Flexibility is an important property of porphyrins, both natural and synthetic. We applied two synchrotron-based techniques, nuclear resonance vibrational spectroscopy and inelastic X-ray scattering, to quantify this property by measuring the bulk modulus of a protein active-site mimic [chloro(octaethylporphyrinato)iron(III] and the resilience of the iron environment. Their values are 6.95 +/- 0.24 GPa and 15.4 +/- 0.5 N/m, respectively.