Let's get picky.
deltaP / L = specific weight
Specific weight is weight (force) per unit volume = pounds force per cubic ft or Newtons per cubic meter, for example.
Density is mass per unit volume, pounds mass per cubic foot or kg per cubic meter for example.
For some calculations it doesn't matter, but or some it does. You should always keep track of your units properly, or you could be in deep doodoo some day.
The formula with the picture posted by Snap25 , for example, has feet and pounds and inches mixed in higgledy piggledy. You'll end up with pig slop for your result if you don't watch your units!
Done right the result of this calculation will be as accurate as the differential pressure measurement accuracy and the tolerane on the distance between the two pressure taps. I would personally take it over ultrasonic or other instrument prone to various error inducing influences. This is fundamental physics.