The feat can be found here (3:04)
Ultron's leg = 102 px ~ 60 cm
2a = 145 px = 85.3 cm
2b = 304 px = 178.8 cm
2c = 380 px = 223.6 cm
a = 42.65 cm
b = 89.4 cm
c = 111.8 cm
I'll use half the volume of an ellipsoid, one can find better approximations but i'll use this one for simplicity.
V = 2/3π*a*b*c
V = 892 805.7 cm^3
Using fragmentation of concrete (6 J/cc)
E = 5.35*10^6 Joules (1.28 kg TNT) Wall level
Using violent fragmentation of concrete (20 J/cc)
E = 1.785*10^7 Joules (4.26 kg TNT) Wall level+