How Prediction Market Prices Work: Understanding Probability and Odds

Prediction market prices have a direct mathematical relationship to probability. Once you understand that relationship, reading a market becomes straightforward.

The $0 to $1 scale

Every prediction market contract resolves to either $1.00 (the event happened) or $0.00 (it didn't). The current price is the market's implied probability expressed in dollar terms.

A contract trading at $0.65 means the collective market — all the buyers and sellers transacting right now — estimates a 65% chance the outcome occurs. A contract at $0.20 implies 20% probability.

This is why prediction market prices are often directly comparable to percentage odds without any conversion. When Kalshi shows a contract at $0.72, you can read that as "the market thinks there's a 72% chance."

Where does the price come from?

The price is set by supply and demand on the exchange. When more traders want to buy Yes, the price rises. When more traders want to sell (or buy No), the price falls.

This is meaningfully different from a sportsbook, where a human odds-maker sets the line and adjusts it based on where money is coming in. On a prediction market exchange, the price is purely the result of trader activity.

New information moves prices. If a candidate drops out of a race, the contracts for other candidates surge. If an economic report comes in far above expectations, the contracts tied to that report resolve immediately and related markets shift.

The bid-ask spread

Like any financial exchange, prediction markets have a bid price (what buyers will pay) and an ask price (what sellers will accept). The spread between them is the cost of entering and exiting a position.

On a liquid market with a tight spread, you might see a bid of $0.64 and an ask of $0.66. You pay $0.66 to buy and receive $0.64 if you sell immediately — a $0.02 round-trip cost on top of any trading fee.

On a thinly traded market, that spread might be $0.55 bid / $0.75 ask. That $0.20 gap is a substantial implicit cost — before you even factor in the platform's trading fee.

Always check the spread before entering a position in a low-volume market. Wide spreads significantly reduce your potential return.

Expected value: the only metric that matters

Expected value (EV) is the calculation that determines whether a trade is worth making. The formula is:

EV = (probability of winning × profit per contract) − (probability of losing × loss per contract)

Example: You think there's a 75% chance an event happens. The market is pricing it at $0.60. If you buy Yes at $0.60:

• Win scenario (75% likely): you profit $0.40 per contract ($1.00 − $0.60)
• Lose scenario (25% likely): you lose $0.60 per contract
• EV = (0.75 × $0.40) − (0.25 × $0.60) = $0.30 − $0.15 = +$0.15

A positive EV of $0.15 means this is a mathematically favorable trade given your probability estimate. The edge comes from the gap between your estimated probability (75%) and the market's implied probability (60%).

If you agreed the probability was 60%, there would be no edge and the trade wouldn't be worth making after fees.

What makes prices accurate or inaccurate?

Prediction markets are generally well-calibrated on events with lots of information — major elections, Fed meetings, Super Bowls. Prices on these markets reflect a large number of informed traders.

Smaller, less-followed markets can have significant mispricings. The trader with the informational edge in a niche market can find real opportunities, but also faces the risk that low liquidity makes entering and exiting costly.

Prices can also be temporarily distorted by large one-sided bets. If an institutional trader takes a big position in a market, the price can move away from what most informed observers would consider fair value — and then correct.

Applying this to a real trade

You're looking at a market on the Fed's next meeting outcome. The Yes contract on "Rate cut in September" is at $0.45.

Before trading, ask:

• What is my probability estimate? Be honest and specific — not "probably" but "I think it's about 60%."
• What is the implied probability? The market says 45%.
• Is my edge large enough to overcome fees? If you're paying 3% in fees, your net return on a $0.55 profit is $0.55 × 0.97 = $0.534. Recalculate EV with the actual net payout.
• How liquid is the market? Is the spread wide? Will I be able to exit if my view changes?

If you can't answer the first question with a specific number rather than a vague feeling, you probably don't have enough of an edge to trade.