Determining definite volume of tank

[KianMan]

Hi guys! ive got a tank that im assuming is 20 gallons. I fill it with 3.5 (more or less) 5 gallon buckets but. that's not all the way to the top. the tank dimensions are
length: 24 in.
breadth: 12 in.
height: 15 in.
Help will be really appreciated

 

[Sauceboat]

Mathematically the dimensions play out to be around 18.7 gallons.. if those were indeed the correct measurements, you can expect this water volume.. an odd sized tank you have there

 

[KianMan]

Mathematically the dimensions play out to be around 18.7 gallons.. if those were indeed the correct measurements, you can expect this water volume.. an odd sized tank you have there

I got it from a friend 'cause he was giving it away. thanks what formula did you use to calculate?

 

[Rye3434]

You can use l x w x h and then convert cubic inches to gallons

 

[MasterPython]

You can use l x w x h and then convert cubic inches to gallons

Measure the inside of the tank. I think the listed volume is based on the outside dimensions for some reason.

 

[Rye3434]

Yes but the difference would be negligible with how thin the glass is