A parametrization of nucleation rates for H2O-H2SO4-NH3 vapors in atmospheric conditions is developed. The parametrization is based on nucleation rates obtained from a self-consistent model of ternary nucleation, including state-of-art thermodynamics, H2O-H2SO4 hydration model, and rigorous treatment of nucleation kinetics. The parametrized nucleation rates are shown to be within one order of magnitude from those given by the full model with enhanced accuracy at higher nucleation rates. Parametrizations for particle numbers and radius of the critical nucleus are given as a function of nucleation rate and temperature. The parametrizations are valid for temperatures 240--300 K, relative humidities 5--95%, sulfuric acid concentrations 104--109 molecules cm-3, ammonia mixing ratios 0.1--100 ppt, and nucleation rates 10-5--106 cm-3 s-1.