NBA Prop Implied Probability and Vig
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The Number Behind Every Prop Price
Every prop price you see on a UK book is two numbers welded together. The first is the bookie’s estimate of the true probability the bet wins. The second is the operator’s margin — the vig, the juice, the overround — added on top so the book pays itself for taking the bet. If you cannot separate those two numbers, you are guessing about whether any prop is good value. Once you can, the entire prop menu starts to look different.
This is not optional knowledge for a serious prop bettor. The Dimers analyst team writes that most days the largest betting edges found by modelling work belong to player prop bets rather than traditional moneylines, spreads or over/unders — and they say there is virtually always great value to be found on their best props page. The reason value exists is that prop pricing has wider vig than spread pricing, and the wider the vig, the more room there is for individual lines to be mispriced by the operator’s model.
The Implied Probability Formula
For a negative price like -125, implied probability is calculated as: -price divided by (-price + 100). So -125 implies 125 / (125 + 100), which is 125 / 225, or 0.556. The implied probability is 55.6%.
For a positive price like +130, the formula flips: 100 divided by (price + 100). So +130 implies 100 / 230, or 0.435. The implied probability is 43.5%.
The decimal-odds equivalent is easier still: implied probability equals 1 divided by the decimal odds. A 2.20 price implies 1/2.20 = 0.455, or 45.5%.
The numbers should be memorable for a handful of common prices: -110 implies 52.4%, -120 implies 54.5%, -150 implies 60%, +100 implies 50%, +120 implies 45.5%, +150 implies 40%. With these anchors, you can eyeball most prop prices without doing the maths in real time.
The Overround on a Two-Way Prop
A two-way prop market has an over price and an under price. If both implied probabilities summed to 100%, the market would be fair. They never do. The sum is always above 100%, and the gap is the operator’s overround — the vig.
The classic two-way price is -110 on both sides. Each side implies 52.4%. The sum is 104.8%. The overround is 4.8%. The operator’s hold per pound staked over a balanced market is roughly 4.5-4.8% – slightly less than the overround because of how the operator splits the action.
Lopsided prices widen the overround. A -150/+125 market implies 60% + 44.4% = 104.4% – similar overall but the operator’s exposure favours the over side. A -130/-110 market implies 56.5% + 52.4% = 108.9% – wider overround, often seen on player props where the book wants to discourage one side.
Prop markets typically run wider overrounds than the headline match markets. A spread market might be -110/-110 with a 4.8% overround. The same operator might price a player points prop at -120/-105 with a 6.5% overround. The wider vig reflects model uncertainty and lower liquidity.
A Simple Devig Method You Can Run on Mobile
Devigging converts a two-way market with overround into a fair no-vig probability. The maths is straightforward: take each side’s implied probability, divide by the sum of the two implied probabilities. The result for each side is its no-vig implied probability.
Example: a -120/-105 player points market. The over at -120 implies 54.5%. The under at -105 implies 51.2%. The sum is 105.7%. Divide each: over’s no-vig probability is 54.5 / 105.7 = 51.6%. Under’s no-vig probability is 51.2 / 105.7 = 48.4%. The fair odds on the over are about +94 (decimal 1.94). The book is taking the over at -120 — meaning anyone who bets the over at -120 needs an edge of about 6 percentage points above the fair price to break even.
The devig calculation is the foundation of value identification. If your own projection says the over should be 56%, and the no-vig fair price is 51.6%, the bettor’s edge is the 4.4-percentage-point gap — but only after the bettor has paid the vig. The bettor’s edge above the fair price is what matters, not the raw difference from the listed price.
Comparing No-Vig Prices Across UK Books
Different operators price the same prop differently. The Dimers analyst team has noted there is virtually always value to be found across the prop menu, and a meaningful fraction of that value comes from price discrepancies between operators rather than from the operators all being wrong in the same direction.
To compare books, devig each operator’s two-way price separately, then compare the no-vig fair probabilities. The operator whose no-vig implied probability is closest to your projection is the one offering the cleanest line — even if the headline price looks similar across books.
Example: Operator A prices a player points at -115/-115, implying 53.5% / 53.5%, no-vig 50% / 50%. Operator B prices the same prop at -125/-110, implying 55.6% / 52.4%, no-vig 51.5% / 48.5%. If your projection says the over is 53%, Operator A’s no-vig 50% on the over is the cleaner side. You bet the over at Operator A despite its slightly worse headline price because the underlying value is in the more honest line.
For the broader strategic frame on how to integrate these calculations into a working bet selection process, see my walkthrough of prop bankroll management.
The Expected-Value Threshold You Need
OddsIndex’s strategy guide recommends bankroll discipline at 0.5-1% of bankroll per prop. That sizing implies a tight EV threshold — you cannot afford to bet thin edges if your unit is small relative to your bankroll. Conventional analysis says the minimum EV threshold for a prop bet should be 2-3%, meaning the bettor expects to win 2-3 pence per pound staked in the long run.
To get a 2% EV at standard -110 pricing, the bettor’s true win probability must be at least 53.5% (vs the fair 50% on a no-vig basis). That is a 3.5-percentage-point edge above no-vig fair — substantial, but achievable with disciplined model work on specific matchups.
Below the 2% EV threshold, the bet still has positive expected value, but the variance can swallow the edge over hundreds of bets. Above the 5% EV threshold, the bettor either has a real model edge or has misread the line. I treat 2-5% EV as the working band — wide enough to bet, narrow enough to require honest scepticism about whether the edge is real.
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Prepared by the HoopMargin editorial staff.
