How to calculate cosmological density

When I calculate cosmological density, I divide mass by volume.
But in a book this comes out wrong. Is rho = mass/ (c t)^3
where t is the age of the universe, right or wrong?

Maybe it should be rho= mass / (scale factor)^3 ?
I think the scale factor is a bit larger than ct, so this
density is smaller.

I am confused what is the official method.
Thank you - Jana

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