Implied probability is the single most useful concept in sports betting, and it is also one of the least understood. Every set of odds implies a probability: the chance the sportsbook thinks an outcome has. If you can calculate that probability, you can compare it with your own assessment, and that comparison is where value betting begins. Without it, you are guessing. With it, you are making a decision based on numbers.

The formula is simple. For decimal odds, the implied probability is one divided by the odds, multiplied by one hundred to get a percentage. Decimal odds of 2.00 imply a probability of 50 percent. Decimal odds of 1.50 imply a probability of about 67 percent. Decimal odds of 4.00 imply a probability of 25 percent. The shorter the odds, the higher the implied probability, and the longer the odds, the lower it is. A quick way to remember it is that odds of 2.00 are a coin flip, odds below 2.00 mean the outcome is more likely than not, and odds above 2.00 mean it is less likely than not.

For American odds, the formula is slightly different. For a favourite at minus 150, the implied probability is 150 divided by 250, which is 60 percent. For an underdog at plus 200, the implied probability is 100 divided by 300, which is about 33 percent. The general rule for favourites is the odds divided by the odds plus 100. For underdogs, it is 100 divided by the odds plus 100. If you find the American format confusing, switch your betting site to decimal odds, do the calculation there, and switch back if you need to.

If you add up the implied probabilities on both sides of a two-way market, you will get more than 100 percent. That extra is the overround, which is the sportsbook's margin. For example, if both sides of a market are priced at 1.91, each side implies a probability of about 52.4 percent, and the two together add up to about 104.8 percent. The extra 4.8 percent is the book's cut. A market with an overround of five percent is offering better value than a market with an overround of ten percent, because less of your stake is going to the book. Comparing the overround is a quick way to judge whether a market is worth betting in.

Finding value is a matter of comparing your probability with the book's implied probability. If you think a team has a 60 percent chance of winning and the odds imply a 50 percent chance, you have found a value bet, because you are being paid for a 50 percent chance on something you believe happens 60 percent of the time. If you think the team has a 40 percent chance and the odds imply a 50 percent chance, you should not bet, because the book is asking you to pay for a probability that is not there. This is the whole game, and it is why serious bettors spend more time on probabilities than on predictions.

Expected value turns that comparison into a number. Multiply your estimated probability by the payout, and subtract the probability of losing multiplied by your stake. If the result is positive, the bet has positive expected value over the long run. If it is negative, the bet loses money over time, even if it wins this time. You do not need to calculate expected value for every bet, but understanding it will change how you think about odds. Implied probability is a tool, not a prediction. It tells you what the market thinks, and your job is to decide whether the market is right, and by how much. Bet responsibly and set a budget you can afford.