Quick answer: An underdog (often called a “dog” or “longshot” depending on the magnitude of underdog status) is the team or player the sportsbook considers less likely to win. Underdogs are listed with positive American odds: +150, +220, +400, +800, etc. The number represents what you’d win on a $100 bet. A +200 underdog returns $200 profit on $100 risked. Underdog status doesn’t mean the team can’t win; it means the implied probability is below 50%. The 2007 Giants beat the 18-0 Patriots as +12-point underdogs. The 2008 Cardinals nearly won the Super Bowl as +10-point underdogs.
How Underdog Pricing Works
American odds for underdogs convert to implied probability via: 100 / (odds + 100). So +200 = 100/300 = 33.3%. +400 = 100/500 = 20%. +1000 = 100/1100 = 9.1%. The implied probability is what the book thinks the underdog’s chance is, including vig. To get the true (no-vig) probability, you divide each side’s implied probability by the total of both sides. So if the favorite is -250 (71.4% implied) and the dog is +200 (33.3% implied), the total implied is 104.7% and the true probabilities are roughly 68.2% favorite vs 31.8% underdog.
When Underdogs Are Good Bets
Three scenarios. First: matchup-driven mispricings where the public has overreacted to recent results. A team that lost 4 straight by lopsided margins gets priced as a longer underdog than the matchup math supports. Second: weather and injury news that shifts the projected score. A favored team’s QB getting downgraded mid-week can flip the moneyline math even if the line moves slowly. Third: contrarian sentiment around primetime games where public money piles on the favorite. PropsBot.AI’s MLB calibrated model produces independent probability estimates that often identify these underdog opportunities. The High ROI Signal at 31.7% verified ROI on 101,881 graded props frequently includes underdog bets where the matchup math says the implied probability undersells the true chance.
Why Public Bettors Avoid Underdogs
Three reasons. First: confirmation bias. Public bettors back recent winners (typically favorites). Second: social proof. Cheering for an underdog when no one else is feels uncomfortable; cheering for the popular favorite feels validated. Third: the perceived “sure thing” of favorites at -200 odds. The math doesn’t support that perception (-200 favorites win 67% of the time, which means they lose 33% of the time, which is enough variance to break a casual bettor’s bankroll). Sharp bettors take underdogs more often than public bettors because the math tends to favor the unpopular side over time, especially when matchup factors compound.
The Sharp Strategy on Underdogs
Three principles. First: only bet underdogs when the implied probability is meaningfully below your model’s projection. A +400 underdog with model probability of 30% has 5% edge over implied 20%; that’s worth betting. Second: bet larger on +200 to +400 underdogs and smaller on +800+ longshots. The variance is enormous on extreme longshots, and even +EV bets in that range produce wild bankroll swings. Third: combine with line shopping. The same underdog might be +220 at one book and +250 at another; the difference is huge over a long sample. PropsBot’s multi-book odds comparison surfaces these gaps automatically.
Notable Underdog Wins in History
The 2007 Giants beating the 18-0 Patriots as +12-point underdogs in Super Bowl XLII. The 2016 Cubs winning the World Series as +200 underdogs in Game 7. The 1985 Villanova Wildcats winning the NCAA Championship as +9.5-point underdogs. Buster Douglas knocking out Mike Tyson at +4200 odds. Leicester City winning the 2015-16 Premier League as +5000 preseason longshots. Each of these underdog wins paid the sharps who took the unpopular side. The casuals who hammered favorites at short odds lost the easy money the lines implied.
Frequently Asked Questions
What is an underdog in sports betting?
The team or player the sportsbook considers less likely to win. Listed with positive American odds (+150, +220, +400, etc.) where the number represents what you’d win on a $100 bet.
What’s the difference between an underdog and a longshot?
Underdog typically refers to anyone with positive odds (+100 to +500). Longshot usually means heavy underdog with odds longer than +500. The terms overlap; “longshot” implies extreme underdog status.
How do I calculate underdog implied probability?
100 / (odds + 100). So +200 = 100/300 = 33.3%. +400 = 100/500 = 20%. +1000 = 100/1100 = 9.1%.
Are underdogs good bets long-term?
With matchup awareness, yes. The math doesn’t favor blindly betting underdogs (they lose more often than favorites by definition), but identifying mispriced underdogs where the matchup math diverges from the implied probability produces edge.
What’s the biggest underdog win in NFL history?
The 2007 Giants beating the 18-0 Patriots as +12-point underdogs in Super Bowl XLII. Closing odds had the Patriots at -12, implying ~75% probability of victory. The Giants won 17-14 in one of the most famous upsets in sports history.
How does PropsBot identify underdog value?
Through calibrated probability modeling. The High ROI Signal at 31.7% verified ROI on 101,881 graded MLB props uses Brier-score calibration (0.1903 vs Vegas 0.1947) to find bets where the model’s probability exceeds the no-vig implied probability. Underdog bets are frequent contributors when matchup math diverges from public sentiment.
Part of the PropsBot.AI Sports Betting Glossary. Updated 2026-05-04.
