RPKM, Reads Per Kilobase of exon model per Million mapped reads, is defined previous in [1]. 
The formula for calculating RPKM is as following:
 
(number of reads (fractional) mapped to an exon x 1,000,000,000)/(NORM_TOTAL x length of an exon) 
where NORM_TOTAL = the total number of reads (fractional) that mapped to exons, excluding those in the mitochondrial chromosome

Reference
[1] Mortazavi, A., Williams, B.A., McCue, K., Schaeffer, L., and Wold, B. (2008). Mapping and quantifying mammalian transcriptomes by RNA-Seq. Nat Methods 5,621-628.