Betting odds are the language of sports betting, and if you cannot read them, you cannot compare prices, calculate payouts, or tell whether a bet is good value. There are three main formats: American, decimal, and fractional. They all describe the same thing, which is how much you win if your bet wins, and how likely the book thinks the outcome is. Learning to read all three takes an afternoon, and it will save you from expensive misunderstandings for as long as you bet.

American odds are the default in the United States. A minus number tells you how much you must risk to win one hundred. A favourite at minus 150 means you risk 150 to win 100 in profit, plus your 150 stake back. A plus number tells you how much you win on a one hundred stake. An underdog at plus 200 means a one hundred bet returns 200 in profit, plus your 100 stake back. The minus sign always marks the favourite, and the plus sign always marks the underdog. The bigger the minus number, the heavier the favourite. The bigger the plus number, the longer the shot.

Decimal odds are the default in most of the world, and they are the simplest format to work with. The number tells you the total return on a one-unit stake, including your stake. Odds of 2.50 mean a one hundred bet returns 250 in total, which is 150 in profit plus your 100 stake back. Odds of 1.50 mean a one hundred bet returns 150 in total, which is 50 in profit. To work out the profit, multiply your stake by the odds and subtract your stake. To work out the implied probability, divide one by the decimal odds. Decimal odds of 2.00 imply a 50 percent chance, and decimal odds of 4.00 imply a 25 percent chance.

Fractional odds are the traditional British format. Odds of 3/1, often written as 3-1, mean you win three for every one you stake, so a one hundred bet returns 300 in profit plus your 100 stake back. Odds of 1/2 mean you win one for every two you stake, so a one hundred bet returns 50 in profit. Fractional odds are less common in online betting now, but you will still see them on horse racing and in British betting shops, so it is worth knowing how to read them.

Converting between formats is straightforward once you know the relationships. American minus 150 is the same as decimal 1.67 and fractional 2/3. American plus 200 is the same as decimal 3.00 and fractional 2/1. Most betting sites let you switch formats in the settings, and it is worth setting the format you find easiest and sticking with it, because mixing formats is a common source of expensive mistakes. If you ever see odds that look wrong, check the format before you check anything else.

The most important thing about odds is that they are probabilities in disguise. Decimal odds of 2.00 imply a 50 percent chance, because one divided by two is 0.5. Decimal odds of 1.25 imply an 80 percent chance, because one divided by 1.25 is 0.8. Learning to convert odds into implied probability is the single most useful skill in betting, because it lets you compare your own assessment with the book's assessment and spot the bets where they disagree. Pick one format, learn it properly, and always check the odds before you bet. A price you do not understand is a bet you should not place. Once you can convert odds into probabilities in your head, you will never look at a betting slip the same way again.