How a spread becomes a probability
A spread is a forecast of the final margin. Real margins scatter around it in a bell curve, and the favorite wins whenever the margin lands above zero. The win probability is the share of that curve above zero.
How wide the curve is decides everything. For the NFL this page uses 13.1 points, the margin spread in the model behind Rivory’s NFL games, fitted on NFL results from 2000 to 2025. For the NBA it uses 12.5 points, the starting value in Rivory’s NBA model. That one is not fitted to results yet, so read NBA numbers as a rough guide.
Example. A 7-point NFL favorite: 7 ÷ 13.1 is about 0.53 widths of the curve, and 70.3% of a bell curve sits below that point. So the favorite wins about 70.3% of the time, and the underdog about 29.7%.
Quick reference
How often the favorite wins at common spreads, from the same curves as the calculator.
| Spread (points) | NFL favorite | NBA favorite |
|---|---|---|
| 1 | 53.0% | 53.2% |
| 1.5 | 54.5% | 54.8% |
| 2 | 56.1% | 56.4% |
| 2.5 | 57.6% | 57.9% |
| 3 | 59.0% | 59.5% |
| 3.5 | 60.5% | 61.0% |
| 4 | 62.0% | 62.6% |
| 4.5 | 63.4% | 64.1% |
| 5 | 64.8% | 65.5% |
| 5.5 | 66.2% | 67.0% |
| 6 | 67.6% | 68.4% |
| 6.5 | 69.0% | 69.8% |
| 7 | 70.3% | 71.2% |
| 7.5 | 71.6% | 72.6% |
| 8 | 72.9% | 73.9% |
| 9 | 75.3% | 76.4% |
| 10 | 77.7% | 78.8% |
| 11 | 79.9% | 81.1% |
| 12 | 82.0% | 83.1% |
| 13 | 83.9% | 85.1% |
| 14 | 85.7% | 86.9% |
What the number leaves out
- Ties. About 0.3% of NFL games end level. This page counts them as someone’s win, the same way Rivory’s NFL model does.
- News after the line was set. The calculator reads only the spread you type, so an injury or a weather change counts only if the spread already reflects it.
- Certainty. A 59.0% favorite still loses about 41.0% of the time. It is a forecast, not a promise.
Put a number on it in Probability Picks
In Rivory’s NFL Probability Picks you set a whole percentage from 1 to 99 on each game instead of just picking a side. Confident right calls earn more and confident wrong calls lose more. A 50% call scores zero either way. The line under the calculator shows what your number would score.
You play it with your crew, and every game keeps its own standings. Nothing on Rivory is a bet.