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

Quick answer: -120 implies a 54.55% break-even probability, while +100 implies a 50% break-even probability, before Get more information accounting for any other costs or uncertainty. Those figures are odds conversions—not predictions that either fighter will win.

That distinction matters in MMA. A fighter can be the likelier winner and still be a poor bet at the available price. The useful question isn’t just “Who wins?” It’s “Does the price offer enough return for the chance I think this fighter has?” That’s the difference between picking winners and value betting.

The Fast Odds Conversion

American odds use a plus sign for underdogs and a minus sign for favorites. To turn them into implied probability, use the appropriate formula:

• Negative odds: implied probability = absolute odds ÷ (absolute odds + 100)
• Positive odds: implied probability = 100 ÷ (odds + 100)

Calculating -120 Implied Probability

For -120, take the absolute value of the number and plug it into the negative-odds formula:

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

Multiply by 100 to get a percentage: -120 implied probability = 54.55%.

In practical terms, a bettor laying -120 needs to win about 54.55% of comparable bets to break even in the long run, assuming no ties or pushes and ignoring other factors. A -120 wager risks $120 to win $100. The stake is larger than the potential profit, which is why the break-even rate is above 50%.

Calculating +100 Implied Probability

For +100, use the positive-odds formula:

100 ÷ (100 + 100) = 100 ÷ 200 = 0.50

So +100 implied probability = 50%. A $100 bet at +100 wins $100 in profit if successful, plus the original stake is returned. If the bettor wins exactly half of a large set of equivalent bets, the wins and losses roughly cancel out before considering any other costs.

A Quick Reference Table

American odds Formula Implied probability Break-even interpretation -120 120 ÷ 220 54.55% Win about 54.55% to break even +100 100 ÷ 200 50.00% Win about 50% to break even

For a mental shortcut, remember that +100 is even money, so it represents a 50% break-even point. At -120, the sportsbook is asking you to risk $120 to make $100; the implied probability is a little above 50%, specifically 54.55%.

Implied Probability Is Not the Same as the Fighter’s True Chance

Odds conversion tells you the probability a price requires to break even. It does not establish the fighter’s actual chance of winning. A line is a market price shaped by sportsbook margins, betting action, limits, information, and the book’s own risk management. It is useful information, not a verdict.

Suppose your careful estimate gives a fighter a 58% chance to win. At -120, the break-even point is 54.55%, so your estimate is above the price’s threshold. That could indicate value, provided your estimate is sound and the offered line is available to you. But if you estimate the fighter at 52%, the same -120 price does not clear your break-even bar—even if you think that fighter is more likely to win than the opponent.

This is where bettors get tripped Find more information up: “I think this fighter wins” is not a complete betting case. The price has to compensate you for the risk. A favorite can win often and still be overpriced. An underdog can lose most of the time and still be worth considering if the payout is generous relative to the underdog’s real chance.

Value Betting Versus Picking Winners

Value betting is about comparing your estimated probability to the probability implied by the odds. In a simplified example, if you estimate a fighter’s win probability at 58% and the line is -120, compare 58% with the 54.55% break-even figure. Your estimate is higher, which may suggest positive expected value. If your estimate is only 53%, the apparent favorite is still the more likely winner, but the price doesn’t meet your threshold.

That comparison depends on the quality of your estimate. A decimal in a spreadsheet doesn’t make an opinion accurate. In MMA, uncertainty can be especially high: a short-notice replacement, a change in weight class, a takedown-heavy matchup, or an unusual fight pace can all make a probability estimate less reliable. Treat your number as a range where appropriate, not a magically precise forecast.

A disciplined bettor can also pass. If your reasonable estimate overlaps the break-even point, or you do not have enough information to price the matchup confidently, there may be no wager worth making. Passing is not a failure to pick a side. It is a decision not to pay a price you cannot justify.

