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How to read prop odds and implied probability
By JB, Slip Surgeon · Updated Oct 26, 2026
Every prop price is a probability in disguise. Once you can read it, you can tell a 60% leg from a 40% one at a glance, and see which leg on a slip is doing the heavy lifting.
American odds in one minute
Negative odds (−150) show how much you risk to win $100. Positive odds (+150) show how much you win on a $100 bet. A player prop usually has two sides, over and under, each with its own price.
The implied probability formula
- Negative odds:
|odds| ÷ (|odds| + 100). −150 → 150 ÷ 250 = 60%. - Positive odds:
100 ÷ (odds + 100). +150 → 100 ÷ 250 = 40%.
That number is the break-even rate: how often the bet has to land for the price to be fair to you, with the book's margin still included.
Cheat sheet: common prop prices
| Price | Implied chance | Price | Implied chance |
|---|---|---|---|
| −300 | 75.0% | +100 | 50.0% |
| −200 | 66.7% | +110 | 47.6% |
| −150 | 60.0% | +120 | 45.5% |
| −130 | 56.5% | +150 | 40.0% |
| −115 | 53.5% | +200 | 33.3% |
| −110 | 52.4% | +300 | 25.0% |
| −105 | 51.2% | +500 | 16.7% |
Reading an over/under pair
Say a points prop shows Over 24.5 −120 / Under 24.5 +100. The over implies 54.5% and the under 50%, which adds up to 104.5%. The extra 4.5% is the book's margin (the vig). Strip it out and the market's fair view is about 52.2% over. That's covered in finding no-vig fair odds.
Half-point lines and milestones
Main props use half points (24.5) so there's no push. Alternate lines and milestones ("25+ points", "100+ receiving yards") are one-sided, usually with no under to compare against, so they tend to carry more margin. A milestone at the same number as the main line should cost about the same as the over; when it's priced noticeably worse, the main line is the better way to bet the same read.
What implied probability doesn't tell you
It tells you what the price requires, not what will happen. To judge a leg, compare the implied chance with your own estimate, built from the player's recent games, a projection and the matchup. Our props pages for the NBA and NFL show FanDuel's line next to a model chance for exactly that reason.
From one leg to a parlay
Multiply the decimal odds for the parlay's price; its implied chance is 1 ÷ that number. The parlay calculator does it for up to 12 legs, and the bet slip checker does it straight from a screenshot.
Frequently asked questions
How do I convert American odds to implied probability?
Negative odds: |odds| ÷ (|odds| + 100). Positive odds: 100 ÷ (odds + 100). −110 is 52.4%; +200 is 33.3%.
What does −110 mean on a player prop?
You risk $110 to win $100. It implies a 52.4% chance, so the bet has to land a bit more than half the time to break even.
Check your own slip
Slip Surgery runs these checks on any NBA or NFL slip for free, then loads it into FanDuel or BetMGM.
Keep reading
Model picks, not locks. No checker can tell you a slip will win; it can only show you the price, the math and what changed. 21+ only. If betting stops being fun, call 1-800-GAMBLER or read our responsible gaming page.