  We precisely determine the infrared (IR) length scale of the no-core shell
model (NCSM). In the NCSM, the $A$-body Hilbert space is truncated by the total
energy, and the IR length can be determined by equating the intrinsic kinetic
energy of $A$ nucleons in the NCSM space to that of $A$ nucleons in a
$3(A-1)$-dimensional hyper-radial well with a Dirichlet boundary condition for
the hyper radius. We demonstrate that this procedure indeed yields a very
precise IR length by performing large-scale NCSM calculations for $^{6}$Li. We
apply our result and perform accurate IR extrapolations for bound states of
$^{4}$He, $^{6}$He, $^{6}$Li, $^{7}$Li. We also attempt to extrapolate NCSM
results for $^{10}$B and $^{16}$O with bare interactions from chiral effective
field theory over tens of MeV.
