Ordinarily I'd pass on this one, because you haven't said what the "21 foot" is ...
whether it's a diameter, a circumference, or the side of a square pool. But this
particular volume question promises an interesting answer:
I'll assume that the pool is round, and its diameter is 21 feet.
Volume = (pi) (radius)2 (depth) = (pi) (10.5)2 (1) = 110.25 pi = 346.36 cubic feet = 2,590.96 gallons (rounded)
At 1 minute per gallon, it'll take 43hours10minutes 57seconds to fill the pool.
4400 gallons, or 4,398.24 gallon rounded off.
7.48 gallons fill one square foot.
1 Gallon
how many gallons of water will a tank that is 5 foot by 4 foot by 8 foot hold
7.518 gallons.
WolframAlpha, a new search engine gives a UK gallon of water at 0.1337 cu.ft so your pool should take 6,462 gallon to fill, give or take a cupful. Check it out!
4400 gallons, or 4,398.24 gallon rounded off.
a gallon or so
One gallon of water is equivalent to approximately 0.1337 cubic feet.
To give you an idea, a 36-foot by 18-foot by 5-foot deep pool takes about 24,000 gallons of water to fill.
7.48 gallons fill one square foot.
7.48052 US gallons per cubic foot.
1 Gallon
it depends on the dimensions of the pool a 5 foot deep tub would fill up a lot faster than an Olympic sized pool.
how many gallons of water will a tank that is 5 foot by 4 foot by 8 foot hold
There are 7.48 gallons in one cubic foot. So, there are 0.13368 cubic feet per gallon
7.518 gallons.