Elastic constants of lizardite [Mg3Si2O5(OH)4] were computed using first‐principles quantum mechanical calculations within the density functional theory. The predicted c‐axis compressibility is much larger than measured. Modeling of the weak O‐H⋯O interactions between layers must be improved in order to better predict layered hydrated mineral elastic properties. The large computed bulk modulus range is consistent with equation of state and seismic velocities in chrysotile‐lizardite serpentinites, but shear wave velocities are lower than the lowest theoretical estimate. The low seismic velocities measured for chrysotile serpentinites can be attributed to specific contribution of the nanotube‐textured chrysotile. For antigorite, the data from acoustic and EoS measurements are consistent. If used for interpreting seismic velocities, the inferred degrees of serpentinization of the mantle wedge are higher than with the commonly used calibrations using chrysotile‐serpentinite properties. Serpentine is likely a dominant phase in low velocity areas of the mantle wedge at 30–50 km depths.