From American odds to implied probability
Odds describe a payout, but they can also be translated into the probability needed to break even. For negative American odds, divide the absolute odds by that number plus 100. For positive odds, divide 100 by the odds plus 100.
At -120, the raw implied probability is 120 ÷ 220, or about 54.55%. At +105, it is 100 ÷ 205, or about 48.78%. These probabilities are useful, but they still contain the operator's margin.
Why do both sides add to more than 100%?
A two-outcome market should theoretically allocate 100% probability between its sides. Published prices commonly sum to more than 100%. That excess is often called vig, juice, margin, or overround.
If higher is -120 (54.55% raw) and lower is -110 (52.38% raw), the total is 106.93%. Reading 54.55% as the market's pure view of higher would overstate it because both sides contain margin.
You cannot reliably remove a two-sided market's margin from only one price. The opposing quote tells you how the total margin is distributed in the published pair.
The proportional no-vig method
A clear, common approach is to normalize each raw implied probability by their sum:
This method preserves the relative relationship between the quoted sides while removing the total overround. It is transparent and reproducible, which makes it appropriate for a comparison board.
Worked example
Suppose a reference market lists higher 6.5 assists at -125 and lower 6.5 assists at -105.
| Step | Higher | Lower |
|---|---|---|
| Published price | -125 | -105 |
| Raw implied probability | 55.56% | 51.22% |
| Total / overround | 106.78% | |
| Proportional no-vig | 52.03% | 47.97% |
The market's normalized lean is about 52.03% toward higher and 47.97% toward lower. This is not an objective forecast. It is an estimate derived from that market's currently published prices.
How PROPD uses no-vig probability
PROPD uses a reference no-vig probability only when it can compare the exact same strike with a target pick'em offer. It then subtracts the target's break-even probability:
If the reference higher side is 52.03% no-vig and the target higher selection requires 50% to break even, the arithmetic difference is +2.03 percentage points. The evidence label still matters: one source is not the same as multi-source agreement.
Pick'em break-even probabilities can depend on the complete entry format and payout. PROPD uses the applicable published offer data when available and distinguishes standard from modified lines rather than applying one universal assumption.
What no-vig probability does not solve
- It is not a prediction model. It reflects a quoted market at a moment in time.
- Normalization method matters. Proportional removal is transparent, but alternative methods can produce slightly different estimates.
- Thin markets can be noisy. Low liquidity, limits, and stale quotes may weaken the signal.
- Different strikes are different outcomes. No-vig math does not make a 6.5 line directly comparable with 7.5.
- Outcomes remain uncertain. A well-priced selection can lose; a poorly priced selection can win.
Inspect the prices, not just the pick
PROPD shows published reference prices, normalized probabilities, strike matches, and evidence states in one board.
Open the live player prop board