How to read American, decimal and fractional odds

Odds look like a foreign language until you learn that they are just three ways of expressing the same idea: how likely an outcome is believed to be and how much a successful stake returns. You will run into American, decimal and fractional formats depending on where you are reading. This article explains each one, shows how to convert them, and ties them to probability. It is educational, intended for adults 18 and over, and it does not recommend placing any wager.

American odds

American odds use a plus or minus sign next to a number built around 100 units.

• A minus number (such as -200) shows how much you must stake to win 100. At -200, a 200-unit stake would profit 100 if it succeeds.
• A plus number (such as +150) shows how much you would win from a 100-unit stake. At +150, a 100-unit stake would profit 150.

Minus numbers describe outcomes seen as more likely, and plus numbers describe outcomes seen as less likely.

Decimal odds

Decimal odds show the total return per unit staked, including the stake itself. A price of 2.50 means a 1-unit stake returns 2.50 in total if it succeeds, which is 1.50 in profit. This format is common in Europe, Canada and Australia, and it is the easiest to convert into a probability.

Fractional odds

Fractional odds, long used in the United Kingdom and Ireland, express profit relative to stake. A price of 3/2 means 3 units of profit for every 2 units staked. Converted to decimal, that is 1 + 3/2 = 2.50, the same as +150 in American terms.

Converting between formats

Here are a few matching examples:

• +150 equals 2.50 decimal and 3/2 fractional.
• -200 equals 1.50 decimal and 1/2 fractional.
• +100 equals 2.00 decimal and 1/1 fractional (often called even money).

To convert American to decimal, a positive number becomes 1 + (number / 100), and a negative number becomes 1 + (100 / the number without its minus sign).

From odds to implied probability

Implied probability is the chance a price suggests. The formulas are short:

• Decimal: 1 divided by the decimal price. A price of 2.50 implies 1 / 2.50 = 40%.
• Positive American: 100 divided by (the number + 100). At +150 that is 100 / 250 = 40%.
• Negative American: the number (without the minus) divided by (the number + 100). At -200 that is 200 / 300, or about 66.7%.
• Fractional: the denominator divided by (numerator + denominator). At 3/2 that is 2 / 5 = 40%.

Notice that +150, 2.50 and 3/2 all produce 40%. Same price, three outfits.

Why prices do not add up to 100%

If you convert every outcome of an event into an implied probability and add them up, the total usually lands above 100%. At -110 on both sides of a two-way market, each side implies 110 / 210, about 52.38%, and the pair adds up to roughly 104.76%. The extra amount is the margin built into the prices. It means the posted odds are not a pure statement of probability, and it is the reason the house expects to come out ahead over time.

Common misreadings

• Treating a big plus number as a bargain. A long price signals a less likely outcome, not a generous one.
• Forgetting that decimal odds already include the stake.
• Assuming implied probability equals true probability. It is a priced-in estimate that also reflects the margin.

No format tells you what will happen. Odds describe pricing and likelihood, not certainty.

Frequently asked questions

Which odds format is best?

None is better. They are interchangeable, so use whichever you read most easily and convert when needed.

What does even money mean?

It means a profit equal to the stake, which is +100 in American, 2.00 in decimal and 1/1 in fractional.

Can implied probability tell me who will win?

No. It reflects how an outcome is priced, not what will occur, and it includes a margin in favor of the party setting the odds.