In metric, the weight of water in grams is equal to the volume in cubic centimeters, by definition. The volume of a cylinder is the area of the circle times the length. The area of a circle is pi * r^2. So my description above skipped a step or two.
> In metric, the weight of water in grams is equal to the volume in cubic centimeters, by definition.
Just to clarify this is not exactly true. It was the original idea, but pretty quickly they realized that it wasn't a practical definition and kilogram ended up being defined by a standard reference object which was carefully stored in a vault in Paris and thus being disconnected from the meter and the density of water. Finally in 2019 kilogram was redefined again in terms of natural constants in the great SI unit upheaval. Although sadly the new definition is not very intuitive to understand.
I'm sure the updates to the Kilogram definition tried to keep it as close to the original as possible, right? The difference pales in comparison to the other shortcuts I was taking.
I have no idea what the GP is doing but you would just do
Pi * D^2 / 4 * H * 62.4
Where D is tank diameter in ft H is tank height in ft and 62.4 is the unit weight of water in lbs per cubic foot. Divide by 144 of D is in inches and 12 if H is in inches.
Or if you want to be real lazy you can see Pi/4*62.4 is ~49 and round to 50.
The unit weight of water being 62.4 pcf is probably not known by most people - most people don’t think in cubic feet. The more likely (and less useful) number most people know is ~8 lbs per gallon.