How to covert RKPM into absolute quantification?

Question

[marongiu.luigi]

Hello, is it possible to convert RPKM values into absolute values? That is to convert a qualitative (relative) measurement into a quantitative (absolute) measurement? If I have an RPKM of, let's say, 375 for a gene of interest (or micro-organism) and the sample was 1 mL of something, can I calculate the number of initial copies of the gene?

[Fabio Marroni]

If you don't have a spike-in of known concentration is impossible. See answers to a similar question here

Answer 1

[Devon Ryan]

This is not possible with regular RNA-seq data.