Calculate how many points, runs or goals a team typically scores. For instance, if the Dallas Cowboys have scored 350 points in 10 games, then the average is 35 points. If their rival that week has scored 300 points over that same span, the average amount is 30.
Identify who the home team is for the game. Many times a team plays better when it is at home than when it is on the road. The factors such as fan noise and knowing the field or playing surface give an advantage to the home team. If a team generally performs well at home, meaning it has a good home record, then extra points, runs or goals should be counted in its favor.
Look at the record of the two teams playing the game. The record is one of the biggest indicators in determining the point spread. A team with a good record will generally be favored over a team with a bad record, and therefore the point spread will be larger. If the teams have similar records, then the point spread will be closer since they are in theory equally matched.
Look for extraneous factors for the teams playing. For instance, if the star athlete of one team is out, then it might stand to reason that the team will score fewer runs, points or goals. This should be factored in when calculating the point spread. For instance, if the star player averages 35 points per basketball game, and his replacement averages 20 when playing a full game, the team might score fewer points. Scoring fewer points would in theory cause the point spread to be tighter if the team is favored or larger if it is the underdog.
Calculate an average margin of victory or loss. For instance, if a team has won 10 games by a combined total of 30 runs in baseball, then it wins by an average of three runs per game. If that same team lost 10 games by 12 runs, then its average margin of loss is 1.2 runs per game. So if the team is favored, it might "give" three points to the opponent and if it is the underdog it might "take" 1 run per game (because you can't take 1.2 runs.)
Combine all of your research to calculate the point spread. There is no exact science when determining this figure. However, use the research to get the figure. For instance, in basketball if the favorite averages 100 points, with a victory margin of five points, and is playing at home, then make it a five-point favorite. Conversely, if a team has an inferior record, is playing on the road, scores 95 points and loses by three points, make it a three-point underdog.