volume = length * width * depth = 7 ft * 7 ft * 4 ft
volume = 196 ft^3
find out how much water is in a cubic foot
196 ft^3 * x gallons/ft^3 = 196x gallons
according to http://wiki.answers.com/Q/How_many_gallons_will_fill_a_cubic_foot
it is "One cubic foot of water contains about 7.48052 US gallons or 6.229 Imperial gallons."
196x = 196 * 7.48052 = 1466.18192 US gallons approximately
196x = 196 * 6.229 = 1220.884 Imperial gallons approximately