Vig: Why the Two Sides Can Add Up to More Than 100%

Sportsbooks generally build a margin into the odds. That margin is often called the vig, or juice. You can see the effect by converting both sides of a market into implied probabilities and adding them together.

Imagine one fighter is -120 and the opponent is +100. The implied probabilities are 54.55% and 50%, which total 104.55%. That extra 4.55 percentage points is the market’s overround: a simplified indication of the margin embedded in those prices.

The overround is not a direct promise of how much a sportsbook will earn on a particular fight. Betting may not be balanced, and actual pricing can vary. But it explains why adding the two displayed probabilities often produces a total above 100%. The market prices are not automatically clean estimates of each fighter’s true chance.

When comparing lines, keep the vig in view. One side may look appealing at a glance, but a better price elsewhere can improve your expected return without changing your opinion of the matchup. Same fighter, same fight, less return—that’s what you accept when you take a worse number than another available book.

Line Movement: A Signal, Not a Winner’s Receipt

Line movement tells you that the market price has changed. It can reflect betting action, new information, a sportsbook adjusting its exposure, or a move by another book that the first book chooses to follow. It does not prove the side attracting action will win. Nor does a line move, by itself, reveal why the market moved.

Keep a simple record of the opening line and the fight-week line. For example, note whether the moneyline opened at -110 and later reached -120, or moved the other way. Then compare the current price with the number you would be willing to bet—not just with the line’s direction. If the price has moved past your estimated value point, a fighter you liked earlier may no longer be a bet at the current odds.

A move can still be useful context. It may prompt you to recheck a weigh-in, injury report, opponent change, or matchup assumption. But don’t treat movement as proof that sharp bettors know the outcome. Markets can react to incomplete information, and even well-informed bets do not erase the variance of a fight.

Moneyline Discipline: When to Bet and When to Pass

A moneyline bet is a wager on the winner, but the odds determine how much you stand to gain or lose. Before placing one, make the price do some work for you. A practical routine is:

1. Convert the odds. Work out the break-even probability for the line in front of you.
2. Form an independent estimate. Consider the matchup, recent form, opponent quality, likely fight paths, and any relevant late news.
3. Compare estimate to price. Ask whether your estimated win chance clears the break-even figure by enough to justify the uncertainty.
4. Shop the number. Compare multiple legal, reputable options where available. A small odds difference can matter over repeated bets.
5. Pass when the edge is unclear. Don’t force action just because the event is on television or a favorite feels safer.

Be especially cautious about parlays built around large favorites. A string of “likely” outcomes is not a guarantee, and every added leg creates another way for the ticket to lose. The payout can look attractive while hiding the combined variance. Evaluate each price on its own before deciding whether a multi-leg bet makes sense.

Tools and Resources for MMA Bettors

For MMA context, LowKickMMA can be one reference point for following fight coverage and fighter news. Use reporting as a starting point, then check the source and timing of any detail that could affect your estimate. A headline is not a probability model, and news alone does not tell you whether a moneyline is good value.

TheRX’s offshore sportsbook guide is another resource bettors may consult when learning about sportsbook options and comparisons. Treat any sportsbook guide as informational rather than an automatic endorsement. Before using a sportsbook, check the laws where you live, the site’s current terms, withdrawal rules, and whether the operator is licensed or otherwise permitted in your jurisdiction. Availability and legal status can change.

The Bottom Line on -120 and +100

The fast odds conversion is straightforward:

• -120 implied probability: 54.55%
• +100 implied probability: 50%

Those are break-even thresholds based on the listed prices, not guarantees of what will happen in the cage. Strip out the vig where you can, make your own estimate, compare it with the price, and track the opening line against the fight-week number. If the price doesn’t offer a defensible edge—or you can’t assess the matchup with enough confidence—passing is a sound decision.

In betting, the goal is not to be right about every winner. It is to make decisions at prices that justify the risk over time, while accepting that any single fight can go sideways. The odds tell you what the wager costs. Your job is to decide whether the return is worth paying for.