About

Every set of betting odds encodes an implied probability. A bookmaker's line at −150 American odds means the market prices that outcome at roughly 60%. The problem: that number is inflated. Sportsbooks embed a margin (the "vig" or "overround") into every line, which means the sum of implied probabilities across all outcomes exceeds 100%. If you compare raw implied probability against your own model without stripping the vig, you systematically overestimate the market's true assessment of each outcome. This calculator converts American, Decimal, or Fractional odds into implied probability P, computes the total overround, isolates the vig margin, and derives the fair (no-vig) probability for each outcome. It handles markets with 2 to 10 outcomes. Note: implied probability is a market-derived figure. It reflects the combined effect of true likelihood, public money flow, and bookmaker risk management. It is not a prediction. Treat it as a pricing signal, not a forecast.

Formulas

Implied probability from American odds:

{

P = |odds||odds| + 100 if odds < 0P = 100odds + 100 if odds > 0

Implied probability from Decimal odds:

P = 1d

Implied probability from Fractional odds (a/b):

P = ba + b

Overround (total implied probability across all outcomes):

O = n∑i=1 P_i

Vig margin:

Vig = O − 1

True (no-vig) probability for outcome i:

P_i^true = P_iO

Where P = implied probability (as decimal, 0 to 1), d = decimal odds, a/b = fractional odds numerator/denominator, O = overround, n = number of outcomes.

Reference Data

American Odds	Decimal Odds	Fractional Odds	Implied Probability	Typical Market
−1000	1.10	1/10	90.91%	Heavy favorite, boxing mismatch
−500	1.20	1/5	83.33%	Strong favorite, top seed tennis
−300	1.33	1/3	75.00%	Clear favorite, home team NBA
−200	1.50	1/2	66.67%	Solid favorite, NFL spread
−150	1.67	2/3	60.00%	Moderate favorite
−110	1.91	10/11	52.38%	Standard vig line (spread/total)
+100	2.00	1/1	50.00%	Even money, coin flip
+110	2.10	11/10	47.62%	Slight underdog
+150	2.50	3/2	40.00%	Moderate underdog
+200	3.00	2/1	33.33%	Clear underdog
+300	4.00	3/1	25.00%	Long shot, MMA upset
+500	6.00	5/1	16.67%	Major underdog, horse racing
+1000	11.00	10/1	9.09%	Long shot, futures bet
+2000	21.00	20/1	4.76%	Deep longshot, league winner
+5000	51.00	50/1	1.96%	Extreme longshot, Leicester 2016
+10000	101.00	100/1	0.99%	Novelty / prop bet territory

Frequently Asked Questions

Bookmakers build a profit margin (vig or juice) into their odds. In a two-outcome market with both sides at −110 American odds, each side implies 52.38%, totaling 104.76%. The 4.76% excess is the overround - the bookmaker's theoretical edge. This calculator strips that margin to reveal the true probabilities the market implies for each outcome.

Estimate your own probability for an outcome using a model or domain expertise. If your estimated probability is higher than the true (no-vig) implied probability from this calculator, you have identified a positive expected value (+EV) situation. For example, if the no-vig probability is 40% but your model says 48%, the edge is 8 percentage points. Consistent +EV betting is the only mathematically sustainable long-term strategy.

Implied probability is the raw conversion of odds to a percentage. It includes the bookmaker's margin. True (or fair) probability is the implied probability after removing the vig by dividing each outcome's implied probability by the total overround. True probability across all outcomes sums to exactly 100%. Use true probability for model comparison; use raw implied probability to understand what the line is actually charging you.

No. American odds of 0 are undefined. Values between −100 and +100 (exclusive) do not exist in the standard American odds format. Negative odds must be −100 or lower (e.g., −110, −200). Positive odds must be +100 or higher. This calculator validates input and rejects invalid ranges. If you encounter such odds on a platform, they are likely using a non-standard format.

All three formats (American, Decimal, Fractional) encode the same underlying probability - the format is a presentation choice, not a precision choice. However, fractional odds can introduce rounding artifacts for irrational probabilities (e.g., 1/3 = 33.33...%). Decimal odds at two decimal places can represent probabilities in 0.01 increments at best. American odds lose granularity at extreme values. This calculator carries full floating-point precision internally and rounds only for display.

Standard two-way markets (NFL spreads, NBA totals) typically carry 4-5% overround (both sides at −110). Sharp books like Pinnacle operate at 2-3%. Recreational books on prop bets or parlays can embed 8-15% or more. Futures markets (season-long bets with 20+ outcomes) routinely carry 15-40% overround. Generally, anything above 10% in a two-way market signals poor value. This calculator makes the margin transparent so you can shop for the tightest lines.