  In this paper we provide an accurate bound on primordial gravitational waves,
i.e. tensor-to-scalar ratio $(r)$ for a general class of single-field models of
inflation where inflation occurs always below the Planck scale, and the field
displacement during inflation remains sub-Planckian. If inflation has to make
connection with the real particle physics framework then it must be explained
within an effective field theory description where it can be trustable below
the UV cut-off of the scale of gravity. We provide an analytical estimation and
estimate the largest possible $r$, i.e. $r\leq 0.12$, for the field
displacement less than the Planck cut-off.
