[airspirit]

I think your measurements are off.

Area of a circle = r^2 * pi
Volume of a cylinder is A*depth
Diameter = 4" (my 4" section at home measures 4")
Radius = 2"

2.54 cm = 1".

4*2.54 = 10.16 cm diameter.
R^2 * pi = 81 cm^2.

ans*60"*2.54 = 12349 cc = 12349 ml

This = 12.35 L. That is a bit over 4 gallons for the whole length. I think you squared you diameter, among other errors. You could not fit 15 gallons into a 5 ft stack 4" wide. Your milk cartons are wider, and you'd only need to stack six or seven to get to 5 ft, if my reckoning is right. Shrink it down to scale, and four gallons seems right. Plus, I'm only going to fill the coolant level to 2" over the height of the highest used input, so there will be an empty foot or so in the top (breathing area for air compression when I screw the lid on).

Is my math right (I did some sick siggy digit clipping ... I am aware of that). The other reason I think you squared the diameter is that your 1.5" measurement is just slightly smaller than my 4". I think you are also using outer diameter. As I said, my measurement of actual in-hand pipe is 4" ID (give or take a couple mm).