The Language of Probability

Every price you see at a sportsbook is a translation. The bookmaker has estimated a probability and converted it into a format you can bet with. Understanding all three major formats, and how to move between them, is the first skill of any serious bettor.

Most U.S.-facing books display American odds by default. You'll see numbers like -150 or +220. The minus sign tells you how much you need to risk to win $100. The plus sign tells you how much you win if you risk $100. A -150 favorite requires a $150 bet to return $100 in profit. A +220 underdog returns $220 in profit on a $100 bet.

This format is intuitive once you grasp it, but it has a flaw: the mathematical relationship between probability and price isn't immediately visible. That's where decimal odds become essential.

Decimal Odds: The Universal Format

Decimal odds, common in Europe and Australia, express the total return per unit staked. A decimal price of 1.67 means a $100 bet returns $167 total. The math becomes transparent: implied probability = 1 / decimal odds. So 1.67 implies a 59.9% chance.

This is why sharp bettors prefer working in decimals. The conversion is immediate. No mental gymnastics about what -185 means versus -190. Just divide one by the number and you have the book's estimated probability.

Conversion formulas:

  • American to Decimal (positive): Decimal = (American / 100) + 1
  • American to Decimal (negative): Decimal = (100 / |American|) + 1
  • Decimal to Implied Probability: Probability = 1 / Decimal

Fractional Odds

Still common in the UK and Ireland, fractional odds like 5/2 or 6/4 represent profit relative to stake. 5/2 means you win $5 for every $2 staked. Converting to decimal: divide the fraction and add 1. So 5/2 = 2.5 + 1 = 3.50 decimal.

Why Implied Probability Is What Matters

Here's the critical insight: the odds format you see is just packaging. The implied probability is the product.

When you see the Chiefs at -200, your first thought should not be "I need to bet $200 to win $100." It should be: "the market is pricing the Chiefs at 66.7% implied probability. Do I think they win more or less often than that?"

"The number on the screen is not advice. It is a price. Your job is to determine whether that price is too high or too low."

The Vig Problem

Bookmakers do not offer fair odds. The implied probabilities of both sides of a bet sum to more than 100%. On a standard NFL point spread, you might see -110 on both sides. The implied probability of each is 52.4%. Added together: 104.8%. That extra 4.8% is the vig — the bookmaker's margin.