How Often Do Underdogs Win After Odds Drop 20%?

If an underdog's odds drop by roughly 20% before kickoff (for example, from decimal 5.00 to 4.00, or from +400 to +300 in American odds), the underdog's implied probability of winning rises from about 20% to about 25%—and the empirical evidence suggests that, on average, the closing price is right. Underdogs in that scenario go on to win at approximately the rate their closing odds imply, not their opening odds.

Put another way: the market has revalued them for a reason. The move is informative. But "informative" is not "guaranteed." A 25%-implied underdog still loses roughly three times out of four. The value was in getting the price before the drop— not in betting after it.

The rest of this article explains why, with real data.

What "Odds Drop 20%" Actually Means

Before anything else, the math. Betting odds are a price, and a "20% drop" needs a clear reference point.

In decimal odds, a 20% drop means the price paid on a winning bet falls by one-fifth. A $100 bet at 5.00 returns $500 ($400 profit). At 4.00, the same bet returns $400 ($300 profit). The implied probability of winning shifts from 1/5.00 = 20% to 1/4.00 = 25%—a five-percentage-point jump that looks small but is actually enormous in betting-market terms.

In American odds, the same move looks like +400 shortening to +300. At +400, a $100 bet wins $400. At +300, it only wins $300. The "price" of backing the underdog has gotten worse because the market now believes that underdog iitikely to win.

This is the scenario the question is asking about: a team that was a long shot has become less of a long shot, and the market is telling you something with its wallet.

Why Odds Drop: Three Real Causes

Odds don't shorten randomly. In well-studied markets (NFL sides, Premier League moneylines, ATP singles, MLB), there are essentially three reasons a previously long-shot underdog sees its price collapse.

First—and most importantly— is sharp money. Professional bettors and syndicates move markets by placing large wagers at sharp sportsbooks. When Pinnacle Sportsbook, CRIS, or Circa—the books that accept high limits and welcome winning players—take respected action on an underdog, the line moves toward those books first. Other sportsbooks follow within minutes. This cascade is what bettors call a steam move. Sports Insights has published data showing that a bettor who had followed every Pinnacle MLB moneyline steam move since 2007, staking $100 per game, would have earned over $30,000 in net profit over roughly a decade — a small but real edge derived purely from line-movement signals.

Second is new information — usually injuries, sometimes weather or lineups. In December 2025, when Micah Parsons tore his ACL in Week 15, the Green Bay Packers went from 3-point favorites against the Chicago Bears to 1.5-point underdogs — effectively a 4.5-point swing. The same week, when Patrick Mahomes suffered an ACL injury, the Kansas City Chiefs dropped from 10.5-point favorites against the Titans to roughly 3-point favorites, a 7.5-point line move almost entirely driven by the quarterback news. Injury-driven moves like these are why sharp bettors obsess over beat-reporter Twitter/X feeds in the hour before kickoff.

Third is reverse line movement (RLM). This is the counterintuitive case: the majority of the bets are on one side (say, 75% of tickets on the favorite), but the line moves toward the other side. Sportsbooks are telling you, through their pricing, that the dollar volume on the minority side—usually from respected accounts—is heavy enough to outweigh the ticket count for recreational players.

Each of these causes carries different predictive weight. Sharp money and injury news are genuinely informative. Public-driven moves (everybody piling on a popular favorite) are noisier signals and sometimes even counter-productive.

What the Research Actually Shows

Most serious work on this question uses the closing line value (CLV)—the difference between the odds you took and the final odds just before kickoff in a sharp book— as its central metric. Marco Blume, former Trading Director at Pinnacle, has publicly stated that Pinnacle's closing line is, on average, extremely accurate — that sharp bettors beat it, and his traders work to make it as efficient as possible. This is why academic and industry researchers consider it the most reliable estimate of a match's "true" probability.

Joseph Buchdahl, a well-known betting analyst, analyzed a sample of 162,672 soccer-match opening and closing odds from Pinnacle. The finding: 35.7% of opening home-and-away odds held a profitable theoretical advantage over their "fair" closing prices, and the margin by which a bettor beats the closing line is a reliable predictor of long-term profitability. In plain English: when the closing price is lower than where the market opened on an underdog, that underdog is, on average, a better bet at the opening price than it was at the closing one—but the closing price itself is approximately right about the actual win rate.

This is the critical nuance most bettors miss. The question "How often do underdogs win after odds drop 20%?" has two completely unique answers depending on when you asked.

If you asked at the opening price (before the drop), the underdog wins more often than the opening odds implied—roughly at the rate the closing odds eventually settle on.

If you asked at the closing price (after the drop), the underdog wins at roughly the rate the closing odds imply. The market has already priced in whatever information caused the drop. The edge is gone.

