Aims: Precise determination of the stellar effective temperature of solar type stars is extremely important for astrophysics. We present an effective temperature calibration for FGK dwarf stars using the line equivalent width ratios of spectral absorption lines. Methods: We used the automatic code ARES to measure the equivalent width for several spectral lines. These measurements were used for calibration with the precise effective temperature derived from spectroscopy presented in a previous work. Results: We present the effective temperature calibration for 433 line equivalent width ratios built from 171 spectral lines of different chemical elements. We also make a free code available that uses this calibration and that can be an extension to ARES for fast and automatic estimation of spectroscopic effective temperature of solar type stars. Full Table 2 and line list are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/512/A13