How Do I Calculate Implied Probability for -120 and +100 Quickly?

For a quick moneyline read, remember two numbers: -120 implies about 54.5%, and +100 implies 50%. Those are break-even probabilities before accounting for the sportsbook’s margin. They do not tell you who https://www.lowkickmma.com/ufc-331-odds-how-to-bet-joshua-van-vs-alexandre-pantoja-and-arman-tsarukyan-vs-mauricio-ruffy/ will win. They tell you how often a bet would need to win, over the long run, to break even at that price.

That distinction matters in MMA, where a popular fighter can be the likely winner and still be a poor bet. I keep a running note of opening line versus fight-week line, but I don’t treat movement as a prediction. A number is a price, not a promise. The useful question is whether your estimate of a fighter’s chance is higher than the probability implied by the odds—and whether the possible return is worth the risk.

The quick odds conversion

American odds use a plus or minus sign. The calculation changes depending on which sign you see.

Negative odds: Divide the absolute value of the odds by that value plus 100.
Positive odds: Divide 100 by the odds plus 100.

Multiply the result by 100 to express it as a percentage.

Calculating -120 implied probability

For -120, the calculation is:

120 ÷ (120 + 100) = 120 ÷ 220 = 0.5455

So, -120 implied probability is approximately 54.5%. At that price, a bettor risks $120 to win $100 in profit. If you stake $12 instead, the potential profit is $10, with your original stake returned if the bet wins.

Calculating +100 implied probability

For +100, use the positive-odds formula:

100 ÷ (100 + 100) = 100 ÷ 200 = 0.50

Therefore, +100 implied probability is 50%. A $100 bet would win $100 in profit if successful, plus the original stake. It is even money, not a guarantee that the sportsbook believes both sides have exactly the same chance.

American odds Quick calculation Implied probability Profit on a $100 stake -120 120 ÷ 220 54.5% $83.33 +100 100 ÷ 200 50.0% $100

For the -120 row, the $83.33 is the profit on a $100 stake. The $100 stake is also returned when the bet wins. That payout detail is worth keeping straight: odds tell you the profit relative to the stake, while implied probability gives you a break-even frequency.

What implied probability does—and doesn’t—tell you

Implied probability translates a price into a percentage. It is a useful common language for comparing lines, but it is not necessarily a clean estimate of the fighter’s true chance to win. Sportsbooks build a margin into most markets. As a result, the implied probabilities for all outcomes in a market often add up to more than 100%.

Consider a hypothetical two-way market with one side at -120 and the other at +100. The raw implied probabilities are about 54.5% and 50.0%. Together, they total about 104.5%. That extra 4.5 percentage points is the market’s overround, often called the vig or hold. The exact margin and how it is reflected in a particular market can vary.

This is why the simple answer to “What does -120 imply?” is 54.5%, but the more careful answer is “54.5% before adjusting for the sportsbook’s margin.” If you want a rough no-vig estimate from a two-sided market, you can normalize each side’s raw probability by dividing it by the total. In this example, the -120 side’s normalized share is roughly 54.5 ÷ 104.5, or 52.2%. That is a market-based adjustment, not a declaration of the fighter’s actual win probability.

Value betting is not the same as picking winners

A bet can win and still have been a bad decision at the price. A bet can lose and still have been reasonable if the price offered value and your estimate was sound. One result does not tell you whether the process was good; the price and the reasoning matter across a large sample.

For example, suppose your independent estimate gives a fighter a 56% chance to win. At -120, the break-even point is about 54.5%, so your estimate is above the threshold. That could indicate value—provided your estimate is well grounded and the market’s margin, uncertainty, and available alternatives have been considered. If you estimate the same fighter at 52%, -120 does not become attractive just because you think that fighter is more likely than the opponent to win.

