not sure if it is working right as i when to put my tank in and got way diff. 10'x6'x2.5'....so i did a test with the standard 180gal dim. of 72"x24"x24" and got 119.96gal not 180gal as with other calculators
I used the same published link as what is in this article. Not sure what to tell you, I can't figure out where you got 120 gallons unless you wrote 16 in place of a 24 or 48 in place of a 72.