



 
Mike, The subject line of your email was "volume differences" so I am going to assume you are looking at the percentage difference in the volumes. The volume of a cylinder is π × (d/2)^{2} × h where d is the inside diameter, h is the length and π is approximately 3.1416. Thus for two pipes of the same length h then the volumes are
and
for a difference of
If you want the difference as a percentage you need to ask "percentage of what?" As a percentage of the 3 in diameter pipe
so the 4 inch diameter pipe has a volume which is 55.6% larger than the volume of the 3 inch diameter pipe. As a percentage of the 4 in diameter pipe
so the 3 inch diameter pipe has a volume which is 31.25% smaller than the volume of the 4 inch diameter pipe. Percentages can be confusing,  


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