  As the strong evidence for inflation, the relic gravitational waves (RGW)
have been extensively studied. Although, it has not been detected, yet some
constraints have been achieved by the observations. Future experiments for the
RGW detection are mainly two kinds: the CMB experiments and the laser
interferometers. In this paper, we study these current constraints and the
detective abilities of future experiments. We calculate the strength of RGW
$\Omega_g(k)$ in two methods: the analytic method and the numerical method by
solving the inflationary flow equations. By the first method we obtain a bound
$\Omega_g<3.89\times10^{-16}$ at $\nu=0.1$Hz, where we have used the current
constraints on the scalar spectral index, the tensor-scalar ratio, furthermore,
we have taken into account of the redshift-suppression effect, the accelerating
expansion effect, the neutrino damping effect on the RGW. But the analytic
expression of $\Omega_g(k)$ depends on the specific inflationary models and not
well applies for the waves with very high frequencies. The numerical method is
more precise for the waves with high frequencies and applies to any
single-field inflationary model. It gives a bound
$\Omega_g<8.62\times10^{-14}$, which is independent of the inflationary
parameters, and applies to any single-field slow-roll inflationary model. After
considering the current constraints on the inflationary parameters, this bound
becomes $\Omega_g<2\times10^{-17}$ (see text).
