And round we go again!
You will not know the exact volume unless you count how many buckets it takes to fill up - and at 6ft long, that could take quite a while! Along with a pen & paper to note down each bucket as you go or you'll lose count!
If you dont want to do that, you can get a fair-ish estimate once the sand and water is in. Measure from the top of the sand line to the water line for the height. You already have the width and depth. Pop the measurements into the calculator and you will get the volume but dont forget to deduct some for decor. Only you can estimate how much the decor takes as only you know what you have. It wouldnt be easy for us to guess from photos or description.
The measurements you give above come back as a volume of 547 litres. Are these measurements the overall size of the tank? If so, then the actual volume will be nowhere near 547 litres. No tank is filled to the brim. Nearly all tanks have sand/gravel and logs/rocks, plants and other decor. You will be amazed how much space is lost from all this. Ideally, you need a fairer "actual volume" to get the filter size needed.
It is generally recommended for a filter to turn the tank five times per hour (ten times if heavily planted). When calculating what size filter you need, bear in mind that manufacturers will quote a maximum performance. Eg, if a filter is alleged to be 1500 litres per hour, then that will be on an empty filter. By the time you add media and the filter starts collecting gunk from your tank then that 1500 PH will be more like, perhaps 1300 maybe even less? There are other debating factors too which make things even more complicated - Sue managed to find a very interesting article about it on the old forum and it pretty much left us all having no clue as to how to work out exactly what a filter would do! The best we can do is go by the 5 times per hour (actual volume) calculation then round it up a notch or three! Having a filter too powerful will turn your tank into a washing machine and your fish will think they're living in a permanent tornado
Erm, may I ask...... are you a skilled carpenter??? The way you said that its your first attempt..... did you mean your first attempt at a fish cabinet or first attempt at woodwork? Sorry - I couldnt figure out which you meant
Although at the time it must have been annoying about the 5ft tank, the consolation is that you now have a 6-footer!!