how to calculate the volume of a tank

[wm23oh]

My neighbor lent me a book called Marine Aquarium. As I was reading the book it gave me a formula to calculate the volume of an empty tank in centimeters. The equation it uses gives different results than any I have seen online. Any idea which is more accurate?
inches L X W X H divided by 231=
centimeters l X W X H divided by 1000 X 0.27=
for example my 20in 50.8cm L 11in 27.9cm W 11in 27.9cm H tank 10 gallons all glass brand gives me these 2 different results.
in 20 X 11 X 11 divided by 231=10.48 gallons
cm 50.8 X 27.9 X 27.9 divided by 1000 X 0.27=10.68 gallons
Their is a difference of .2 gallons per 10 gallons. That can make an estimate quite a bit off.
Does anyone know which is more accurate?

 

[bawood]

There are some rounding errors...
For example, 11 inches is 21.94cm So if it's really 21.94 cm, it's less than 11 inches.
231 cubic inches exactly equals a gallon, so I'd use this formula, the only issue would be the reliability of the measurements.
The cm to gallons formula is an rounded value, .27 is not an exact number.

 

[wm23oh]

Ok. I was just wondering. Thanks

:happyfish

 

[jacksdad]

For rectangle, cubes, etc...
length x width x height x .004329
For cylinders....
.0034 x (height x diameter squared) if height and dia are in inches
Bob