Implied Probability Calculator

A price is a probability wearing a disguise. Converting it back is the single most useful calculation in betting, because until you have a percentage there is nothing to compare your own opinion against.

The first calculator turns a price into a probability and a break-even win rate. The second runs it backwards: give it a probability and it returns the price that opinion is worth.

The Calculation

Implied probability is one divided by the decimal price. Everything else is bookkeeping.

At -110 the decimal price is 1.9091, so the implied probability is 1 ÷ 1.9091 = 52.38%. At +250 the decimal is 3.50, so it is 1 ÷ 3.50 = 28.57%.

Straight from American odds, without the detour: for a negative price, probability = |odds| ÷ (|odds| + 100). For a positive price, probability = 100 ÷ (odds + 100). So -150 gives 150 ÷ 250 = 60%, and +150 gives 100 ÷ 250 = 40%.

Running it backwards, a fair price is 1 ÷ probability in decimal. A 58% shot is worth 1 ÷ 0.58 = 1.7241, which is -138 in American terms.

Implied Probability Is Also Your Break-Even Rate

The same percentage answers a second question: how often you have to be right to stop losing money.

At -110, 52.38% is both what the price implies and the win rate you need. Win 52 of 100 bets at -110 with 100 units at stake each time and you finish slightly down; win 53 and you are slightly up. The gap between 50% and 52.38% is the whole business model of a sportsbook.

Rewriting it as a record makes it concrete. "You need 52.38%" is abstract. "You need 52-48 across a hundred bets, every hundred bets, forever" is the actual requirement, and it is much harder than it sounds.

Why Both Sides Add Up To More Than 100%

Add the two sides of any market and the implied probabilities total more than one. That surplus is the book's margin, and ignoring it is how bettors talk themselves into bets that were never live.

A -110 pair implies 52.38% twice, or 104.76% in total. The extra 4.76% is the overround; the book's actual hold on equal action is 4.55%. Split it out proportionally and each side's fair probability is 52.38 ÷ 104.76 = 50%, exactly as it should be for a coin flip.

This matters whenever the two sides are unequal. Take -140 on one side and +120 on the other: raw implied probabilities of 58.33% and 45.45%, totalling 103.79%. The favourite's fair probability is 58.33 ÷ 103.79 = 56.20%, so the fair price is about -128 rather than -140. If your own estimate is 57%, you have a bet at -140 only by a hair, and no bet at all if you were rounding.

The vig calculator does this across two-way and three-way markets, and closing line value uses the same de-vigging step to judge a bet after the fact.

What To Do With The Number

Once a price is a percentage, three checks become possible, and all three are quick.

Compare it to your own estimate. A bet exists when your probability is higher than the fair implied probability by more than your own margin of error. If your process is accurate to within three points, a two-point disagreement is not an edge, it is noise. The expected value guide covers the discipline this requires.

Compare it across books. -105 and -115 on the same side are 51.22% and 53.49% — more than two full points apart on an identical opinion. Over a season that gap is larger than most people's edge, which is why line shopping outranks handicapping for most bettors.

Compare it to the close. If the price you took implies less than the market's eventual fair number, you were ahead of the market, whether or not the bet won. That is the most reliable short-run signal available, and it is what closing line value measures.

Frequently Asked Questions