How to Read Prediction Market Prices and Find Edge

Updated May 2026 · 9 min read

Every profitable prediction market trader knows one thing reflexively: the price is the probability. Once that idea is wired into how you look at a market, everything downstream — expected value, position sizing, knowing when to skip a trade — falls into place. This guide walks through reading prices, finding edge, and turning that edge into a sustainable process.

The four-step price read

Step 1 — Read the YES price

Open a market. The YES contract has a current price — say $0.42. That's the last price someone paid for YES, or the midpoint of the bid and ask. Prices on binary markets always live between $0.00 (impossible) and $1.00 (certain).

Step 2 — Convert to implied probability

The price IS the probability. $0.42 means the market is implying a 42% chance YES happens. You do not need to multiply by anything. You do not need to subtract a vig. On a clean binary contract that pays $1 to the winner, dollars and probability are the same number.

For a NO contract at $0.58, that means a 58% implied chance of NO — which is the same as 42% YES. The two prices always sum to ~$1 (minus tiny exchange fees).

Step 3 — Form your own estimate

Cover the price for a moment and ask: knowing what I know, what is my honest probability? Write down a number. Don't anchor on the market price; you'll lose money if you only ever shade slightly off the consensus.

Sources for your estimate: news, base rates from past similar events, expert forecasts you trust, models you've built, private knowledge. The discipline of writing your number before looking at the price is the single best calibration habit you can build.

Step 4 — Compare and decide

Now compare. Three cases:

• Your number > market price by >5% → you have edge buying YES.
• Your number < market price by >5% → you have edge buying NO.
• Your number is within ~5% of the price → skip the trade. Your edge is smaller than the bid-ask spread plus your own modeling noise.

The 5% threshold is a heuristic, not a law. On highly liquid markets with tight spreads, 3% edge is tradable. On illiquid markets with wide spreads, even 10% edge can be unprofitable to execute.

Expected value: turning edge into a number

Once you've found edge, calculate expected value (EV) to know what a single contract is worth. The formula is simple:

EV per contract = (your probability × $1) − price you paid

Worked example. You buy YES on a market at $0.42. Your estimated true probability is 55%.

• EV = ($0.55 × $1) − $0.42 = $0.13 per contract.
• If you buy 100 contracts ($42 cost), EV of the trade is $13.
• This isn't your guaranteed profit. On any single trade you win $0.58 (with 55% chance) or lose $0.42 (with 45% chance). Across many such trades, you average $0.13.

Markets reward repeated positive-EV decisions. One $0.13 EV trade is statistical noise. A hundred $0.13 EV trades, sized appropriately, is a real strategy.

The bid-ask spread (the silent cost)

On every market, the "price" you see is usually the midpoint. What you actually pay depends on two numbers: the bid (highest someone is buying at) and the ask (lowest someone is selling at). The difference is the spread.

• Liquid market: bid $0.41 / ask $0.43, spread 2¢. Buying at market means paying $0.43.
• Illiquid market: bid $0.38 / ask $0.46, spread 8¢. Buying at market means paying $0.46.

In the second case, even if your edge was 5% above the midpoint, executing at the ask eats most of it. Liquid markets are almost always more profitable than illiquid ones for retail-size traders. The full comparison of Polymarket vs Kalshi covers where liquidity lives on each venue.

Position sizing: how much to bet

Edge is only profit if you survive the variance. The Kelly criterion gives the mathematically optimal size for a given edge:

Kelly fraction = (p × b − q) / b

where p is your probability of winning, q is 1−p, and b is your net win-to-loss ratio. On a binary prediction market where you buy at price c, b equals (1−c)/c.

Most experienced traders use fractional Kelly — typically 25% to 50% of the full Kelly fraction — because perceived edge is usually overstated, and full Kelly is brutally volatile. A practical rule for retail: never risk more than 1–3% of bankroll on a single market.

Resolution risk: the part most beginners miss

Before any of the math, read the resolution rule. Carefully. Twice. The market's wording determines what gets paid out, not your intuition about the event.

Common traps:

• Ambiguous source. A market that resolves on "official government data" might be ambiguous if multiple agencies report different numbers.
• Time-zone fine print. "By December 31" usually means midnight UTC, not your local time.
• Cancellation edge cases. "Will event X happen by Y" markets often default to NO if the event is delayed past Y, even by a day.
• Subjective wording. "Major" or "official" without a tight definition leaves resolution discretion to the moderator.

Resolution risk is the most common reason traders lose money on positions where their core thesis was correct. Read the rules.

Putting it together: a worked trade

Market: "Will the Fed cut rates at the December 2026 FOMC meeting?" YES is trading at $0.42 with a $0.41 bid and $0.43 ask.

1. Read the resolution rule. Resolves YES if the FOMC announces a rate cut in their December 2026 statement; NO otherwise. No ambiguity.
2. Form your estimate. Based on recent inflation data, recent Fed commentary, and market expectations from rate futures, you put the probability at 60%.
3. Compare. Your 60% vs market 42% (or 43% at the ask) → 17%+ edge buying YES.
4. Size the trade. Kelly fraction = (0.60 × (0.58/0.42) − 0.40) / (0.58/0.42) = 0.31, or 31% of bankroll at full Kelly. You use quarter-Kelly: 7.8% of bankroll. On a $1,000 bankroll: ~$78. That's about 180 contracts at $0.43.
5. Place the trade. Limit order at $0.43, partial fill at $0.43, rest at $0.44 — you accept it.
6. Calculate trade EV. ($0.60 × $1 − $0.435) × 180 contracts = $29.70 expected.

The practice loop

Reading prices and doing the math is easy in writing. Doing it under live conditions, on a market that feels emotionally charged, while resisting the urge to over-trade, is the entire skill. Polynate's course is built around exactly this loop:

1. You see a real market question.
2. You write down your probability before seeing the price.
3. The lesson reveals the market price and the resolution.
4. You build calibrated intuition trade by trade.

Frequently asked questions

What does it mean when a YES contract is at $0.63?

The market is implying a 63% chance YES happens. Buying YES at $0.63 returns $1 if you win ($0.37 profit) and $0 if you lose ($0.63 loss).

How do I calculate expected value?

EV per contract = (your probability × $1) − price paid. Buy YES at $0.42, your probability 55%, EV = $0.13 per contract. Positive EV trades compound across many bets.

What is the bid-ask spread?

The gap between the highest bid and lowest ask. It's the cost of an immediate fill. Liquid markets have 1–2¢ spreads; illiquid markets can have 5–10¢ spreads that eat most retail edge.

How big should each trade be?

Most retail traders should risk 1–3% of bankroll per market resolution. Kelly criterion gives a mathematically optimal size, but fractional Kelly (25–50% of full Kelly) is safer because perceived edge is usually overstated.