What A Price Is Actually Saying
Three formats, one underlying question: how much comes back if this wins. They just disagree about what to measure it against.
American quotes against 100. A minus number is what you'd need to stake to win 100; a plus number is what a 100 stake would profit. -150 means risk 150 to win 100. +150 means risk 100 to win 150.
Decimal quotes your total return per unit staked, profit and stake combined. A price of 2.50 returns two and a half times your stake, which is 1.50 in profit per 1 risked. It's the easiest format to actually calculate with, which is why the tool above converts to it internally before doing anything else.
Fractional quotes profit against stake as a ratio. 3/2 wins three for every two risked. It's the older British convention and still shows up in horse racing and futures markets.
Worked Example: The Standard -110
The two-way price on a typical spread or total is -110. Put it into the calculator above and here's the full row it produces:
| Format | Value | Reads as |
|---|---|---|
| American | -110 | Risk 110 to win 100 |
| Decimal | 1.9091 | 1.9091 back per 1 staked |
| Fractional | 10/11 | Win 10 for every 11 risked |
| Probability | 52.38% | Break-even win rate |
A 100 stake returns 190.91 in total, a profit of 90.91. The row that actually matters, though, is the last one: at -110 you need to win 52.38% of the time just to break even, not 50%.
Converting By Hand
The conversions are short enough to do on paper, which is worth knowing so a mispriced screen is obvious at a glance.
- American to decimal. Positive price: decimal = (odds ÷ 100) + 1. So +250 becomes 3.50. Negative price: decimal = (100 ÷ |odds|) + 1. So -200 becomes 1.50.
- Decimal to American. At 2.00 or above: American = (decimal − 1) × 100. Below 2.00: American = −100 ÷ (decimal − 1).
- Fractional to decimal. Divide the fraction and add one: 7/4 becomes 1.75 + 1 = 2.75.
- Anything to probability. Probability = 1 ÷ decimal. This is the one step worth memorizing: every judgment about whether a price is worth taking runs through it.
Why The Probability Row Is The Useful One
Converting between formats is clerical. Converting to probability is the analytical step, because it turns a price into something you're allowed to disagree with.
-150 doesn't mean anything on its own. 60% is simply a claim; the market thinks this happens three times in five. You're free to hold a different view, and if your view is 66%, you've found a bet worth looking at. That comparison is impossible to make while the price is still written as -150.
One caution worth carrying forward: the implied probability from a single price is always a little inflated, because the book's margin is baked into it. Both sides of a -110 market imply 52.38%, and when multiplied by two, add up to more than 100%. Stripping that margin out and finding the fair number underneath is exactly what the implied probability calculator teaches you.
Frequently Asked Questions
How do I convert American odds to decimal odds?
Positive price: divide by 100 and add 1. +250 becomes 3.50. Negative price: divide 100 by the number without its sign and add 1. -200 becomes 1.50.
What does -110 mean in decimal and fractional odds?
-110 is 1.9091 in decimal and 10/11 in fractional. It implies a 52.38% chance, which is also the win rate you'd need to break even at that price.
Which odds format should I actually calculate in?
Decimal. The implied probability is just 1 divided by the price, and multiplying decimal prices together gives you a combined price directly. American is what most U.S. books display, so reading in American and converting to decimal for the math is the practical middle ground.
How does the calculator know which format I typed?
A slash reads as fractional. A leading + or - (or "even"/"pk") reads as American. A decimal point under 1.0 reads as a probability, like 0.52 for 52%; at 1.0 or above it reads as decimal odds. A bare whole number at 100 or more is read as American with the sign assumed; anything from 2 to 99 with no sign or point is read as decimal odds, since real American prices are never below 100.
Does this tool need an internet connection?
Only to load the page once. Every conversion happens locally in your browser. Nothing you type is transmitted, stored, or logged anywhere.