I have a 2500 gallon horizontal cylindrical tank that is 11 feet long and has a diameter of 6 feet 4 inches. How much liquid is in it when I measure 27 inches of water ?

I assume the tank is lying on its side.

Consider the cross-section.
The area of a segment of a circle subtended by angle θ is

1/2 r^2 (θ - sinθ)

The radius is 38 inches, so we have
cos(θ/2) = 11/38 = 0.289
sin(θ/2) = 36.37/38 = 0.957
V/2 = 1.276
θ = 2.552

so, the area of the water in the cross-section is

1/2 * 38^2 (2.552-.554) = 1442.556 in^2

The volume in the bottom of the tank is

1442.556 * 11 * 12 = 190417 in^3 = 824.3 gal

makes sense, since a depth of 38 inches would be half full, or 1250 gal