When someone says a prediction market is "pricing in a 63% probability," they're doing a simple calculation: the YES contract is trading at 63¢, and YES pays $1 if the event happens. 63 divided by 100 is 0.63, or 63%. That's the implied probability.
But implied probability is a market-price-derived number, not a scientific measurement. It has real limitations. Here's how to use it properly.
What implied probability IS
It's the collective current opinion of the traders participating in that market, expressed as a probability. If the market is deep and liquid, and if traders are rational profit-seekers, the price tends to track actual likelihood closely. That's the theory. Most of the time, it holds.
What implied probability IGNORES
Implied probability doesn't account for:
- The bid-ask spread: the "true" probability is somewhere between the YES bid and YES ask. In thin markets, that spread can be 10-20 points wide. A market "trading at 50%" might really be 45% to 55% depending on which side you're trading.
- Fees: Kalshi takes 1-7% per trade. Polymarket takes gas and maker/taker fees. After fees, the "true" break-even probability is somewhat worse for the retail participant than the raw price suggests.
- Liquidity risk: if you want to exit before settlement, you may have to cross the spread or sit in the order book. That's a cost.
- Settlement risk: is the resolution rule unambiguous? Some prediction markets have settled controversially (Polymarket's Ethereum ETF market, various Kalshi ones where the data source was ambiguous).
What implied probability ISN'T
Not a forecast. The market isn't predicting anything , it's reflecting the current collective bet. If fresh information arrives, the price moves. What the price was 10 minutes ago is irrelevant to what it is now.
Not a sports line with vig. Sports betting lines include the bookmaker's house edge (usually 4-10% built in). Prediction market contracts settle at $1 or $0 cleanly; the equivalent of "vig" is the exchange fee, which is separate from the price.
Not a lock on any individual event. A 70% implied probability means 30% of the time you lose. Over many bets on 70% markets, you'd expect to win ~70% of them. On any single one, you won't know until resolution.
When implied probability is most trustworthy
- Deep markets: high volume, narrow bid-ask spread, significant open interest
- Events with clear resolution: "Did the Fed cut rates at the May meeting?" = yes or no, no gray area
- Time close to resolution: short-dated markets have less time for new information to upend the price
- Multiple independent venues agreeing: if Kalshi and Polymarket both price an event at ~65%, that's a stronger signal than one market alone
When to be suspicious
- Wide bid-ask spreads (>5 points) , the price isn't stable
- Low volume (<$10k total) , a single trader can move it
- Volatile prices , big swings hour-to-hour suggest people are speculating on thin info
- Ambiguous resolution rules , you may be betting on something slightly different from what you think
- Long time to resolution , more time for new information to change the picture
How to cross-check a market's implied probability
If you want to know whether a market is pricing an event correctly, compare it to:
- A qualified external model (NOAA for weather , see our mispricings page)
- Historical base rates (how often has this type of event happened before?)
- Another prediction market for the same event (see our arbitrage pairs)
- Expert analyst surveys (Bloomberg consensus for economic data, for instance)
When those agree within a few percentage points, the implied probability is probably right. When they disagree a lot, something interesting is happening , either the market knows something the model doesn't, or the market is wrong.