Tank size

[Tacoeater]

Looking to buy a tank locally, he has it advertised as a 80g but my calculations come out closer to a 90g.
He has it listed as 48x18x23, googled that size and sure enough some call it a 80g or 90g. Want to update my 60g so a 90g would be my choice. Thoughts?
Using online calculator I get 86g so would it be a 90g?

 

[J.Mcpeak]

From the external dimensions here is what you lose,
If the base panel is raised into the tank (common practice on manufactured tanks) you will lose some volume, same thing for rocks gravel etc. for example my rocks work out to about 150 litres worth of water. Also consider the thickness of the glass. Take an internal measurement.