Odds are just numbers, not fortunes
Betting markets throw numbers at you like cheap tricks. The real magic? Turning those odds into a percentage that tells you how likely an event actually is. No mysticism—just math.
The core formula, plain and simple
Implied probability = 1 ÷ decimal odds. If the decimal price is 2.50, the probability is 1 / 2.50 = 0.40, or 40 %.
American odds
Positive odds (+150) mean: you win $150 on a $100 stake. Implied probability = 100 ÷ (odds + 100). So, 100 ÷ 250 = 40 %.
Negative odds (‑200) mean: you must bet $200 to win $100. Implied probability = odds ÷ (odds + 100). That’s 200 ÷ 300 = 66.7 %.
Fractional odds
Odds of 3/1 translate to 1 ÷ (3 + 1) = 25 %. The denominator is always the sum of numerator and denominator, then invert.
Why the numbers never add up to 100 %
The bookmaker’s margin—also called the vig—inflates the implied probabilities. If you sum the probabilities from both sides of a match and get 105 %, the extra 5 % is the cut they keep. Spotting that excess is the first step to finding value.
Quick cheat sheet
Decimal → 1 / d
American (+) → 100 / (a + 100)
American (‑) → a / (a + 100)
Fractional → 1 / (n + d)
Practical application, no fluff
Imagine a football game where Team A is listed at 1.80 decimal, Team B at 2.20. Team A’s implied prob = 1 / 1.80 = 0.5556 (55.56 %). Team B’s = 1 / 2.20 = 0.4545 (45.45 %). Add them: 101 %. The extra 1 % is the bookmaker’s edge.
Now ask yourself: where does the market disagree with your own assessment? If you think Team B has a 50 % chance, that’s a 4.5 % edge right there. Bet accordingly.
Last move
Pull a calculator, plug the odds into the formula, strip away the vig, and you’ve got the real probability. Use it, trust it, and let the numbers guide your next stake.
