[Plantoid]

USE milimetres  then do ...
     pi x r squared x the length of the pipe to get the volume in Cubic Centimetres .... thus the weight of water in the pipe in grammes  I think that's right  (  a past stroke clouds my head some days )

 Pi= 3.142
r 2= radius of pipe multiplied by it'self
l= length in centimetres

1 cc of water  = 1 gramme