In zero electric field, the interactions energy between two Rydberg atoms is $U=C_6 r^{-6}$. If the Rydberg transition is driven with an (effective) Rabi frequency $\Omega$, then the blockade radius is $$R_b = \left( \frac{\hbar\Omega}{C_6} \right)^{-1/6}$$ If one atom is excited to the Rydberg state, a second atom will be detuned by just enough to prevent a complete transition.