SLG = (s + 2d + 3t + 4hr)/ AB or SLG = (h + d + 2t + 3hr) / AB, where AB is the number of at-bats for a given player, and s, h, d, t, hr, are the number of singles, hits, doubles, triples, and home runs, respectively. The following site provides information on calculation, total bases, total official at bats, slugging, and other baseball statistics: ESPN's MLB statistics glossary.
For example, in 1920, Babe Ruth was playing his first season for the New York Yankees. In 458 at bats, he had 172 hits, including 73 singles, 36 doubles, 9 triples, and 54 home runs, which brings the total base count to (73 × 1) + (36 × 2) + (9 × 3) + (54 × 4) = 388. He had 458 at bats, so his total number of bases (388) divided by his total at-bats (458) is .847, his slugging average. The next year he slugged .846, and for 80 years those records went unbroken until 2001, when Barry Bonds hit 411 total bases in 476 at-bats, bringing his average to .863, unmatched since. Babe Ruth led the league 13 times (1918-1931, except 1925) - the most times which any batter of pitcher led the league in any major category.
Slugging average's significance Edit
Long after it was first invented, the slugging percentage gained new significance when baseball analysts realized that it combined with on-base percentage (OBP) to form a very good measure of a player's overall production. A predecessor metric was developed by Branch Rickey in 1954. Rickey, in Life Magazine, suggested that combining OBP with what he called "extra base power" would give a better indicator of player performance than typical Triple Crown stats. EBP was a predecessor to slugging average.
Allen Barra and George Ignatin were apparently the early adopters in combining the two modern-day statistics, multiplying them together to form what is now known as "SLOB" (Slugging × On-Base). Bill James applied this principle to his Runs Created formula several years later (and perhaps independently), essentially multiplying SLOB × At-Bats (the actual formula for Runs Created is: $ RC = [(Hits + Walks)(Total Bases)]/[At Bats + Walks] $). In 1984, Pete Palmer and John Thorn developed perhaps the most widespread means of combining slugging and on-base average: OPS. "OPS" simply stands for "on-base plus slugging", and is a simple addition of the two values. While less accurate than SLOB and Runs Created, OPS is extremely easy to calculate, and has become the unofficial shorthand form of player evaluation in recent years.