Replacement
Batter
Runs an average hitter is worth over a replacement one, across his plate appearances.
Pitcher
Runs an average pitcher is worth over a replacement one, across his innings.
Formula
Hitters: his plate appearances × the amount per plate appearance that makes all hitters total 570 wins × the season’s share of a full schedule. Pitchers: (0.12 × starts share + 0.03 × relief share) wins per 9 innings × his innings / 9, plus a shift per inning so all pitchers total 430 wins × that share, in runs.
Our own model
Not checked against Savant. A season is written only when every check on the aWAR page passes.
Notes
The 570 and 430 split is FanGraphs’ convention, kept as an assumption. 2020’s share is 60 / 162.
Era caveats
aWAR begins in 2016
Savant’s fielding leaderboard begins in 2016, so 2015 has no aWAR and no part of it.