Odds Converter | Fraction to Decimal Odds & Probability

Published: Sep 6, 5:34pm Last Updated: Jul 30, 9:31am man o bong on Strategy 2 Comments 22751 Views

Fraction to Decimal to Probability

Odds Conversion Table

This odds conversion table can work in many different ways for you when checking prices from bookmakers in the UK or making up your own prices. 

Each betting market had odds and these odds convert to probability. 

It is useful for both pre-match value bets and in-running bets.

We know how important getting the right value is, the betting school article shows how you can become an expert in value betting.

As a matter of fact one of the guidelines we mention in the in-running betting basics article is quick decision making, if you know both fractional, decimal and probability percentages this will surely assist you in making quick betting decisions when betting in running. 

You can either convert fractional odds to decimal, or vice versa, and you also can see the probability for each of the odds in either format too.

Also, this is very handy when creating your own tissue prices. If you worked out a probability, you can convert probability to odds here too.

Fractional
Decimal
Probability
1/5
1.20
83.3%
2/9
1.22
81.8%
1/4
1.25
80%
2/7
1.29
77.8%
3/10
1.30
76.9%
1/3
1.33
75%
4/11
1.36
73.3%
2/5
1.40
71.4%
4/9
1.44
69.23%
40/85
1.47
68%
1/2
1.50
66.67%
8/15
1.53
65.22%
4/7
1.57
63.64%
8/13
1.62
61.9%
4/6
1.67
61.54%
8/11
1.73
57.89%
4/5
1.80
55.56%
5/6
1.83
54.55%
10/11
1.90
52.38%
21/20
1.95
51.22%
EVENS
2.00
50%
21/20
2.05
48.78%
11/10
2.10
47.62%
6/5
2.20
45.45%
5/4
2.25
44.44%
13/10
2.30
43.48%
11/8
2.38
42.11%
7/5
2.40
41.67%
6/4
2.50
40%
8/5
2.60
38.46%
13/8
2.63
38.1%
7/4
2.75
36.36%
9/5
2.80
35.71%
15/8
2.88
34.78%
2/1
3.00
33.33%
85/40
3.13
32%
11/5
3.20
31.25%
9/4
3.25
30.77%
12/5
3.40
29.41%
5/2
3.50
28.57%
13/5
3.60
27.78%
11/4
3.75
26.67%
14/5
3.80
26.32%
3/1
4.00
25%
16/5
4.20
23.81%
10/3
4.33
23.08%
17/5
4.40
22.73%
7/2
4.50
22.22%
18/5
4.60
21.74%
4/1
5.00
20%
9/2
5.50
18.18%
5/1
6.00
16.67%
11/2
6.50
15.38%
6/1
7.00
14.29%
13/2
7.50
13.33%
7/1
8.00
12.5%
15/2
8.50
11.76%
8/1
9.00
11.11%
17/2
9.50
10.53%
9/1
10.00
10%
19/2
10.50
9.52%
10/1
11.00
9.09%
11/1
12.00
8.33%
12/1
13.00
7.69%
13/1
14.00
7.14%
14/1
15.00
6.67%
15/1
16.00
6.24%
16/1
17.00
5.88%
20/1
21.00
4.76%
22/1
23.00
4.35%
25/1
26.00
3.85%
28/1
29.00
3.45%
33/1
34.00
2.94%
40/1
41.00
2.44%
50/1
51.00
1.96%
66/1
67.00
1.49%
100/1
101.00
0.91%


Many people are able to make their "own book" on sporting events and look for value within the odds and percentages. 

By comparing their odds against those that the bookmaker is offering if they have a runner priced at (2/1 or 33%) and the bookmaker has gone (4/1 or 25%) chance, one might consider this price to be larger than the chances you have given the runner and are therefore getting a bigger price than you should. 

It is a worthwhile exercise in creating your "own book" as it instills in you the need to always hunt for value, again for both pre-match betting and in running betting.

This is not the most extensive guide to odds setting but does illustrate what I consider to be essential to find your "value" bets. 

I learned it like I learned my times table at school. It comes very naturally after you've practiced it a few times.

Good luck, and enjoy your new-found, odds compiling knowledge.

Understand Betting Odds

An essential tool in ensuring you are getting value with your bets is too fully understand Betting Odds. 

I don't mean knowing that 7/2 will return three and half time your stake as profit, but understanding what 7/2 is in percentage probability terms. 

It is also important not to mix this up with decimal odds which I see happening time and time again.

Spot the Value in Bookmakers Prices

Knowing how to convert odds to probability, or probability to odds can make spotting value prices and bookmaker mistakes EASY!

