On the basis of the second-order moment of the power density and in the use of the series expansion, the expressions for the beam width, far-field divergence angle and M 2 factor of nonparaxial Hermite–Gaussian (H–G) beams are derived and expressed in a sum of the series of the Gamma function. The theoretical results are illustrated with numerical examples. The M 2 factor of nonparaxial H–G beams depends not only on the beam order m , but also on the waist-width to wavelength ratio w 0 / λ . The far-field divergence angles of nonparaxial H–G beams with even and odd orders approach their upper limits θ max = 63 . 435 ∘ and 73 . 898 ∘ , respectively, which results in M 2 < 1 as w 0 / λ → 0 . For the special case of m = 0 our results reduce to those of nonparaxial Gaussian beams. Some problems related to the characterization of the nonparaxial beam quality are also discussed.