Muhammad:  You need a relatively concentrated DNA sample to be measured with a regular spectro(photo)meter or a plate reader.  We usually use 5 microliters of mini-prep DNA sample and diluted to 300 microliters (60x) and read at A260 and A280 with a microcuvette of 1.0 cm light pah to get ODs and their ratio for purity (good DNA has 1.8-2.0 of A260/280).  After that, you simply  calculate with C = A x 60 (dilution factor)/50 (microgram per ml for double stranded DNA).  Good luck.

Hi Mohammed

• I cannot add to Ke answer in terms of what to do
• However, I will say that you also need to look at the 260/230 ratio as well as the 260/280 ratio if you can: 260/230 < 1.0 indicates salt contamination of your DNA and/or phenol or Trizol if you have extracted your DNA the old fashioned way
• What is more absorption at 260nM is not specific to DNA: It will also detect RNA and indeed any organic, e.g. phenol/Trizol with an aromatic ring  (in fact @ 260nM you are inducing resonance of your delocalised pi  electrons in your aromatic ring)
• Thus specs or Nano drops @ 260nM can over estimate the amount of DNA
• To elaborate, if your 260/280 ratio is > 2.2 (up to 2.2 is actually OK) and your 260/230 ratio is < 1.0 then you might have trizol contamination of your DNA (if used to extract) where as a 260/280 ratio < 2.0 with a 260/230 ratio < 1.0 implies salt contamination
• Finally, remember for UV absorption (DNA/RNA) to use a quartz and not a plastic cuvette 

Above all clearly explains it.

Also, make sure to dilute your DNA in order to get absorptions values in between 0.2 and 0.8. And make sure all the DNA is dissolved. 

Thanks all. I will perform the step as the suggestion given.