  Accurate spectroscopic lithium abundances are essential in addressing a
variety of open questions, such as the origin of a uniform lithium content in
the atmospheres of metal-poor stars (Spite plateau) or the existence of a
correlation between the properties of extra-solar planetary systems and the
lithium abundance of their host stars. We have developed a tool that allows the
user to improve the accuracy of standard lithium abundance determinations based
on 1D model atmospheres and the assumption of Local Thermodynamic Equilibrium
(LTE) by applying corrections that account for hydrodynamic (3D) and non-LTE
(NLTE) effects in FGK stars of different metallicity. Based on a grid of
CO5BOLD 3D models and associated 1D hydrostatic atmospheres, we computed three
libraries of synthetic spectra of the lithium line at 670.8 nm for a wide range
of lithium abundances, accounting for detailed line formation in 3D NLTE, 1D
NLTE, and 1D LTE, respectively. The resulting curves-of-growth were then used
to derive 3D NLTE and 1D NLTE lithium abundance corrections. We demonstrate
that 3D and NLTE effects are small for metal-poor stars on the Spite plateau,
leading to errors of at most +/-0.05 dex if ignored. The largest corrections
are found at the coolest effective temperature, Teff=5000 K, where they can
reach -0.4 dex for the strongest lines. We present analytical functions for
evaluating the corrections and for directly converting a given lithium
abundance into an equivalent width, or vice versa, a given equivalent width
(EW) into a lithium abundance. For convenience, a Python script is made
available that evaluates all fitting functions for given Teff, log g, [Fe/H],
and A(Li) or EW. By means of the fitting functions, the results of complex 3D
and NLTE calculations are made readily accessible and quickly applicable to
large samples of stars across a wide range of metallicities.
