As mentioned above, bookmakers will add a betting margin after calculating the real probability of each outcome of an event.
This margin will vary depending on the type of event, with major sporting events usually having a margin of between 2-5% and rare sporting events possibly going up to a margin of 20%.
The reason for this is that more people bet on major events, so the bookmaker can afford to provide odds using a smaller margin. The rarer events will have less bets placed on it and therefore the bookie will want to make a bigger profit by using a bigger margin.
Using the above example, with real probability giving odds of 4/6 (1.66) for the home team, 9/2 (5.68) for the away, and 7/2 (4.50) for the draw, we can work out how much of a margin the bookmaker has added.
If the odds were changed to 4/9 (1.45), 11/2 (6.50), and 4/1 (5.00), the margin used would be 4.64%. With a balanced book, that would mean the bookmaker making £4.64 for every £100 wagered.
There are sites out there with margin calculators, which enable you to find out the margin being used for every sporting event. They are usually easy to use, although the explanations of each method may sound extremely complicated to more novice gamblers.