The spectral lines of high and low excitation potentials respond differently to the change in effective temperature (Teff). Therefore, the ratio of their depths (or equivalent widths) is a very sensitive temperature indicator. A set of 100 relations is obtained which rely Teff on line depth ratios, calibrated against previously published precise (one per cent) temperature estimates of giants. Highly precise temperatures have been determined from this relations for a set of 110 giants of about solar metallicity, based on high resolution (R=42000), high S/N echelle spectra. The application range of the calibrations is 4000-7000K (F2III-K4III). The internal error of a single calibration is less than 95K, while the combination of all calibrations for a spectrum of S/N=100 reduces uncertainty to only 5-25K (1{sigma}). The big advantage of using line-depth ratios is the independence on the interstellar reddening, spectral resolution, rotational and microturbulence broadening.