Political betting odds and prediction market prices say the same thing in two languages: how likely something is. By-election odds, next Labour leader odds and a market price in cents can all be converted to a percentage, which is the only way to compare them properly.
Turning odds into probability
| Fractional odds | Decimal odds | Implied probability |
|---|---|---|
| 1/4 | 1.25 | 80% |
| 4/6 | 1.67 | 60% |
| Evens | 2.00 | 50% |
| 2/1 | 3.00 | 33.3% |
| 5/1 | 6.00 | 16.7% |
| 20/1 | 21.00 | 4.8% |
The formula for fractional odds a/b is b ÷ (a + b). Use the odds converter to do it quickly.
The bookmaker's margin
Add up the implied probabilities for every candidate in a next-leader market and you'll usually get well over 100%, often 115% or more in political markets with many runners. The excess is the overround: the bookmaker's built-in margin. It means the implied probabilities are inflated, so each price pays slightly less than a fair bet would.
How a prediction market differs
On a prediction market, a YES share pays $1.00 if the event happens. A price of 33¢ is an implied probability of about 33%.
| Bookmaker odds | Prediction market | |
|---|---|---|
| Who sets the price | The bookmaker | Traders, through an order book |
| Cost to you | Margin inside the odds | Trading fee and spread |
| Getting out early | Only if cash-out is offered | Sell at the market price |
| Betting against | Limited | Buy NO |
Because traders set the price, a liquid market's YES and NO prices sit close to 100% combined. A thin one can be as distorted as any bookmaker's price, so check volume first.
What moves political odds
- By-elections. Local polling is scarce, so odds lean on national polls, the previous result and constituency reports. Prices can swing hard on a single constituency poll.
- Leadership contests. The electorate is party members or MPs, not the public, which makes national polls a weak guide.
- General elections. Under first past the post, vote share and seats diverge. "Most seats", "overall majority" and "next Prime Minister" are three different questions with three different prices.
A question like "will Reform win the next election" needs defining before it can be priced: most votes, most seats, or forming a government?
Access in the United Kingdom
On Polymarket's geographic restrictions list, the United Kingdom is close-only: existing positions can be closed, new ones can't be opened, on the website and the API (checked 30/09/2026). Prices remain public and are widely quoted as a probability estimate. See United Kingdom and Polymarket UK. We never explain ways around country restrictions.