Pi*r^2*L
Pi*r^2 being the area of a circle, L being the length
So, 3.14159 * 4^2 * 18
Assuming 8" and 18" are the internal dimensions. If not, replace those numbers (4 & 18) with the actual internal dimensions.
PS: This is junior high math here (that stuff you thought you'd never use in real life.......). You should know this.