The sum is just one divided by the decimal odds. A price of 1.50 implies 66.7%. A price of 4.00 implies 25%. Fractional odds need converting first: 6/4 is 2.50 in decimal, so the implied chance is 40%.
I use this constantly, because "looks big" is not a number. Once a price is a percentage, you can compare it with the chance you actually give the outcome. If you think a team wins 45% of the time and the price implies 38%, the price is the longer of the two. That gap is the whole idea behind a value bet.
The bookmaker's margin
Add the implied probabilities of every outcome in a market and you will get more than 100%. That extra is the overround, the bookmaker's margin. A home/draw/away market might imply 104% or 108% in total. None of those three prices is a pure probability until you account for that.
A fair comparison is your probability against the implied probability of the price you can actually take, not against a price you have already stripped the margin out of in your head and then forgotten. The odds calculator does the conversion if you do not want to do the division yourself.
Where it shows up on the site
Most of the probability tools start from the other direction. We publish a model chance, then you compare it with the implied chance in the bookmaker's price. On value bets that comparison is already done for you: the feed is fixtures where the price implies less chance than the model.
Dropping odds change the implied probability as they move. A price that shortens from 2.20 to 1.90 has gone from about 45% to about 53%. Whether that new number is still worth taking is a separate question.
FAQ
What is the implied probability of 2.50?
40%. One divided by 2.50 is 0.40. In fractional odds, 2.50 is 6/4.
Why do the percentages in a market add up to more than 100%?
Because the bookmaker prices each outcome a bit shorter than a fair chance. The amount over 100% is the margin. It is not a sign that the maths is wrong.
Related Terms
A value bet is a price whose implied chance is lower than the chance you give the outcome. Closing line value compares the implied chance you took with the implied chance of the closing price.