We have two responses for you.
There are 231 cubic inches in an American gallon so your 26600 gallons is 231 26600 = 6144600 cubic inches. 18 feet is 18 12 = 216 inches so
6144600/(216 216) = 131.7 inches which is 131.7/12 = 11 feet.
Hence your tank needs to be 11 feet deep.
Does this help?
There are two different sized "gallons": the UK and the US ones. The
UK gallon holds 277.4 cubic inches and the US gallon holds 231 cubic
inches. You didn't say which international definition you are using
so I'll show you the calculation with the UK gallon.
Whenever I convert things like this, I try to track the units and
cancel things out as needed. That way I'm sure of my answer:
Hope this helps,
Stephen La Rocque.