Any container 30 inches long 13 inches wide and 13 inches high has a volume of 22 gallons
Approximately 21 Imperial gallons.
168.311 US gallons.
138.5281 gallons.
= 93.09 gallons (US) = 77.51 gallons (imperial)
It holds 632.73 US gallons, approx.
Approximately 201 gallons.
70.1298701
Assuming it is a rectangle, 42.077 US gallons
Probably 25-30 gallons, be we cannot say for sure, since you did not say how TALL it is.
Its a long explanation how but it should be about 25 gallons
36x16x20=11520 cu inches, =6.6667 cu ft=49.87 gallons