virus copy number - how to calculate from mass (Oct/30/2008 )

I am doing some endpoint titration PCRs using purified virus stocks. If I know the mass in pg of the assay's LOD and the MW of the virus. How do I calculate the number of copies this represents?

Thanks from a numbers-challenged molecular biologist

the same as you do for any chemistry calculation where you want to find the number of moles:

n=mass/MW