Units for expression level. FPKM - number of Fragments Per Kilobase of feature length per Million reads FPKM is calculated by dividing the fragment count per feature by the total number of reads in millions (FPM - Fragments Per Million). FPM is then divided by feature length in kilobases to obtain FPKM. TPM - Transcripts per kilobase Per Million reads TPM is calculated by first dividing the fragment/read count by feature length in kilobases (RPK - Reads Per Kilobase). The count of all RPKs in the sample are then divided by a million to generate a ‘per million’ scaling value. For each feature RPK divided by the ‘per million’ scaling factor generated TPM.