The formula used to calculate RPI is:
A unique feature of this RPI is that winning percentage is weighted based on the caliber of opponent. This approach to calculating winning percentage gives teams credit for beating better opponents, while not punishing a team for losing to the same strong opponent.
The standard way to calculate wins would be to count all wins and losses the same and dividing the number of wins by the total number of games played. However, a win over a top-5 team should give more credit than beating an average or below average team.
Wins and losses are adjusted using this formula:
\begin{equation} \text{Adj. Win} = 1 + \frac{\text{Opponent RPI}}{2} \end{equation} \begin{equation} \text{Adj. Loss} = 1 - \frac{\text{Opponent RPI}}{2} \end{equation}The weighted winning percentage, then, is calculated as:
\begin{equation} \text{Win%} = \frac{\text{Adj. Wins}}{\text{Adj. Wins} + \text{Adj. Losses}} \end{equation}This weighted winning percentage is what is used in the RPI formula.
Weighted winning percentage is simply a way to capture a team's strength of record.
If a team goes 25-5 while facing mostly top teams in the state, that is a more impressive record than a team going 25-5 while facing lower caliber teams.
Last season, the top-RPI rated team in Alabama was Hunstville, with an RPI rating of 0.8562. Here is a team would get credited for facing them:
Out of state opponents are not included in the RPI calculation.