As popular as i figured this would be, search didn't find anything!

I found this online on how to find n digits of pi

Pi = SUMk=0 to infinity 16-k [ 4/(8k+1) - 2/(8k+4) - 1/(8k+5) - 1/(8k+6) ]

I thought I knew how to use it until i tested it out manually on my calculator and got some funky answer. Has anyone seen this before? How is it used? (heh i thought i knew )