SLG in baseball is shorthand for slugging percentage, which is a rough measure of a hitter’s power ability.

Specificaly, slugging percentage is defined as the number of bases a hitter produces per at-bat and is calculated by dividing a player’s total bases by his number of at-bats:

slugging percentage = total bases/# of at-bats

In turn, the number of total bases is defined as:

total bases = (# of singles) + 2*(# of doubles) + 3*(# of triples) + 4*(# of home runs)

So then the full formula for calculating slugging percentage becomes:

slugging percentage = ((# of singles) + 2*(# of doubles) + 3*(# of triples) + 4*(# of home runs))/(# of at-bats)

As an example, consider a hitter who collects 100 singles, 25 double, 4 triples, and 10 home runs in 600 at-bats. The total bases he produced would be:

total bases = 100 (singles) + 2*25 (doubles) + 3*4 (triples) + 4*10 (home runs)
total bases = 100 + 50 + 12 + 40 
total bases = 202

This batter’s slugging percentatge would be:

slugging percentage = 202 total bases/600 at-bats
slugging percentage = 202/600
slugging percentage = .337

So this hypothetical batter has a .337 slugging percentage. For context, the average Major League Baseball player slugged at a .414 rate in 2023, which makes this batter subpar when it comes to SLG.