  Dust polarization is a powerful tool for studying the magnetic field
properties in the interstellar medium (ISM). However, it does not provide a
direct measurement of its strength. Different methods havebeen developed which
employ both polarization and spectroscopic data in order to infer the field
strength. The most widely applied methods have been developed by Davis (1951),
Chandrasekhar & Fermi (1953) (DCF), Hildebrand et al. (2009) and Houde et
al.(2009) (HH09). They rely on the assumption that isotropic turbulent motions
initiate the propagation of Alvf\'en waves. Observations,however, indicate that
turbulence in the ISM is anisotropic and non-Alfv\'enic (compressible) modes
may be important. Our goal is to develop a new method for estimating the field
strength in the ISM, which includes the compressible modes and does not
contradict the anisotropic properties of turbulence. We use simple energetics
arguments that take into account the compressible modes to estimate the
strength of the magnetic field. We derive the following equation:
$B_{0}=\sqrt{2 \pi\rho} \delta v /\sqrt{\delta \theta}$, where $\rho$ is the
gas density, $\delta v$ is the rms velocity as derived from the spread of
emission lines, and $\delta \theta$ is the dispersion of polarization angles.
We produce synthetic observations from 3D MHD simulationsand we assess the
accuracy of our method by comparing the true field strength with the estimates
derived from our equation. We find a mean relative deviation of $17 \%$. The
accuracy of our method does not depend on the turbulence properties of the
simulated model. In contrast DCF and HH09 systematically overestimate the field
strength. HH09 produces accurate results only for simulations with high sonic
Mach numbers.
