How is slugging percentage calculated?

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.

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.

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.

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).

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

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