I'm not sure why you converted it to metric units and then to gallons...that just makes it more difficult / less accurate. But yeah, it's just slightly under 15 gallons, actually 14.9610389.
Because we use the metric system and it was easier that way, the conversions .6m &.3m were only rough as 1" is actually 25.4mm but as it wasn't critical I only ran through it quickly and roughly
My ex whore of a gf's dad has a 150+ gallon tank in the middle of the living room. It's actually built around one of the main support beams for the house.