  We propose a new method for measuring the top-quark width based on the
on-/off-shell ratio of $b$-charge asymmetry in $pp\to Wbj$ production at the
LHC. The charge asymmetry removes virtually all backgrounds and related
uncertainties, while remaining systematic and theoretical uncertainties can be
taken under control by the ratio of cross sections. Limited only by statistical
error, in an optimistic scenario, we find that our approach leads to good
precision at high integrated luminosity, at a few hundred MeV assuming 300-3000
fb$^{-1}$ at the LHC. The approach directly probes the total width, in such a
way that model-dependence can be minimized. It is complementary to existing
cross section measurements which always leave a degeneracy between the total
rate and the branching ratio, and provides valuable information about the
properties of the top quark. The proposal opens up new opportunities for
precision top measurements using a $b$-charge identification algorithm.
