  We present a calculation of the kaon $B$-parameter, $B_K$, using lattice QCD.
We use improved staggered valence and sea fermions, the latter generated by the
MILC collaboration with $N_f=2+1$ light flavors. To control discretization
errors, we use four different lattice spacings ranging down to $a\approx
0.045\;$fm. The chiral and continuum extrapolations are done using SU(2)
staggered chiral perturbation theory. Our final result is $\hat{B}_K = 0.727
\pm 0.004 (\text{stat}) \pm 0.038 (\text{sys})$, where the dominant systematic
error is from our use of truncated (one-loop) matching factors.