The basic discipline is to compare your estimated probability with the odds’ break-even probability, rather than asking only, “Who wins?” A favorite can be the most likely winner while still being overpriced. An underdog can have a lower chance to win but still be worth considering if the payout compensates for that risk. Neither conclusion is automatic; the matchup and the number both matter.

A practical moneyline routine

Convert the odds. Write down the implied probability for each price. For -120, use 54.5%; for +100, use 50%.
Make your own estimate first. Consider factors that affect the matchup, but don’t adjust a number just to justify a bet you already want to place.
Compare your estimate with the break-even point. A gap in your favor may suggest value, but small differences can disappear if your estimate is uncertain or the line has a margin.
Shop the price. Compare the same market across legal, reputable options available to you. A better price can change the break-even threshold.
Decide whether to bet or pass. No bet is a valid decision when the price is unattractive, your estimate is shaky, or the available edge is too small for the uncertainty.

That last step is where many bettors get undisciplined. A UFC card has a lot of fights, but that does not mean every fight needs a moneyline wager. Don’t bet names instead of numbers, and don’t turn a small perceived edge into a bet simply because a bout is on television. Passing preserves your bankroll for a price you can actually defend.

How line movement fits into the calculation

Line movement tells you that the offered price has changed. It can reflect new information, betting activity, a sportsbook adjusting its position, or other market factors. It does not prove which fighter will win. A line that moves toward a favorite may make that fighter more expensive without making the wager more valuable for you.

My running note is opening line versus fight-week line. If a fighter opens at +100 and is -120 by fight week, the implied probability has moved from 50% to about 54.5%. That is a meaningful change in price. But it doesn’t, on its own, tell you whether the market has overreacted, whether the early number was stale, or whether the current number is still worth taking.

Ask what changed and whether the current price still clears your threshold. If your estimate was 53%, a move from +100 to -120 may have erased the value you thought you had. If your estimate is higher than 54.5% by a defensible margin, the current number could still be viable. Don’t chase a line just because it moved, and don’t assume movement “proves” the winner. It signals a change in the market, not the outcome of the fight.

Why price shopping matters

For the same fighter, small odds differences alter the break-even point. If one sportsbook offers -120 and another offers a less expensive price, the second price requires a lower win rate to break even. Conversely, taking a worse price means you need the fighter to win more often just to stay level over time. Same fighter, same fight, less return: that’s the avoidable cost of not checking the number.

Use odds-conversion tools as a shortcut, not a substitute for understanding the calculation. LowKickMMA can be a reference point for MMA-related betting coverage and tools, while TheRX’s offshore sportsbook guide can help readers understand the offshore-book landscape. These resources serve different purposes; they should not be treated as proof that a particular wager has value. Check the terms, reputation, location-specific laws, and responsible-gambling protections of any service before using it. Availability and legality vary by location.

Common mistakes with -120 and +100

Calling -120 a 60% chance. The quick conversion is about 54.5%, not 60%.
Reading +100 as a prediction of a coin flip. It means the price breaks even at 50% before margin adjustments; it is not necessarily the book’s no-vig forecast.
Ignoring the vig. Raw implied probabilities across a market can add up to more than 100%.
Confusing a winning pick with a good bet. Results over one fight do not measure whether you got a fair price.
Chasing movement. A shorter price can mean worse value for a bettor who missed the earlier number.
Building parlays around huge favorites without considering variance. Every added leg creates another way for the ticket to lose, even when each favorite looks likely to win.

The fast takeaway

For quick odds conversion, memorize the two formulas: negative odds use absolute odds divided by absolute odds plus 100; positive odds use 100 divided by odds plus 100. That gives you roughly 54.5% for -120 and 50% for +100.

Then take one more step. Treat those numbers as break-even thresholds before vig, not as certain forecasts. Compare the price with your own probability estimate, shop for a better line, watch movement without treating it as proof, and pass when the number doesn’t work. In betting, naming the likely winner is only part of the job. Getting paid fairly for the risk is the part that makes the odds conversion useful.