  Quantifying coherence is an essential endeavor for both quantum foundations
and quantum technologies. In this paper, we put forward a quantitative measure
of coherence by following the axiomatic definition of coherence measures
introduced in [T. Baumgratz, M. Cramer, and M. B. Plenio, Phys. Rev. Lett. 113,
140401 (2014)]. Our measure is based on fidelity and analytically computable
for arbitrary states of a qubit. As one of its applications, we show that our
measure can be used to examine whether a pure qubit state can be transformed
into another pure or mixed qubit state only by incoherent operations.