This is why professional bettors care far more about "getting the better of the number"—betting before the move—than about chasing steam after it happens.

Real Match Evidence

Case 1: Chicago White Sox vs. Houston Astros (MLB), documented by Action Network. The White Sox opened as a +145 underdog. Houston was drawing roughly 60% of the betting action. But in the hours leading up to the first pitch, the Chicago line shortened sharply—from +145 down to +130— while the Astros lengthened. Pinnacle moved first; other books followed. The White Sox won the game 3–1. This scenario is textbook reverse line movement: public money on the favorite, sharp money on the dog, and the dog cashed. At +145, the implied probability of a White Sox win was about 40.8%; at +130, it was about 43.5%. The market's sharper estimate was the winning one.

Case 2: Leicester City, Premier League 2015–16 — the slow-motion steam move. At the start of the season, Leicester were priced at 5,000-to-1 to win the title (implied probability: 0.02%). By the October 2015 international break—after they sat fifth in the table with 15 points from eight games— their price had already been slashed to 1,500-to-1 (0.067%). By December, after they went top of the table, Leicester were 100-to-1 (~1%). By the time they finished the season as champions 10 points clear of Arsenal at 81 points, they had traced one of the largest multi-month price drops in major-sport history. UK bookmakers absorbed roughly £25 million in losses. The market's steady downward pressure on Leicester's odds reflected accumulating information — Jamie Vardy scoring in 11 straight matches, N'Golo Kanté's emergence, Riyad Mahrez's form — that the opening line had failed to capture. Every drop along the way was, in hindsight, justified.

Case 3: NFL Week 16, 2025 — the Patrick Mahomes injury move. When Mahomes was ruled out for the season with a torn ACL, the Kansas City Chiefs' line against the Tennessee Titans collapsed from -10.5 to -3 at DraftKings Sportsbook. In moneyline terms, Tennessee's implied win probability—as a significant home underdog—roughly doubled overnight. This is a classic case of information-driven drop: new, material information entered the market, the line promptly shifted across all sportsbooks, and the number settled at a level that accounted for the quarterback's absence. Bettors who held Titans tickets purchased before the injury news captured enormous CLV—regardless of the final score—because they had bought them at a price the market no longer believed was correct.

The Honest Bottom Line

If you put the three cases together and strip out the noise, a consistent pattern emerges.

When an underdog's odds drop by around 20%, the underlying win probability has genuinely shifted. Sharp money, injury news, or new information has revised the market's estimate. Over thousands of such games, the closing line is the most accurate single predictor of outcome that exists. Research on Pinnacle data across 162,672 matches supports this conclusion.

But three honest cautions follow.

First, a 25%-implied underdog is still an underdog. Even after a 20% odds drop, the team is expected to lose roughly three times out of four. If you chase steam at the new price, you are paying the revised price for the revised probability. That is fair, not profitable.

Second, chasing steam at your sportsbook is not the same as being on the sharp side. If the market-moving books have already adjusted and your book is lagging, you may have a genuine edge. If your book has already moved, you do not need to. This is why beating the closing line—not following it—is the metric professionals track.

Third, variance is brutal over small samples. Short-term luck overwhelms long-term edges measured in single-digit percentages. Bettors who see three steam-move underdogs lose in a week and conclude the signal is broken are making a sample-size error. Bettors who observe three cash and conclude they've discovered a cheat code are also making a sample-size error.

Practical Framework

If you want to use line movement in your decision-making, a disciplined approach works better than a reactive one.

First, record the opening line for any match you care about. Without such data, you can't tell whether an underdog's current price is a bargain or already reflects every piece of public information.

Second, identify why the line moved. An injury to a star player is a causal move — the new price is the right price. A wave of recreational Monday-night money on a popular favorite is a noise move — the resulting reverse line movement on the dog may represent value. Weather changes in outdoor sports (NFL, MLB) are causal. Social-media hype is noise.

Third, compare across sportsbooks. If one market-setting book has moved an underdog from +400 to +300 and your book still has +375, you may have a genuine edge. If every book has already moved in concert, the edge is priced in.

Fourth, track your closing line value, not your wins and losses. Over 100-plus bets, a positive CLV is a more reliable indicator of skill than a W-L record. Most of the professionals interviewed by industry outlets like Sports Insights and Pinnacle have said some version of the same thing: they pay more attention to how the line moves after their bet than to whether the bet wins.

The One-Sentence Takeaway

When an underdog's odds drop 20%, it wins at roughly the rate its new, shortened price implies—which means the move is real information, but the profit opportunity belongs almost entirely to the bettors who got the price before the drop, not the ones chasing it afterward.

The market is not always right for a single game. Over thousands of games, it is closer to right than almost anything else you can consult. Respecting that—without worshipping it—is the foundation of disciplined betting.