Gambling Online 101
beginner
7 min readUnderstanding Odds
How to read American, Decimal, and Fractional odds.
BonusBell Team
Odds are the language of sports betting. They tell you two things: how much you can win and the implied probability of that outcome. Master reading odds, and you stop seeing prices as random sportsbook labels. You start seeing break-even thresholds, hidden vig, and whether one book is quietly offering a much better number than another.
Three Odds Formats
The same probability can be expressed three ways:
Same Odds, Different Formats
| American | Decimal | Fractional | Meaning |
|---|---|---|---|
| +150 | 2.50 | 3/2 | Bet $100, win $150 |
| -150 | 1.67 | 2/3 | Bet $150, win $100 |
| +100 | 2.00 | 1/1 | Bet $100, win $100 |
American Odds (Moneyline)
Most common in the US. Based on $100 bets.
Positive Numbers (+)
Show profit on a $100 bet:
+250 Odds
$100 stake × (250 ÷ 100)=$250 profit ($350 total return)
Underdog. You risk $100 to win $250.
Negative Numbers (-)
Show how much to bet to win $100:
-175 Odds
$175 stake to win $100=$100 profit ($275 total return)
Favorite. You risk $175 to win $100.
Decimal Odds
Common in Europe and Australia. Shows total return per $1 bet.
Decimal 2.75
$100 stake × 2.75=$275 total return ($175 profit)
Multiply your stake by the decimal to get total return.
Strategy Insight
Decimal odds are the easiest for calculating returns. Just multiply your bet × odds = total payout.
Fractional Odds
Traditional UK format, especially for horse racing. Shows profit relative to stake:
- 5/1 = Win $5 for every $1 bet
- 1/4 = Win $1 for every $4 bet
- 11/8 = Win $11 for every $8 bet
Converting Between Formats
Good to Know
American → Decimal:
- Positive: (American ÷ 100) + 1
- Negative: (100 ÷ |American|) + 1
+200 → Decimal
(200 ÷ 100) + 1=3.00
-200 → Decimal
(100 ÷ 200) + 1=1.50
Calculating Implied Probability
Odds imply how likely the sportsbook thinks an outcome is:
American Odds to Probability
Positive: 100 ÷ (Odds + 100) | Negative: |Odds| ÷ (|Odds| + 100)=Implied Probability
Odds and Implied Probability
| American | Implied Probability | True Meaning |
|---|---|---|
| -300 | 75% | Very likely to happen |
| -150 | 60% | More likely than not |
| +100 | 50% | Even odds |
| +200 | 33% | Underdog |
| +500 | 16.7% | Long shot |
The Vig Hidden in Odds
Sportsbooks don't offer fair odds. The implied probabilities on both sides add up to more than 100%:
Typical Line
Team A (-110) = 52.4% + Team B (-110) = 52.4%=104.8% (4.8% is the vig)
The extra 4.8% is the sportsbook's built-in profit margin.
Strategy Insight
Look for reduced juice books offering -105 instead of -110. Over a betting career, this saves thousands of dollars.
Worked Example: Comparing Three Books on the Same Game
Suppose the Lakers are playing the Celtics and you want to bet the Lakers moneyline. DraftKings posts Lakers at +145, FanDuel posts +150, and BetMGM posts +155. On a $100 bet, the difference between +145 and +155 is $10 of profit — an extra 6.9 percent on every winning ticket. Over 100 bets that is $1,000 left on the table for someone who does not line shop. The implied probabilities are 40.8 percent, 40.0 percent, and 39.2 percent respectively, meaning BetMGM is giving you the most generous price while DraftKings has baked in the most margin. Always place your bet at the best available number, not the first one you see.
Practice It: Line Shopping Lab
Best available price
Book C
+155 returns $155 profit on this stake.
Extra profit vs worst book
$10
Probability gap
1.6 pts
BookOddsProfitImplied probability
Book A+145$14540.8%
Book B+150$15040.0%
Book C+155$15539.2%
Use this only for the same side of the same market. A better price changes both your payout and the break-even probability you need to beat.
Removing the Vig to Find a Fair Line
To estimate what a fair, no-vig line would be, add both sides' implied probabilities, then divide each by that sum. For a -110 versus -110 line (52.38% + 52.38% = 104.76%), the de-vigged probability for each side is 52.38 / 104.76 = 50.0 percent. That is mathematically equivalent to a pick-em game. Comparing the de-vigged line at a soft book against the posted line at a sharp book is a foundational method for identifying +EV spots, and it is why professional bettors spend so much time building consensus prices across multiple books.
Common Beginner Odds Mistakes
- Confusing +150 with 1.5x payout. +150 means $150 profit on $100 staked, for a total return of $250. Decimal 2.50 is the equivalent and makes this easier to see.
- Assuming a -200 favorite is safe. -200 implies a 66.7 percent win rate. One in three of those favorites will lose on average. Expect cold streaks.
- Stacking heavy favorites in a parlay. Four -300 favorites produce combined odds of roughly +97, which pays less than even money. The vig compounds and the house edge balloons.
Try It: Odds Converter
Related Reading
- Removing Vig— how to turn sportsbook prices into fair probabilities instead of accepting the margin at face value
- Expected Value— what to do once you know whether the market price is better or worse than your number
- Line Movement— why the best price early is often worth more than a “sharp opinion” found too late
Good to Know
Try Our Free Tools
Use our Universal Bet Calculatorto instantly convert between American, Decimal, and Fractional odds and see exactly what you'll win.
Good to Know
Try It Live
Compare odds across sportsbooks to find the best price with BonusBell's Odds Comparisontool. See every book's line side by side and never overpay for a bet again.
Key Takeaways
- 1American odds: + shows profit on $100, - shows stake needed to win $100
- 2Decimal odds: multiply by stake for total return
- 3Fractional odds: numerator/denominator shows profit ratio
- 4Implied probability reveals how likely books think outcomes are
- 5Both sides' probabilities exceed 100%—the difference is vig
Sources & References
- American to Decimal: positive = (American ÷ 100) + 1; negative = (100 ÷ |American|) + 1. Decimal to Fractional: (Decimal − 1) as fraction. Standard mathematical conversions, independently verifiable.
- Implied probability from American odds: positive = 100 ÷ (Odds + 100); negative = |Odds| ÷ (|Odds| + 100). Independently verifiable derivation from the definition of odds.
- Vig (overround) calculation: sum of implied probabilities on all outcomes minus 100%. A standard −110/−110 line produces 104.8% total implied probability, yielding ~4.5% hold. Independently verifiable.
- American Gaming Association (AGA). AGA state-tracking resources provide the current U.S. sports-betting market context that makes American odds the default format for most domestic bettors. (AGA State of Play map; AGA State of the States 2025)
Mathematical claims are independently verifiable. BonusBell platform analysis reflects our tracked platform directory and dated evidence checks as of March 2026.