From a spread to a win probability
A point spread is an estimate of the final margin. Real margins scatter around it, and for NFL games that scatter is close to a normal distribution. Hal Stern measured its standard deviation at 13.86 points in 1991; across 7,017 games from 2000 to 2025 in the nflverse data it is 13.2 points. The probability that the favorite wins is the share of that curve above zero:
A 3.5-point favorite therefore wins about 60 percent of the time, a 7-point favorite about 70 percent and a 14-point favorite about 86 percent. The real record is close: favorites of 6.5 to 7.5 points won 74.3 percent of those games since 2000, favorites of 13.5 to 14.5 points 85.3 percent. The spread explainer draws the curve and shades the area.
From a probability to a moneyline
American odds express the same probability as a price on 100. Favorites carry a minus sign, the amount risked to win 100; underdogs a plus sign, the amount won on 100:
Reading the margin in two prices
Turn each quoted price back into a probability and add them. Anything above 100 percent is the margin built into the pair. Dividing each implied probability by the total removes it proportionally, which gives a no-margin estimate of each side. Proportional removal is the simplest method; others shift more of the margin onto the longer price.
These conversions sit underneath the point spread guide and the win probability calculator, which uses the pregame spread as its measure of team strength.
Frequently asked questions
How do you convert a point spread to a win probability?
Treat the final margin as normally distributed around the spread. With a standard deviation of about 13.2 points, a 3.5-point favorite wins with probability Φ(3.5 / 13.2), about 60 percent.
Where does the standard deviation come from?
Hal Stern measured 13.86 points in 1991 from NFL results against the spread. Across 7,017 games from 2000 to 2025 in the nflverse data, actual margins scatter around the closing spread with a standard deviation of 13.2 points. The converter defaults to 13.2 and lets you change it.
What is a fair moneyline?
The price with no margin built in. For a probability p above one half it is −100·p/(1−p); below one half it is +100·(1−p)/p. A 60 percent favorite is fair at −150.
What is the vig or overround?
When two quoted prices imply probabilities that add up to more than 100 percent, the excess is the margin. Two sides at −110 each imply 52.4 percent apiece, 104.8 percent in total, a margin of 4.8 percentage points.
Why is a 3-point spread different from a 3.5-point spread?
NFL margins cluster on 3 and 7 because of field goals and touchdowns. The normal curve smooths over that, so conversions near key numbers are approximations.