Fractional Odds Explained

Fractional Odds Explained

British Odds

Explained elsewhere on this site, Fractional (or ‘British’) odds indicate both the offered probability of an event and the reward for a successful wager at those odds in the format of a fraction. And how is a fractional odd calculated? Well like all odds it stems from an understanding of the underlying probability of the event happening. This might be theoretically calculable with absolute precision, such as with the probability of winning events on a Roulette wheel or slot machine, or estimated in skill-based events such as sports based on past performance and other metrics.

Let’s imagine that the fractional odds offered for Team A to win in a game against Team B are 3/2, (or ‘3 to 2’ if spoken aloud). The implied probability of that can be calculated in the following way:

Denominator ÷ (Denominator + Numerator)

Slotting the number in from this example that would be 2 ÷ (2+3) = 2 ÷ 5 = 0.4

Multiplying that result by 100 gives the probability as a percentage, in this case 40%, so you can see that the bookmaker is willing to take your bet on its evaluation that Team A has a 40% chance of winning.

Assuming you think the bookmaker’s underestimated the performance of Team A and you take up the bet at those odds, what do you stand to gain from Team A meeting your expectations and winning the match? Well the fractional odds represents the net gain / the stake, and so for this example that would work out as follows:

Numerator ÷ Denominator translates to 3 ÷ 2 = 1.5

This result then becomes the multiplier of your actual stake, so if you bet 1 credit you’d get a net profit of 1.50 credits back. Betting 13.30 credits would bring in a net profit of 19.95 credits (13.30 × 1.5). Note too that you will also get your stake back in these situations – hence the reason we’ve specifically described the payout as the net gain – so if you did wager 1 credit the total paid back to you on winning the bet would be 2.5 credits.

Fractional odds also offer a very quick way to convert them to decimal odds; simply express the fraction in decimal format and add ‘1’! So, using the example above, fractional odds of 3/2 become decimal odds of 2.5 ((3 ÷ 2) +1).