Every two-way price has the book's margin built in. A prop listed at over -115, under -105 isn't the book saying "53.5% over, 51.2% under". Those add up to more than 100%. Take the margin out and you're left with the book's actual estimate: the no-vig, or fair, price.
The fair price is the single most useful number in prop betting. It's how you tell whether another book's price is good, bad, or just different.
Step 1: turn each price into a probability
A price's implied probability is the win rate it needs to break even.
- For a negative price: risk ÷ (risk + 100). So -115 is 115 ÷ 215 = 53.49%.
- For a positive price: 100 ÷ (price + 100). So +110 is 100 ÷ 210 = 47.62%.
For our prop, -115 is 53.49% and -105 is 105 ÷ 205 = 51.22%.
Step 2: add them up
53.49% + 51.22% = 104.71%. The extra 4.71% over 100% is the book's margin, often called the vig or hold. The hold calculator shows it for any line.
Step 3: scale back to 100%
Divide each side by the total:
- Over: 53.49 ÷ 104.71 = 51.08%
- Under: 51.22 ÷ 104.71 = 48.92%
Those two now add up to 100%. That's the book's real view: a slight lean to the over.
Step 4: turn the fair probability back into a price
For the favored side, fair odds = -(p ÷ (1 - p)) × 100. For the other side, the same number with a plus sign.
- Over: 0.5108 ÷ 0.4892 = 1.044, so -104.
- Under: +104.
So the fair price of this prop is about -104 / +104. The no-vig calculator does all four steps at once.
A second example
A more lopsided prop: over -130, under +110.
- -130 is 130 ÷ 230 = 56.52%. +110 is 47.62%. Total: 104.14%.
- Fair: 56.52 ÷ 104.14 = 54.27% over, 45.73% under.
- Fair price: 0.5427 ÷ 0.4573 = 1.187, so about -119 / +119.
Using the fair price
The point of a fair price is to judge someone else's number. Say the fair price on the first prop is 51.08% over (about -104), and a second book is offering over at +100.
At +100 you win $100 for every $100 risked. With a 51.08% chance of winning, the expected value per $100 is:
(0.5108 × $100) - (0.4892 × $100) = +$2.16
That's a positive bet: you're being paid even money on something slightly more likely than not. The same logic works for any price. If the other book's implied probability is lower than your fair probability, the price is better than fair.
Which book's price to trust
A fair price is only as good as the book it comes from. A few rules of thumb:
- Use a sharp book, or several. Prices from books that take big bets and move quickly are closer to the truth. Averaging the fair price across several books helps too.
- Use the main line. Fair prices from far alt lines carry extra margin and are less reliable. See alt lines explained.
- Check the time. A fair price from a book that hasn't moved since injury news is a fair price for a world that no longer exists.
Betravon lines up every book's price for each prop side by side, which is the quickest way to see where one book's number is out of line with the rest. Try it on today's NBA player props, or check any two-way price with the no-vig calculator.