Why Betting Lines Move

Online sportsbooks make their profits through the juice or vig. The vig is a small fee online sportsbook take on bets to cover operating costs and generate a profit. Of course, the vig is (theoretically) only profitable if there is an even amount of bets on each side of the wager. However, it is rare that an even amount of wagers are on both sides. Therefore, to avoid taking a significant loss on a game, online sportsbooks adjust their odds and lines.

How are Opening Lines Determined?

When lines open on games, sportsbooks use a multitude of different factors to determine the moneylines, point spreads and totals. Below are some of the main factors that go into setting opening lines on upcoming games.

Main Factors in Determining Opening Lines

  • Record of each team
  • Which team is at home
  • Weather forecast (often affects totals more than point spreads)
  • Injuries
  • Recent head-to-head record
  • Perceived quality of teams

*Lines on futures (such as the winner of the Super Bowl, a golf tournament or March Madness) can include more aspects, such as the strength of schedule and the quality of the field. 

Reasons Betting Lines Move

Opening lines do not mean final lines. Between the time the game opens for wagering and the time the game starts, lines frequently move. As mentioned earlier, the amount of money bet on each side is a significant factor in lines moving. However, it is not the only reason why betting lines move.

Imbalance in Bets

If after lines open, bettors jump massively on one side of the action, then online sportsbook will adjust their lines. For example, say the Golden State Warriors are a 3.5-point (-110 odds) underdog on the road against the Houston Rockets. After the lines open, $100000 of bets are made on the point spread, with a majority of bets on the Golden State Warriors (65-35 split).

If Golden State wins or losses by three points or less, the sportsbook pays out $59090. Since they only took in $35000 in bets on Houston, the sportsbook loses $24090. The sportsbook adjusts the spread to encourage bettors to wager on Houston, to avoid losing such a large amount of money.

The spread may move to -2.5 Houston. If the movement in the point spread is not enough to balance out the wagers, the online sportsbook may move the spread more, until they are satisfied with the number of bets on each side.

Adjusting the Odds, not the Line

In some scenarios, an online sportsbook may change the odds instead of the point spread or total. If in the example above, $58000 in bets are on Golden State and the other $42000 are on the Houston Rockets, the sportsbook may move Houston to +100 odds and Golden State to -120 odds, instead of changing the point spread.

Injuries, Roster Transactions and Coaching Decisions

Even if there is a relatively even amount of bets on both sides, betting lines can still move. The most common reason lines move when there is a balance in wagers is due to injuries, roster transactions, player suspensions and coaching decisions (i.e. resting a player).  

For example, say the Green Bay Packers are playing the Chicago Bears. The Green Bay Packers open as a 7.5-point favourite on the road. However, just a few days before the game, Packers' quarterback Aaron Rodgers is ruled out with an injury. Betting the Bears as a +7.5 point underdog at home is suddenly a much more attractive wager.

The online sportsbook may briefly close the game and adjust the odds to avoid a massive amount of wagers on the Bears. The odds will re-open, reflecting the strength of the Packers without Aaron Rodgers compared to the Bears at home. Depending on the quality of the player will dictate how much the odds change. A superstar quarterback like Aaron Rodgers will change the odds much more than if the Packers were without their starting left guard. 

Following a Sharp Sportsbook

Lines move first on sharp sportsbooks. If you are using a square online sportsbook, they will often adjust their lines according to what a sharp sportsbook is doing. While the bets the square sportsbook receive on a game could be more balanced than the sharp, leaving the line unchanged opens them up for line shoppers to flood one side of the action.

If you are quick to notice a sharp sportsbook change their odds, it is a smart strategy to bet the opposite on the square before they have a chance to adjust their lines and odds. 

Live Betting Line Movement

When live betting on a game, you should expect lines to change consistently. However, the amount of money bet on either side of the action has less to do with line movement than what is transpiring in the game. If a team goes up a quick 14 in the first quarter, the spread and moneyline will reflect it and the probability of each team winning the game.

It is harder for sharps and the public to flood one side of the action when live betting as the lines move much quicker. The only times it is usually possible for an imbalance in bets to sway the odds is during an intermission or at half-time. 

Frequently Asked Questions

Have more questions or looking for more information? If so, check below for some answer to frequently asked questions sports bettors have on betting line movement.

How much do Lines Move?

Who Influences Line Movement?

Do Sharp Bettors ever Manipulate Betting Lines?

Can you Win Betting on a Line Before and After it Moves?

If you have not heard of Black Sunday, it is the perfect example to explain how it is possible to win betting on lines before and after they adjust.

Black Sunday is the name given to 1979 Super Bowl between the Pittsburgh Steelers and Dallas Cowboys. The Steelers opened the game as a 3.5-point favourite. The majority of early bets were on the Steelers, causing Vegas to adjust the line to 4.5 points for the Steelers. After the line moved, most bets were on the Cowboys. The Steelers won the 35-31 and sportsbooks lost big time.

This type of loss was a much bigger deal 40 years ago. There were not nearly the same number of betting options available in the 1970s, and sportsbook could not recover losses from other bets such as Super Bowl MVP and even parlays.

Nowadays sportsbooks are smarter and avoid these types of mistakes. It can still happen from time to time, but the overall impact of such a foley (called "middled" in the industry) is mitigated through profits on other bets, parlays and live betting.[/faq