Polymarket Odds Converter: Price to American, Decimal and Fractional Odds

A Polymarket price, a probability and the three bookmaker odds formats all describe the same thing: how likely an outcome is and how much it pays. Here is a 40% chance written every way.

• Price to decimal odds: 1 ÷ price. At 40¢: 1 ÷ 0.40 = 2.50.
• Decimal odds to price: 1 ÷ decimal odds. At 1.91: 1 ÷ 1.91 = 52.4¢.
• Price to American odds: under 50¢, (100 − price) ÷ price × 100 with a plus sign. At 50¢ or more, price ÷ (100 − price) × 100 with a minus sign.
• American odds to price: for + odds, 100 ÷ (odds + 100). For − odds, odds ÷ (odds + 100) using the number without its minus.
• Price to fractional odds: (100 − price) ÷ price, reduced. At 40¢: 60 ÷ 40 = 3/2.

A bookmaker is the other side of your bet, so it prices both outcomes to earn a margin whichever one wins. That margin is the vig (vigorish). Take a common two-way line of −110 on both sides:

1. Each side implies 110 ÷ 210 = 52.38%.
2. Add them: 52.38% + 52.38% = 104.76%. The 4.76% above 100% is the overround.
3. The vig is 1 − 1 ÷ 1.0476 = 4.55%. If equal money lands on both sides, the bookmaker keeps that share of everything staked.
4. Remove it by dividing each side by the total: 52.38% ÷ 1.0476 = 50%, the fair chance. A fair 50% would pay $100 profit on $100. At −110 you get $90.91.

Repeat the steps for any line and you get its margin and a fair price you can compare with Polymarket. The comparison tool above does the arithmetic for you, and our Polymarket vs sportsbooks guide walks through a longer example.

Polymarket is an exchange, so there is no margin in the price. It charges takers (orders that fill at once) a fee of shares × fee rate × price × (1 − price), at a rate of 0.05 for sports. The fee peaks at 50¢: on $100 at the fair price of 50% that is $2.50, or 2.5% of the trade. At 90¢ the same $100 pays only $0.50. Makers, whose limit orders wait to be filled, pay nothing.

The fee is not the whole cost. Buyers also pay the spread between the buy and sell price, which is small in busy markets and wider in quiet ones. Put both together and the all-in price on Polymarket can be above or below a bookmaker's no-vig price. See Polymarket fees explained for every category and the rebate tiers, or try the Polymarket fee calculator.

A sportsbook is the counterparty to your bet. Sportsbooks set their own limits and can restrict individual accounts, and data from the UK Gambling Commission, as reported by iGamingBusiness, showed 643,779 of 14,923,840 active accounts (4.31%) restricted at operators in Great Britain in 2024. That is a UK figure, not a worldwide rate, and it is the number our sportsbooks guide uses.

On Polymarket you trade against other users through an order book. Its documentation says the book has no trading size limits and matches buyers and sellers of any amount. The catch is liquidity: a large order can move the price, so check how deep the market is before you trade. If you have been limited at your bookmaker and your country is on the availability list, that is one reason some bettors look at an exchange.

Once you know what a price means, a first trade is three steps: check that Polymarket is open in your country, open an account and add a small deposit, then pick a market and compare its price and fee with your bookmaker's line. The first-trade guide covers sign-up and deposits, and the Polymarket calculator shows what a given amount returns. A $100 trade at 40¢ returns $250 if it wins and loses the $100, plus any fee, if it does not.