What is implied probability?

Every betting price is a percentage in disguise. Learning to read that percentage, called the implied probability, is the single most useful math skill for a bettor. It tells you how often a bet needs to win just to break even, which is the starting point for deciding whether a price is worth taking. This guide covers the two formulas for American odds and shows why the numbers on a real board always add up to more than 100 percent.

The two formulas

American odds come in two flavors, and each has its own conversion. The good news is that both are simple arithmetic you can do in your head with a little practice.

For a negative (favorite) price, divide the odds by the odds plus 100. Take -150. That is 150 divided by (150 plus 100), which is 150 divided by 250, or about 60 percent.

For a positive (underdog) price, divide 100 by the odds plus 100. Take +200. That is 100 divided by (200 plus 100), which is 100 divided by 300, or about 33 percent.

So this made-up board says the Foxes have about a 60 percent implied chance and the Miners about a 33 percent implied chance. That number is also your break-even line. If you think the Foxes win more often than 60 percent of the time, the price may hold value. If you think they win less often, the price does not.

Why the totals pass 100 percent

Add the two implied probabilities above and you get roughly 93 percent, which looks too low. That is because +200 is an unusually generous underdog price for illustration. On a normal two-sided market, the two sides add up to more than 100 percent, not less.

Look at a game where both sides sit at -110. Each -110 price is 110 divided by 210, or about 52.4 percent. Add the two together and you get about 105 percent. A fair market with no margin would total exactly 100 percent, so that extra 5 percent is the vig, the sportsbook's built-in margin.

This is why implied probability matters so much. The number the odds hand you is not the true chance of the outcome. It is the true chance plus a slice of margin. To find whether a bet has value, you have to compare the price against your own honest estimate, knowing the posted number is already inflated.

Turning the skill into a decision

Implied probability by itself does not tell you to bet. It tells you the bar the outcome has to clear. Your job is to form an independent view of how likely the outcome really is, then check it against the implied probability the price is quoting. When your estimate is meaningfully higher than the implied number, the bet may be worth a closer look. When it is lower, the price is asking too much.

That comparison, done honestly and repeatedly, is the whole foundation of value. It also feeds directly into expected value, the next tool for judging a bet as a decision rather than a result.