Take readings at 260 and 280. The OD at 260nm allows calculation of the concentration of the concentration of nucleic acid in the sample. An OD of 1 corresponds to 50 micrograms/mL for dsDNA, 40 micrograms/mL for single stranded DNA and 33 micrograms/mL for RNA. The ration of OD260/OD280 gives you an estimate of the purity of the DNA. Pure DNA preps are from 1.8. Pure RNA is 2.0. If there is contamination of protein or phenol your ratios will be much lower than 1.8 for DNA.
OD260 X 50ug/mL X Dilution factor= ug/mL of DNA
Note: 1bp of the sodium salt of DNA weighs 635daltons (g/mol)