For 2-way markets like tennis or football over/under a typical bookie might have 1.83 on each of the 2 outcomes, which is about 109.3%.
For an average race, it is usually between about 110% and 125% depending on the number of runners – it will be lower for fewer runners or where there are only a couple of very fancied horses against longshots. If you calculate the book percentage on a bookmaker’s odds it will normally be over 100%. When a bookmaker prices up an event, he factors his profit into the odds, and this is called the “overround”.
In reality, the correct odds of all the possibilities add up to make a 100% book (identifying the correct odds for each possibility is the tricky bit)… The book percentage here is 100% – it is a perfect book. The probability of an event happening can be expressed as a percentage, for example, if you toss a coin the probability of heads or tails is 50% or evens for each.
