Fractional odds make it easy to calculate how much a winning player will win but the decimal odds system makes it easier for the player to calculate how much the bookie will pay if the player is successful.
Converting Fractional British odds to European decimal odds
This is very simple. You first use division to convert the fraction and then you add the number one (1).
So, if the fractional odds are 20-1 (pronounced 20-to-1), we divide 20 by one and add one (20/1 + 1 ), which gives decimal odds of 21.0. If the fractional odds are 1-20 (pronounced “1-to-20 on”), we divide one by twenty and add one, so 1/20 + 1, giving decimal odds of1.05. When the odds are 1-to-1 also known as “evens” or “even”, the decimal odds are 1/1 + 1 or 2.0.
An example is when we bet on a horse in UK races. If the horse is favorite and pays out 4/7 then we divide 4/7 = 0.57 and we add 1. We get European decimal odds of 1.57.
Now if we bet on an underdog being offered at 15/1 we divide 15/1=15 and we add 1 and we get European odds of 16.0. In other words the bookie will payout 16 times your stake for a profit of 15 times your stake.
Converting Decimal odds to Fractional British odds
To convert decimal odds to fractional odds we reverse the process.
Say for example, we want to bet on a horse that is being offered at 3.0, that is if we bet $100, we would receive a payout of $300 from the bookie, should the horse win. To convert this to fractional odds subtract one and put the resulting number on the left hand side of the formula. So, 3.0-1 = 2, giving us odds of 2:1, or two to one.
Obviously, it can get a bit more complicated if the fractions are more complicated. So, if the decimal odds are 2.75, subtracting one gives us 1.75. However odds of 1.75:1 are not nice to read or look at. Instead we can multiply both sides by 4 giving odds of 7:4 (seven-to-four), that is, if I bet $4 dollars and win, I get a profit of $7, taking home a total of $11.
The process is similar if the decimal odds are less than 2.0. Say, a win on a horse is being offered at 1.5. If we subtract one, we get 0.5. However, odds of 0.5:1 are not very easy to understand (if I bet $1 I get 50 cent profit). Here, we would multiply both sides by two, giving odds of 1:2 or one-to-two on. That is if I bet $2, I would profit by $1 and take home $3.
The conversion is very simple and you don’t need to use a calculator most of the times. It is also very easy and fast.