Odds converter: American, decimal, fractional and probability

How do I convert American odds to decimal?

For positive odds add the number to 100 and divide by 100; for negative odds divide 100 by the number and add 1. The decimal price includes your stake back, so it is what $1 returns.

+A:  decimal = 1 + A / 100      +150 -> 2.50
-A:  decimal = 1 + 100 / A      -110 -> 1.909
decimal -> American: at 2.00 or more, (decimal - 1) x 100; below 2.00, -100 / (decimal - 1)

American odds must be at least 100 away from zero, so +50 or -50 is not a valid price. The calculator rounds a converted American price to the nearest whole number.

How do I convert odds to implied probability?

Divide 1 by the decimal odds. At −110 (decimal 1.909) the implied probability is 52.38%; at +150 (2.50) it is 40.00%. It is also the break-even win rate, the share of bets you must win to come out even at that price.

What do +200 and -110 mean?

A plus sign is the profit on a $100 bet, and a minus sign is the stake needed to win $100. At +200 a $100 bet wins $200, for 33.33% implied. At −110 you stake $110 to win $100, which is 52.38% implied and a 4.76% total margin when both sides of a market are priced that way.

How do I convert a Kalshi or Polymarket price to odds?

A contract that costs 80¢ and pays $1 if it wins is decimal 100/80 = 1.25, which is −400 American. Put 80 in the calculator with “Contract price” selected. The exchange’s taker fee lowers the payout per dollar spent: at Kalshi the effective decimal price becomes 1.2327, about -430, and the break-even probability rises from 80.0% to 81.12%. The fee models are on the exchanges page, and the prediction markets guide explains how contracts settle.

What is the difference between implied and fair probability?

Implied probability is what a price says; fair probability is what remains after the book’s margin is taken out. At −110 on both sides each implied probability is 52.38% but each fair probability is 50%. Remove the margin with the no-vig calculator before you compare a price with a probability you believe.

What is the break-even win percentage?

It is the implied probability of the price you pay, after any fee: 1 divided by the effective decimal odds. At −110 you need to win 52.38% of your bets; at Kalshi’s fee-adjusted 80¢ price you need 81.12%.

How do fractional odds work?

Fractional odds a/b show the profit a on a stake b: 5/2 pays $5 for every $2, so it is +250 and decimal 3.50, and 10/11 is −110. Add 1 to a/b to get the decimal price. The calculator shows the closest simple fraction, and for odds that are not a tidy fraction it says so.

Related calculators and guides

• No-vig calculator

  Remove the margin to get a fair probability.

• Arbitrage calculator

  Combine prices across venues, net of exchange fees.

• Expected value guide

  How implied and fair probability turn into expected value.

• Prediction markets guide

  Contract prices, fees and settlement.