Firebox Calculation Question

I am making a smoker and I was double checking my dimension to a calculator for intake, firebox volume, chimney volume, etc. I was just curious if the total firebox volume included the ash pan below the grate or if it is just from the grate up? Right now I have it designed from the grate up and if I could use the ash pan volume below the grate I could lower the height of my firebox 3" and save myself some steel.
Thanks ahead for the help!