We need to get nanogram quantities of phages for DNA sequencing, but how much does the genome of a single phage weigh, and how many phages do we need? The math is quite simple!
One base pair of DNA weighs 650 Daltons. This means if you have one mole of DNA it would weigh 650 grams.
One mole is 6 x 1023 molecules, so in this case 6 x 1023 bp of DNA weighs 650 grams.
The average phage genome is 60,000 bp (OK, its not. I’m making the numbers easy! The modal size is about 40kb, but there are some longer ones like crAssphage at 100kb and some shorter ones like phiX174 at 5kb).
Therefore, each phage weighs
(650 ÷ 6 x 1023) * 60,000 = 6.5 x 10-17 g or 65 attograms or 0.000000065 nanograms
Therefore to get one nanogram of DNA, you need to purify about 15 x 106 phages.
Assuming that your DNA extraction efficiency is 100% and you don’t lose any DNA during the extraction and purification process!