See below. If you do the calcualtions in inches, you need to divide by 231 to get gallons since there are 231 cubic inches per gallon.

Hope this helps.

Here is a link that might be useful: Volume of a trapazoidal cone

Excellent formula, grandad.

First a gallon of water is different than a gallon of soil. not the same volume.

Pot manufactures give the contents of all their pots. Greenhouse places publish the data in their catalogs. For example the Griffins Greenhouse Catalog gives the correct info. But remember they will list dry volume and not wet volume.

A 5 gallon nursery pot is probably closer to a 3 1/2 gallon paint bucket. A 5 gallon paint bucket is much larger than a 5 gallon nursery pot.

Ah the difference between a real gallon and a trade gallon. A trade gallon for some reason is only 0.734 of a gallon. Unfortunately the percentage does not stay true to the 0.734 of a gallon as pot size increases. How this is legal is beyond me. One would think that weights and measures would have come down on this long ago. But alas its not a gallon is it? No its a trade gallon.

Below is a company that actually lists the volume of the pots they sell. There really is no standardization. A company could list a 2 gallon pot and as long as it has more than 1 gallon and less than 3 gallons its OK to market it that way.

I'm not recommending the company in the link. I have never purchased from them. They were just the first company I could find that listed the volume of all their containers which is why I provided the link.

Here is a link that might be useful: Nursery Pots