Odds Converter: How to Convert Between American, Decimal & Fractional Odds
Free odds converter with formulas to convert between American, decimal, fractional odds, and implied probability. 6 worked examples, a reference table, and an instant converter tool.
Why you need to convert odds fluently
Every sportsbook, prediction market, and exchange displays prices differently. DraftKings shows American odds. Polymarket shows contract prices (implied probability). Kalshi uses cent-denominated contracts. European books use decimal. UK books use fractional. If you only read one format, you are blind to value on every other platform.
Converting odds is not a nice-to-have skill. It is the prerequisite for line shopping, cross-platform arbitrage, and any quantitative approach to betting. You cannot compare a -110 American line to a $0.52 Kalshi contract without converting both to the same scale. The odds converter handles this instantly, but understanding the math behind it makes you a better bettor.
This guide covers every conversion you will encounter. Six formulas, worked examples for each, and a reference table you can bookmark.
American odds to decimal odds
American odds center around $100. Positive odds show what you win on a $100 stake. Negative odds show what you must stake to win $100. To convert to decimal:
Positive American (+150): Decimal = (American / 100) + 1
Negative American (-200): Decimal = (100 / |American|) + 1
Worked example: +150 to decimal
Decimal = (150 / 100) + 1 = 1.50 + 1 = 2.50
A $100 bet at +150 returns $250 total ($150 profit + $100 stake). Decimal odds of 2.50 say the same thing: multiply your stake by 2.50 to get total return.
Worked example: -200 to decimal
Decimal = (100 / 200) + 1 = 0.50 + 1 = 1.50
A $200 bet at -200 returns $300 total ($100 profit + $200 stake). Decimal 1.50 confirms: $200 x 1.50 = $300.
This conversion is the most common one US bettors need. American odds are intuitive for single bets but terrible for math. Decimal odds make every calculation simpler because the total return is just stake x decimal odds. The EV calculator and Kelly Criterion calculator both work in decimal odds internally.
Decimal odds to implied probability
Implied probability is the break-even win rate embedded in the odds. It strips away profit and loss and answers one question: how often does this bet need to win to break even?
Formula: Implied Probability = 1 / Decimal Odds
Worked example: decimal 2.50 to probability
Probability = 1 / 2.50 = 0.40 (40%)
At decimal odds of 2.50, this bet needs to win 40% of the time to break even. If your estimate of the true probability is higher than 40%, the bet is +EV. If lower, it is -EV. This is the foundation of expected value.
Worked example: decimal 1.50 to probability
Probability = 1 / 1.50 = 0.667 (66.7%)
At -200 (decimal 1.50), you need to win 66.7% of the time to break even. Heavy favorites priced at -200 are often overbet by the public, making the true probability closer to 60-63%. That gap is where sharp bettors find edge.
American odds to implied probability (direct)
You can skip the decimal step and go straight from American to implied probability:
Negative American (-200): Probability = |American| / (|American| + 100)
Positive American (+150): Probability = 100 / (American + 100)
Worked example: -110 to probability
Probability = 110 / (110 + 100) = 110 / 210 = 0.524 (52.4%)
The standard -110/-110 line implies 52.4% probability for each side. Add them up: 52.4% + 52.4% = 104.8%. That extra 4.8% is the vig. The sportsbook has priced both sides as if each is more likely than it actually is. Strip the vig with the de-vig calculator to find the true implied probability.
Worked example: +300 to probability
Probability = 100 / (300 + 100) = 100 / 400 = 0.25 (25%)
A +300 underdog is priced at a 25% implied probability. Long shots like this are where the biggest pricing errors tend to occur. Public bettors undervalue long shots, and sportsbooks sometimes do too.
Fractional odds to decimal odds
Fractional odds (5/2, 7/4, 1/3) show profit relative to stake. The first number is profit. The second is stake.
Formula: Decimal = (Numerator / Denominator) + 1
Worked example: 5/2 to decimal
Decimal = (5 / 2) + 1 = 2.50 + 1 = 3.50
A 5/2 bet pays $5 profit per $2 staked. Total return: $7 per $2, or $3.50 per $1. Fractional odds are common in UK racing but rare in US sports betting. If you encounter them on an exchange like Betfair, convert to decimal immediately for cleaner math.
Worked example: 1/3 to decimal
Decimal = (1 / 3) + 1 = 0.333 + 1 = 1.333
Fractional odds of 1/3 mean you risk $3 to win $1. In American odds, this is -300. In implied probability, 1 / 1.333 = 75%.
