How many Gallons can I put in aquarium measure 40X20X16?

I came up with 47.5......:)
 
If CM 12.8 litres, 2.8 UK Gallons, 3.3 US Gallons

If inches 209 litres, 46 UK Gallons, 55 US Gallons
 
And it also will have the decor and substrate....:)
 
I would add water until it was full.
 
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