contract like a Bayesian belief. Every new piece of information should nudge the price by an amount proportional to how surprising it is. If nothing surprising happens and the price still moves 10 cents in an hour, either you are missing news, or someone is trading on information you do not have.
Extreme Prices Are Where Amateurs Get Wrecked
Contracts trading near 5 cents or 95 cents behave differently from midrange contracts. Selling a 95 cent contract risks 5 cents to make 95, which sounds terrible until you realize the market says the outcome will happen 95% of the time. In the long run, mechanically fading extreme favorites loses money. The same logic applies to buying 5 cent longshots hoping for a payoff.
The trap: extreme prices tend to overstate the probability of “obvious” outcomes and understate the probability of tail events, but only slightly. Empirical studies of Kalshi and Polymarket show implied probabilities above 90% resolve YES around 92 to 94% of the time. The edge is real but tiny, and it evaporates the moment you factor in fees. Beginners are better off staying in the 20 to 80 cent range where price movements are meaningful and fees are less punishing.
Cross-Market Probability Checks
The fastest way to spot a mispriced contract is to compare the same event across venues. If Kalshi has a Fed rate cut at 67% and Polymarket has the same cut at 71%, one of those markets is offering a better price on YES and the other on NO. The gap will not always be arbitrageable, because of fees, withdrawal timing, and jurisdiction rules, but it tells you where liquidity is more informed.
Serious traders also cross-check prediction markets against related instruments. Fed rate contracts against Fed funds futures. Election contracts against sportsbook lines where legal. Crypto contracts against options-implied moves. When prediction market probabilities diverge from a deeper, more liquid market, the prediction market is usually the one that needs to correct.
Turning Probabilities Into a Trading Plan
Reading probabilities well is not just about spotting an edge. It is about sizing correctly. The Kelly criterion, simplified for binary markets, says the fraction of your bankroll to bet equals your edge divided by your odds. If a contract is priced at 60 cents and you believe fair value is 65 cents, your edge is 5 cents on a $1 payoff, or roughly 8% of a full Kelly position. Most pros trade at a quarter to a half of Kelly to survive variance.
Do this consistently and prediction market prices stop looking like numbers and start looking like opportunities. The traders who consistently profit are not the ones who guess the future best. They are the ones who read the market’s probability, subtract fees, size appropriately, and only pull the trigger when the math is on their side.
Start Trading With Confidence
Now that you can decode implied probabilities, put the skill to work on the two largest venues in the market. Open a Kalshi account to trade regulated event contracts across politics, economics, and sports. Or get started on Polymarket for the broadest global contract selection. For a full comparison of every major venue, see our ranked list of the best prediction markets and pick the platform that fits your strategy.