The bookies all construct their books based on percentage probability and understand it fully. 

They can convert a percentage to fractional or decimal odds without a moment's hesitation; if we as punters aim to get one over on our satchel filling friends, we need to be able to do the same.

I'll start with the easiest example I can think of linked to sport. The video above pertains to this also.

If you have a tennis match, you have only two possible outcomes, player A will win or player B will win. If for the sake of simplicity we imagine these two players are absolutely matched in terms of ability and no advantage is given to either player, we have a 50/50 chance. 

50/50,  is simply a way of expressing a 50% chance for player 1 to win and an equal 50% chance that player 2 will win. 

So looking at the Odds table above and looking up the conversion of a 50% chance we can see that both players will be EVENS. 

This is before the bookie adds in his percentage (edge). (more of this later). For now, we have a price of Evens on each side of the book. In decimal terms, this is expressed as 2.

To have both players priced up at 50%, you have 50% + 50% which adds up to a 100% book. Something the bookie will never provide as there is no profit in the book for them.

Bookmakers Always Add an 'Edge'

This is what is known as the 'Over-Round'

Let's extend this now to a four-horse race. It may be considered that the three runners have been allotted the following prices.


True Probability Betting Market

Horse
Fractional
Decimal
Probability/Chance
1
6/4
2.50
40%
2
4/1
5.00
20%
3
4/1
5.00
20%
4
4/1
5.00
20%


Again, you can see that by adding the four percentage probabilities together, we find a 100% book.

It is more likely in the real world that this book would be made up thus.


Bookmakers Market with Over-Round [Edge] Built In

Horse
Fractional
Decimal
Probability
1
6/4
2.50 40%
2
3/1
4.00
25%
3
3/1
4.00
25%
4
4/1
5.00
20%


You can see how Horses 2 & 3 have been given higher probability scores and thus shorter prices than in the first table

This book adds up to an Over-round (self explanatory) of 110%. 

This is what gives the Bookmaker the edge, In this case 10%. 

In the fluctuation of the odds from here on in, the book would be attempted to be balanced by the weight of money coming into the book. 

If a lot of bets were taken on the 6/4 favourite, the price would likely shorten, maybe to 5/4 (44.4%). 

The book will now have an over-round of 114.4%, Horse 4 may not have attracted any financial interest, so to balance the book, and to attract money for it, to level out the liabilities, the price can be pushed out to 9/2, (half a point as the punter would call it), but in percentage terms, the 20% chance is now 18.2%. 

The book now looks like this


New Market After 'Weight of Money' From Punters

Horse Fractional Decimal Probability
1 5/4 2.25 44.4%
2 3/1 4.00 25%
3 3/1 4.00 25%
4 9/2 5.50 18.2%


This book is now, 44.4+25+25+18.2 = 112.6 or 112.6% over-round

This is how odds are affected and moved to maintain the "edge" the bookmakers have to have to operate and make a profit.

How Bets Affect Betting Markets

The above example shows why betting odds change depending on how bets are being placed

Bookmakers do not think in fractional or decimal prices, they think solely in percentages and adjust their fractional/decimal odds accordingly to remain over-round. 

There will be occasions when they may fleetingly go under 100%, this is rare, and is done simply to attract money to the market, this is the exact opposite to over-round, and is under-round, or more commonly known as over-broke. 

Once the required amount of money has been attracted into their book, the liabilities will be reassessed and the book adjusted accordingly to reflect an over-round once again. Of course these days, this is can all be done with the click of a laptop button and changed in an instant.

As an example of how the over-round works in the bookmakers favour, imagine a book was made up to represent an over-round of 125%. Now, in the theoretical event of this bookmakers customer staking to the book, they could take 125 units of bets, and the most the bookmaker would have to pay out would be 100, making a 25 unit profit, and therefore the 25% is converted to profit. 

This is, of course, the plainest of examples and it would not happen this way, which is why the prices for a race will change, ebb and flow during the betting before the race begins as the bookmaker continues to change his liability by taking varying amounts of stakes at various prices whilst readjusting the prices in order to control his liability and "lock in" as much profit as he can in any given event in the race. 

Again, one has to take their hat off to the guys that used to do this years ago with pen and paper whilst the bookie is shouting the stakes and odds at him from the podium in all kinds of weather. Nowadays, relatively simple software will do this job in an instant.

Computers Have Helped Bookies With Odds Making

Hundreds of calculations all done in the head in the old days provided more opportunities to take advantage of bookmaker mistakes

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