Probability Calculator
Convert between probability percentages and American, decimal, or fractional odds — instantly.
Enter any value — others update instantly
Quick Reference
| Probability | American | Decimal | Fractional | Example |
|---|---|---|---|---|
| 75% | -300 | 1.33 | 1/3 | Heavy favourite |
| 66.7% | -200 | 1.50 | 1/2 | Strong favourite |
| 50% | +100 | 2.00 | 1/1 (evens) | Coin flip |
| 33.3% | +200 | 3.00 | 2/1 | Underdog |
| 20% | +400 | 5.00 | 4/1 | Long shot |
| 10% | +900 | 10.00 | 9/1 | Very unlikely |
How to Read Prediction Market Odds
Prediction markets display prices as probabilities — a contract trading at 65¢ implies a 65% chance of that event happening. This is more intuitive than traditional betting odds, but it helps to know how to convert between formats.
Probability %
The most direct format. A 65% probability means the market thinks there is a 65-in-100 chance the event occurs. On Polymarket and Kalshi, you'll see prices like $0.65 — that's 65%.
American Odds
Used in US sports betting. Negative odds (e.g. -186) mean you bet $186 to win $100. Positive odds (e.g. +150) mean you win $150 on a $100 bet. Anything below -100 is a favourite; above +100 is an underdog.
Decimal Odds
Popular in Europe and Australia. Decimal odds of 1.54 mean you get back $1.54 for every $1 wagered (including your stake), so your profit is $0.54. A 50% probability = 2.00 decimal odds.
Fractional Odds
Traditional UK format. Odds of 4/1 mean you win $4 for every $1 staked. Odds of 1/2 (a short-priced favourite) mean you win $1 for every $2 staked.
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