Odds converter: fractional to decimal and implied probability

To go from fractional to decimal, divide the first number by the second and add one. Odds of 5/2 become 5 ÷ 2 = 2.5, plus 1, giving decimal odds of 3.50. The reason for adding one is that fractional odds describe your profit while decimal odds describe your total return including the stake back.

That single difference causes most of the confusion between the two formats. A 2/1 shot returns £30 from a £10 stake: £20 profit plus your £10 back. In fractional terms it is 2/1 because the profit is twice the stake. In decimal terms it is 3.00 because the total return is three times the stake. Neither is more correct — they are describing different halves of the same transaction.

Converting fractional to decimal is the step you need most often, because decimal is the format you multiply. Going the other way, subtract one from the decimal price and express what remains as a fraction. Decimal 4.50 minus 1 is 3.5, which is 7/2. British bookmakers round to a conventional ladder of fractions, so a decimal price of 4.40 will usually appear on the board as 7/2 rather than as the exact 17/5. A betting odds converter is worth keeping to hand precisely because that rounding is not obvious.

Implied probability is one divided by the decimal odds. A price of 4.00 implies a 25% chance; a price of 1.50 implies 66.7%. This is the most useful number on this page, because it converts a betting price into something you can actually have an opinion about.

The catch is that implied probabilities in a real market always add up to more than 100%. That excess is the bookmaker margin, and it means the implied probability of any single price is slightly overstated. If you want the market's genuine estimate rather than the padded one, run the whole market through the no-vig calculator instead.

Once you are comfortable moving between prices and probabilities, most betting questions become easier to answer. "Is 11/4 a good price?" is unanswerable on its own. "Is this a better than 26.7% chance?" is a question you can actually reason about.

American odds are built around a 100-unit stake. A positive line such as +150 means a 100 stake profits 150, which is 5/2 or 3.50. A negative line such as -200 means you must stake 200 to profit 100, which is 1/2 or 1.50. The switch between positive and negative happens at evens.

You will meet the format on US sportsbooks and in most published betting models, so it is worth being able to read it even if you never bet into it. The converter shows all three formats at once precisely so you do not have to hold the conversion in your head.