How real-time odds are calculated from prediction market share prices
A YES share trading at 0.63 USDC implies a 63% probability. That is the simplest version of the relationship. But the real picture is messier, and the word "implies" does a lot of work. This article explains the math, the data sources that feed it, and why the displayed percentage is never exact.
The basic calculation
Prediction markets use binary outcome tokens. A YES token pays 1 unit of collateral if the event resolves as true, and 0 if false. The market price of that token therefore represents the market's collective estimate of the event's probability. If a YES share costs 0.63 USDC, the implied probability is 63%. You are betting that the event is more than 63% likely, because you expect to receive 1 USDC for your 0.63 USDC outlay.
This relationship is linear. Price = probability. It is the simplest pricing model in finance.
Why the "roughly" matters
No prediction market price is a perfect probability. Several factors distort the number.
Spread. The difference between the best bid and best ask creates a range. If YES bids are 0.62 and asks are 0.64, the "price" you see depends on which side you look at. Platforms typically display the midpoint, which is 0.63 in that example. But no trade has ever happened at 0.63. The market is saying the probability is somewhere between 62% and 64%.
Liquidity. Thin order books magnify spreads. A market with 100 USDC of depth might have a 2-cent spread. A market with 10 USDC of depth might have a 10-cent spread. Low liquidity makes the displayed price less reliable as a probability estimate.
Last-trade price. Some platforms show the most recent trade. That is stale the moment it prints. In fast-moving events - election nights, sporting outcomes - a last-trade price from 30 seconds ago can be completely disconnected from current sentiment.
Midpoint price. This is more accurate than last-trade because it uses both sides of the book. But the midpoint is still a mathematical artifact. No one has traded at that price.
The misconception
Many people believe "share price equals percentage probability exactly." This is wrong. It is wrong for every market, every second.
The price is a transaction record, not a truth machine. It reflects what the last buyer and seller agreed on, filtered through spreads and liquidity constraints. A market with one buy order at 0.63 and nothing else shows 63% implied probability. That is almost meaningless. The same price in a market with 500,000 USDC of bids and asks at 0.62 and 0.64 carries vastly more information.
Price equals probability only if the market is perfectly liquid, frictionless, and constantly traded. No on-chain market meets those conditions.
From on-chain price to displayed odds
Raw on-chain price comes from the smart contract's exchange function. For AMM-based prediction markets, that is the constant product formula output for the YES/NO pool. For order book markets, it is the midpoint of the top bid and ask.
Platforms like Polymarket apply a conversion layer. They take the raw on-chain price, round it to a percentage, and display it. That conversion is straightforward arithmetic: multiply by 100 and add a percent sign. The real work is in choosing which price to use.
Polymarket displays the midpoint by default. Users can also see the order book. Some third-party analytics sites show last-trade price or volume-weighted average price. Each method produces a different number.
Other odds formats
Implied probability is universal. But many users prefer American odds or moneyline odds.
American odds for a 63% probability are roughly -170 (the negative means you risk 170 to win 100). The formula is:
- For probabilities above 50%: odds = -(probability / (1 - probability)) * 100
- For probabilities below 50%: odds = ((1 - probability) / probability) * 100
A 63% probability gives -170. A 37% probability gives +170.
Moneyline odds are the same thing by a different name. They are standard in sports betting. Prediction markets that cater to sports traders often display both implied probability and American odds.
The conversion from on-chain price to American odds is exact math. It introduces no extra uncertainty. The uncertainty is already baked into the price.
Real-time updating
Prediction market prices change constantly. Every new trade, every order placed or cancelled, updates the displayed odds. On Polymarket, the price refreshes whenever the order book changes. On AMM-based markets, every swap moves the curve.
The displayed percentage updates in real time because the underlying price updates in real time. There is no smoothing or averaging in most implementations. The number you see is the current state of the market, warts and all.
Low-liquidity markets see wild swings. A single 50 USDC buy can move the implied probability by 10 percentage points. That movement reflects genuine market action, but it does not reflect any change in the underlying event's true likelihood. The market is just thin.
The honest summary
The price of a YES share implies a probability. It does not equal a probability. The difference is not academic. It matters every time you place a trade.
Check the order book. Look at the spread. See how much liquidity sits behind the price. A clean 0.63 with 200,000 USDC on each side is a meaningful estimate. A 0.63 with 200 USDC of depth is a guess wearing a number.
Displayed odds are a starting point, not a final answer. Treat them accordingly.
Not financial advice. wswap.site publishes market data and general information about digital assets. Crypto assets are volatile and you can lose everything you put in. Nothing here is a recommendation to buy, sell or hold, and we make no price predictions.
Prices are sourced from third parties and may be delayed or wrong. Verify anything you intend to act on against a primary source.