Obviously I’d rather do the first. And no, 9 cubic feet is 1 cubic yard.
But I wouldn’t have built a 20m x 20m x 25cm hole in my back yard. I’d have built a 60ft x 60ft x 1 inch hole. And then I’d rather do that in feet than do it in exact metric conversions. I can do the 60x60xinch in my head, but can’t do the metric version of that.
It’s exactly as easy to do round-ish metric numbers in your head as it is to do round-ish imperial numbers in your head, when it comes to volume defined as “cubic units” when the dimensions of the volume are “units”. Neither side has an argument for superiority here, and to claim that it’s somehow easier to do it in meters just because the hole happens to be conveniently metric is just ridiculous.
And I’m being serious when I say that it’s 7 feet. Nobody builds a wall that is 6’7.322" tall. Because that would be insane. In imperial-unit places, walls are sensible imperial units. Just like we use letter sized paper and Europe uses A4. This is important to realize when arguing either side of the issue.
That works fine if you happen to have a perfectly round number to work with - I can also work out something like 21.4m x 27.8m x 46mm with mental arithmetic if I happen to have an odd shaped area.
Its not a matter of superior its a matter of easier - its not hard to do 214 x 278 x 46 mentally, then put in the decimal point as you see fit.
I do find it hard though to do something like 79’5" x 123’ 7" x 1.5" unless I first convert the inches in decimal feet - which if you’re going to do that, why not just use metric in the first place?
I’d love to, because I’m an idiot. Of course, that doesn’t mean the average person knows how many cc are in a liter, either, but yeah. That hurts my argument.
Hmm…depends on you meaning of “average” and “know” - but yeah, if you stopped a European on the street and asked, how many cc in a litre, I don’t doubt that the majority would give a blank look. I would bet though that if people were told to guess at what might be the right answer, a lot would get it right.
I would further bet that a whole hell of a lot more people know how many cc in a litre than could tell you the number of cubic inches in a gallon.