Input resistance of a BJT common-emitter amplifer
If you are considering the Giacoletto (hybrid-\$\pi\$) small signal model, you should also add the base diffusion (bulk) resistance \$r_x\$, therefore you have
$$
R_{in}=R_B||(r_x+r_\pi)
$$
The base diffusion resistance \$r_x\$ models the real (not incremental) contact and base region silicon resistance and is \$\leq100\Omega\$ in the best BJTs, but it is not easily predictable since it varies between different production batches and it depends on the BJT constrution technology and on its layout: for those reasons it is also not easily measurable. The incremental resistance \$r_\pi\$ has the following known expression
$$
r_\pi=\frac{I_B}{V_T},
$$
where 


\$I_B\$ is the base bias current,
\$V_T=\frac{k_BT}{q}\$ and \$k_B\$ is the Boltzmann constant, \$T\$ is the absolute temperature and \$q=1.60217662\cdot 10^{19}\mathrm{C}\$ is the elementary charge. 


As said above, \$r_x\$ is not usually found on the datasheet (there are however notable exceptions (MAT03 datasheet, page 2, \$r_{BE}\$ parameter)), but if you know the h-parameters of the BJT \$T_1\$, (very easily found in the datasheet of every low frequency BJT), then 
$$
R_{in}=R_B||h_{ie}
$$
since  \$h_{ie}=r_x+r_\pi\$.