Caluculating the weight of DNA

[lup0]

This may seem like a simple concept to some, but I need some help.
How would I go about calculating the weight of a DNA sample, for example in ng or pg. I'm primarily getting in to the field of forensic
genetics, and low copy number DNA is something i've been researching, frequently authors will refer to low amounts of DNA being in the picogram region, with good amounts being in nanograms.

How would you determine the weight of the DNA sample?

Thanks.

[JackBean]

weight it?

Measure absorbance of your solution and calculate the concentration.