Prediction market prices to odds
Prediction markets display prices as contract costs between $0.01 and $0.99 (or equivalent). A $0.45 Yes contract is identical to saying the implied probability is 45%. Converting to betting odds:
To decimal: Decimal = 1 / Price
To American (price < 0.50): American = ((1 / Price) - 1) x 100
To American (price ≥ 0.50): American = -Price / (1 - Price) x 100
Worked example: Kalshi $0.40 to odds
Decimal = 1 / 0.40 = 2.50 (equivalent to +150 American)
If you find a Kalshi contract at $0.40 and your model says the true probability is 50%, you are getting 2.50 decimal odds on something you believe has a 50% chance. That is a significant edge. Run it through the Kelly Criterion calculator to size the position.
Worked example: Polymarket $0.72 to odds
Decimal = 1 / 0.72 = 1.389 (equivalent to -257 American)
At $0.72, you need the event to happen 72% of the time to break even (before fees). After Polymarket's 2% profit fee, the break-even probability shifts slightly higher. The prediction market fee calculator computes this precisely.
The complete conversion reference table
| American | Decimal | Fractional | Implied Prob |
|---|---|---|---|
| -500 | 1.20 | 1/5 | 83.3% |
| -300 | 1.33 | 1/3 | 75.0% |
| -200 | 1.50 | 1/2 | 66.7% |
| -150 | 1.67 | 2/3 | 60.0% |
| -110 | 1.91 | 10/11 | 52.4% |
| +100 | 2.00 | 1/1 | 50.0% |
| +110 | 2.10 | 11/10 | 47.6% |
| +150 | 2.50 | 3/2 | 40.0% |
| +200 | 3.00 | 2/1 | 33.3% |
| +300 | 4.00 | 3/1 | 25.0% |
| +500 | 6.00 | 5/1 | 16.7% |
| +1000 | 11.00 | 10/1 | 9.1% |
Bookmark this or use the odds converter for instant conversions. The tool also handles edge cases like very short or very long prices that are harder to calculate mentally.
Common conversion mistakes and how to avoid them
Mistake 1: forgetting that implied probabilities include vig. When you convert -110/-110 to probability, you get 52.4% for each side. That totals 104.8%, not 100%. The extra 4.8% is the vig. To find the true probability, you need to de-vig. The de-vig calculator uses seven different methods to strip vig from bookmaker lines.
Mistake 2: confusing profit with total return. American odds show profit relative to a $100 reference. Decimal odds show total return including your stake. +150 American means $150 profit on $100. Decimal 2.50 means $250 total return on $100 ($150 profit + $100 stake). If you mix these up, every downstream calculation is wrong.
Mistake 3: treating prediction market prices as true probabilities. A Kalshi contract at $0.55 implies 55% probability, but that does not mean the true probability is 55%. Market prices include fees, spreads, and the collective bias of market participants. Sharp prediction market traders compare the implied probability to their own model and only trade when the gap is large enough to overcome fees. For the full framework, read the prediction market strategy guide.
Mistake 4: rounding too early. When chaining conversions (American to decimal to probability to Kelly), rounding at each step compounds errors. Carry at least four decimal places through intermediate calculations. Only round the final output.
Putting conversions to work in a real workflow
Odds conversion is step zero. It feeds every other calculation in the sports betting math pipeline.
- Convert the odds to a common format (decimal is best for math). Use the odds converter.
- De-vig the line to find the true implied probability. Use the de-vig calculator.
- Compare your estimated probability to the de-vigged line. If your probability is higher than the line implies, the bet is +EV.
- Calculate EV to quantify the edge. Use the EV calculator.
- Size the bet using the Kelly Criterion based on your edge and bankroll.
If you are comparing prices across sportsbooks and prediction markets, the market converter translates directly between sportsbook odds and prediction market contract prices. For cross-platform arbitrage, you need both prices in the same format before you can identify the opportunity.
Frequently asked questions
- How do you convert American odds to decimal?
- For positive American odds: Decimal = (American / 100) + 1. For negative: Decimal = (100 / |American|) + 1. Example: +150 becomes 2.50, and -200 becomes 1.50.
- What is the implied probability of -110 odds?
- The implied probability of -110 is 52.4%. The formula is |American| / (|American| + 100) = 110 / 210 = 0.524. This includes the vig. The true break-even probability after de-vigging a -110/-110 market is 50%.
- How do prediction market prices relate to betting odds?
- A prediction market contract price IS the implied probability. A $0.40 contract implies 40% probability and equals +150 American or 2.50 decimal odds. Convert with: Decimal = 1 / Price.
- Why do implied probabilities add up to more than 100%?
- The excess over 100% is the vig (also called juice or overround). A -110/-110 market has 52.4% + 52.4% = 104.8% total implied probability. The 4.8% overround is the sportsbook's margin. Use a de-vig calculator to strip it out.
- Which odds format is best for doing math?
- Decimal odds. Total return = stake x decimal odds. EV = (probability x decimal odds) - 1. Kelly = (p x d - 1) / (d - 1). Every formula is cleaner in decimal. Convert everything to decimal before calculating.