Slugging percentage is simply total bases divided by number of at-bats... technically it isn't a percentage as such, it is a kind of batting average which is weighted to give more credit to extra-base hits, which is why it is used as a measure of power. If every base hit you have is a single, then your slugging percentage will be identical to your batting average; at the other extreme, if every hit you have is a home run then your slugging percentage will be 4 times your batting average.

Walks do not count in slugging percentage.

Batting average and slugging percentage are similar. Whereas batting average measures the percentage of the time you get a hit, it counts all hits equally. Slugging percentage counts them as 1 for singles, 2 for doubles, 3 for triples, and 4 for home runs. So it's the same information as batting average, with a little more info because bigger hits count more.

The formula (from baseball-almanac.com) is:

Number of (Singles + [2 x Doubles] +[ 3 x Triples] + [4 x Home Runs]) divided by At Bats.

Walks count as plate appearances, but NOT as At Bats.Only hits are used in calculating SLG%.

For example, in 2003 Barry Bonds had 133 hits in 390 At Bats. The breakdown was as follows:

45 HR
1 3B
22 2B
65 1B

Using the above formula we get (65 + [2 x 22] + [3 x 1] + [4 x 45]) / 390

(65 + 44 + 3 + 180) / 390
(292) / 390 = .7487 SLG%

MLB.com also lists the SLG% as .749 Walks do not come into the calculation at any point.

In baseball statistics, slugging percentage (SLG) is a measure of the power of a hitter. It is calculated as total bases divided by at bats. Total bases can be calculated from commonly used baseball statistics by using the formula TB = 1B + (2*2B) + (3*3B) + (4*HR).

If I have a.420 batting average through 10 games. What percent of the time do I get a hit?

Slugging percentage is a ratio of bases per plate appearance. If a batter hits a home run in his only plate appearance, his slugging percentage is 4.000 If a player draws a walk, that counts as a plate appearance even though it is not an at-bat, and his slugging percentage is 1.000 If a player has a double and a home run in 3 plate appearances, his slugging percentage is 2.000 The walks are what allowed Barry Bonds to break Babe Ruth's record for highest slugging percentage in 2001 by slugging over .800