As far as I can ascertain, using simple geometry, the West Fertilizer plant explosion was approximately equal in explosive force to a 1.2 kiloton nuclear blast. The nuclear bombs dropped on Hiroshima and Nagasaki completely leveled those cities to an extent of a 1-mile radius. The authorities in West, Texas have consistently reported that everything within a 4-block radius of the fertilizer plant was completely leveled. Four city blocks are about 1/4 of a mile.
Using the formula to calculate the area of a circle (pi multiplied by the square of the radius), about 3.14 square miles of Hiroshima and Nagasaki were completely destroyed. In the West blast, about 1/5 of a square mile was flattened, which is about 6 percent of the area wiped-out by the nuclear bombs dropped on Japan.
If the explosive yield of the Hiroshima bomb was equal to 20 kilotons of TNT, then the fertilizer plant explosion would be 6 percent of that figure, 1.2 kilotons of TNT. Even if that figure is too high, it does give an indication of a huge amount of explosive material being stored inside the plant, perhaps more than the 270 tons of ammonium nitrate and 24,000 gallons of anhydrous ammonia reported by the company to the EPA in 2012.