In providing how to calculate the amount of drinking water supplied, the SI specifies that the amount is to be calculated by reference to an average amount over a specified period.
Wrth bennu sut i gyfrifo faint o ddŵr yfed a gyflenwir, mae’r offeryn statudol yn nodi y dylid cyfrifo’r swm drwy gyfeirio at swm cyfartalog dros gyfnod penodedig.