We propose a numerical procedure, addressed as copula integration method, to calculate quanto implied volatility adjustments. The method consists in a direct integration of the quanto vanilla payoff, using the bivariate terminal probability distribution of the asset and the relevant foreign exchange rate. The bivariate terminal distribution is obtained by coupling the marginal distributions of the two underlyings by means of a Gaussian copula. The asset and the foreign exchange rate marginal distributions are directly inferred from the corresponding Black-Scholes market volatility smiles. In order to obtain well defined marginal distributions, we propose an extrapolation method for the standard implied volatility outside the quoted region, which does not allow arbitrage opportunities. The validity of the copula integration method is established by comparing its predictions to exact results for quanto option prices, obtained by numerical computations in two realistic test cases, in which the dynamics of the assets is driven by a local volatility and a Heston stochastic volatility model.