How to convert featureCounts to FPKM?
I have seen many posts regarding counts to RPKM and TPM.


There’s your answer then: FPKM = RPKM. It’s simply a more accurate name.

Speaking of RPKM for paired-end data is discouraged because the reference to “read” in this context lends itself to ambiguity. But mathematically the quantity is the same: we are counting fragments, not individual reads (of which each fragment has two, for paired-end data).

But as mentioned in the comments, there are good reasons against using FPKMs.