We present precise effective temperatures (Teff) of 215 FGK giants determined using the method of line-depth ratios. For each star, we have measured the line depths and equivalent widths of a large number of spectral lines of low and high excitation potentials and established 100 relations between Teff and their ratios. Starting with an initial value Teff, the relations are then self-calibrated by an iterative process. Our final estimates have been compared with very accurate (1 per cent) published temperatures and show a good agreement. Using our calibrations, we derive precise temperatures for 215 giants with near-solar metallicity, from high-resolution (R = 42 000) and high signal-to-noise ratio (S/N = 100–250) echelle spectra, obtained with the ELODIE spectrometer at the Observatoire de Haute Provence (OHP). The range of application of the method is 3500–5700 K (G0 III–K4 III). The internal error for a single calibration is less than 95 K, while the combination of all 100 calibrations reduces the uncertainty to only 5–20 K (1?). A big advantage of the line ratio method is its independence of interstellar reddening, and its modest sensitivity to spectral resolution, abundance, macroturbulence and other factors.