Ever see a bet at +200 and wonder, “What are the chances of this winning?” Or you looked at a -150 favorite and thought, “Is this a smart bet, or am I risking too much for a likely win?”

Welcome to the world of odds conversion and implied probability.

This is where smart bettors live. Because once you know how to translate odds into win probabilities, you stop betting based on gut feels and start betting based on math. And when the math is on your side, you’re not just betting—you’re investing.

Odds Are Just Prices

Let’s start here: Odds = price.

Just like the sticker on a gallon of milk or the price tag on a pair of Jordans, odds are what you pay to “buy” a bet.

But the twist? In sports betting, odds also tell a story—specifically, how often the sportsbook thinks that outcome will happen.

That story is called implied probability.

And your job is to ask: Do I agree with the book’s version of reality? Or do I think they’ve priced it wrong?

If your answer is the latter, then you may have found value.

American Odds

If you bet in the U.S., you’re probably used to American odds—those lovely pluses and minuses:

  • +150 = Underdog

  • -200 = Favorite

But what do those numbers mean?

Let’s break it down.

Positive Odds (e.g., +150):

Use this formula to find implied probability:

Implied Probability = 100 / (Odds + 100)

Example: +150: 100 / (150 + 100) =0.4 = 40%

The book is saying: “This outcome has a 40% chance of happening.”

Negative Odds (e.g., -200):

Use this formula:

Implied Probability = Odds (as positive) / (Odds + 100)

Example: -200: 200 / (200 + 100) =0.667 = 66.7%

That’s the book telling you: “You need this to win about 2 out of 3 times just to break even.”

Why The Percentages Matter

Let’s say you think a team listed at +150 has a 50% chance to win (not 40% as implied). You believe the true odds should be around +100.

In that case, you’ve spotted a value discrepancy—the sportsbook is offering more payout than they should. That’s a +EV bet (positive expected value), and that’s how you win long-term.

Real Example

Picture this. It’s Week 6 in the NFL. A scrappy home team is listed at +170 against a playoff contender.

Most casual bettors ignore it—“No way they win.”

But you’ve been tracking this team. You’ve watched their pass rush click, you know the matchup’s favorable, and you believe they win this game about 42% of the time.

Let’s check the math:

  • +170 odds → Implied probability = 100 / (170 + 100) = 100 / 270 = 37%

Your estimated win chance: 42%

Book’s implied win chance: 37%

You’re seeing an edge. A 5% edge might not sound like a lot, but in betting, that's a big advantage over the book.

Decimal & Fractional Odds

Not every book uses American odds. If you’re betting on international sites, you’ll run into some different formats:

Decimal Odds (e.g., 2.50)

These are used in Europe, Canada, and elsewhere. To calculate implied probability:

Implied Probability = 1 / Decimal Odds

Example: 2.50: 1 / 2.50 = 0.40 = 40%

Decimal odds also show your total return, not just your profit.

A 2.50 line means for every $1 you bet, you get $2.50 back (profit + stake).

Fractional Odds (e.g., 5/2)

Common in the UK and horse racing. To find implied probability:

Implied Probability = Denominator / (Numerator + Denominator)

Example: 5/2: 2 / (5 + 2) = 2 / 7) = 28.6%

Spotting Value: The Only Way to Win Long Term

Here’s the golden rule:

If your estimated probability of an outcome is higher than the implied probability of the odds, you’ve found a value bet.

Let’s go quick-fire:

  • Book offers +200 → Implied probability ≈ 33%

  • You believe the true chance of winning = 40%

    You’re beating the number with a margin of safety built in, so you take it

  • Book offers -130 → Implied probability ≈ 56.5%

  • You think it’s 50/50

    Odds are overpriced, so you pass; consider looking at the other side.

Line by line, edge by edge, this is how smart bettors build long-term profit. It’s not about winning tonight—it’s about winning consistently, by backing numbers that make sense.

Bonus Tip: Use an Odds Calculator

You don’t have to do the math in your head. There are dozens of odds conversion calculators online—bookmark one and use it until the numbers feel second nature. Better yet, build a simple spreadsheet that converts odds and compares them to your personal win probabilities. It’ll change how you bet forever.

Final Word: Stop Betting, Start Evaluating

Odds conversion isn’t sexy. There’s no highlight reel. But it’s the foundation of every sharp bettor’s edge.

Once you start translating odds into what the book is saying, you become more than just a bettor—you become an evaluator of risk and return.

Don’t just ask “Who’s going to win?”

Ask: “Is this price worth the risk?”

If the answer is yes? Fire away.

If not? Pass. Or find a better number. Because the more you understand implied probability, the more you bet on your terms, not the book’s.