Odds Explained for Beginners

What Are Odds, Really?

When you see odds, whether it’s “+150” on a game or “35 to 1” on a roulette number, you are looking at two pieces of information:

• How much you win if the bet hits.
• How likely that outcome is in the long run.

Casinos and sportsbooks love to show the first part – the big payout – and hope you don’t think much about the second. Once you learn to translate odds into actual probabilities, the fog lifts.

The Three Main Odds Formats

There are three common ways of writing odds. They all describe the same reality, just in different languages.

Decimal Odds (1.80, 2.25, etc.)

Decimal odds are popular in Europe and Canada, especially online.

• They show your total return per 1 unit staked, including your stake.
• Example: 1.80 means you get 1.80 back for every 1 you bet.
• If you bet $10 at 1.80, total return is $18 ($8 profit plus your $10 stake).

To convert decimal odds to implied probability:

Implied probability = 1 / decimal odds

So:

• 1.80 → 1 / 1.80 ≈ 55.6%
• 2.00 → 1 / 2.00 = 50%
• 3.50 → 1 / 3.50 ≈ 28.6%

American Odds (+120, –150)

American odds are common in North American sports betting.

• Positive odds (+120): how much you win if you risk 100.
• Negative odds (–150): how much you must risk to win 100.

Examples:

• +120 → a $100 bet wins $120 (total return $220).
• –150 → you must bet $150 to win $100 (total return $250).

To convert American odds to implied probability:

• For positive odds: implied prob = 100 / (odds + 100)
• For negative odds: implied prob = odds / (odds + 100), ignoring the minus sign

So:

• +120 → 100 / (120 + 100) ≈ 45.45%
• –150 → 150 / (150 + 100) = 60%

You can use the odds tools on our Calculators page to convert between American and decimal formats automatically.

Fractional Odds (5/1, 7/2)

Fractional odds are the traditional format in the UK and at some horse tracks.

• They tell you how much you win compared to what you risk.
• Example: 5/1 means “five to one” – you win 5 for every 1 staked.
• A $10 bet at 5/1 returns $60 ($50 profit plus $10 stake).

To find the implied probability:

Implied probability = denominator / (numerator + denominator)

So:

• 5/1 → 1 / (5 + 1) ≈ 16.67%
• 7/2 → 2 / (7 + 2) ≈ 22.22%

Implied Probability: The Key Behind Every Price

“Implied probability” is just the chance of an outcome as suggested by the odds. Once you see that number, everything becomes clearer.

Why it matters:

• You can see how often you’d need to be right to break even.
• You can compare your own estimate to the book’s number.
• You can spot when a long-shot is even longer than it looks.

Example: a team is priced at 3.00 decimal (2/1 fractional, roughly +200 American).

• Implied probability = 1 / 3.00 = 33.3%.
• That means you’d need to win this type of bet more than 33.3% of the time to come out ahead long-term.

If your realistic estimate of their chances is closer to 25%, the bet is bad – even if it wins tonight.

Our odds converters and implied probability calculators can do this math for you in a few clicks.

Examples from Casino Games

Casino games don’t always show odds as clearly as sports betting, but the same ideas are hiding under the surface.

Roulette

On a European wheel (single zero), there are 37 pockets: 18 red, 18 black, 1 green zero.

• Betting on red has a true probability of 18/37 ≈ 48.65%.
• If the game were “fair”, winning would pay slightly less than even money.
• In reality, it pays 1:1, which bakes in a 2.7% house edge.

Slots

Slot machines don’t show you odds directly. Instead, they have:

• RTP (Return to Player): e.g., 96% over a very long run.
• House edge: 100% – RTP (so 4% in this case).
• Volatility: how big and rare wins are versus small and frequent.

You can think of a 96% RTP slot as a game where, in the long run, 4% of all money wagered stays with the house. In the short run, the result can be all over the place because of volatility.

We break this down more in our Slots RTP & Variance guide.

Examples from Sports Betting

Sportsbooks bake their edge into the prices they offer. A common setup is a point spread with both sides at –110.

At –110, you risk $110 to win $100. The implied probability is:

110 / (110 + 100) ≈ 52.38%

That means:

• If you win exactly 50% of these bets, you will lose money over time because of the vig.
• You need to win about 52.4% just to break even.

If you see a book offering slightly better prices (for example –105 or reduced juice), the implied probability needed to break even goes down, which is why serious sports bettors shop for lines.

For more on this, see our sports section starting with Sports Betting Bankroll 101.

Odds vs. House Edge vs. Your Real Chances

Odds tell you what you get paid. House edge tells you how much the game keeps on average. Your real chances sit somewhere in the middle.

In pure-chance casino games (roulette, slots, keno), you can’t change the underlying probabilities. The house edge is built into the rules and paytable.

In skill-influenced or opinion-based markets (blackjack, poker, sports betting):

• Your decisions affect your real chances (for better or worse).
• Bad strategy or poor picks effectively give the house or book an even bigger edge.

Our House Edge by Game guide compares the main games side by side, so you can see where your money lasts longer and where the odds are brutal.

If you want to understand how expected value and variance work across all these games, jump into the Complete Odds & Probability Guide.

Common Mistakes When Reading Odds

Even smart people misread odds. Here are a few traps to watch out for:

• Focusing only on payout size. Big potential wins usually mean low probability.
• Assuming a short-term streak changes the math. Past results don’t affect future odds in independent games.
• Ignoring implied probability. If you don’t know what % the price is implying, you can’t judge if it’s reasonable.
• Confusing “unlikely” with “impossible.” Very low-probability events still happen regularly across many players and spins.

A little bit of number sense goes a very long way. Once you’ve translated odds into probabilities a few dozen times, you’ll start doing it by feel.

Quick Summary

You don’t have to love math to understand odds well enough to protect yourself. Just keep these points in mind:

• All odds formats (decimal, American, fractional) describe the same thing in different ways.
• You can always convert odds into an implied probability – the chance the book or casino is assuming.
• Casino games have a built-in house edge, which is why they win long-term.
• Your decisions affect your real chances more in blackjack, poker and sports betting than in pure chance games.
• The bigger and flashier the payout, the lower the probability behind it.

From here, most people will get a ton of value from pairing this page with:

• Complete Odds & Probability Guide (advanced math and game-by-game breakdowns).
• Ultimate Guide to Bankroll Management (how much to risk on all this).
• House Edge by Game (which games drain you fastest).

Understanding odds doesn’t guarantee you’ll win – it just means you’ll know exactly what you’re getting into, and that’s a huge step up from guessing.