We present a direct comparison between experimental data and ab-initio calculations for the electrostrictive effect in the polar LaAlO₃ layer grown on SrTiO₃ substrates. From the structural data, a complete screening of the LaAlO₃ dipole field is observed for film thicknesses between 6 and 20 uc. For thinner films, an expansion of the c axis of 2% matching the theoretical predictions for an electrostrictive effect is observed experimentally.