Formation of coherent Cu precipitates in supersaturated ferrite (1.5 at.%Cu) at 500°C is simulated using the Monte Carlo method. Bond energies used in the atomistic simulation are calibrated on the mutual solubilities given on the Fe-rich and Cu-rich side of the Fe(bcc)-Cu(bcc) phase diagram. The spatial extension of the precipitate phase is defined on basis of a composition criterion of the nearest neighbor shells. Various definition conditions are examined in terms of resulting particle densities, mean radii and composition of the precipitates, as well as the composition profiles across the precipitate/matrix interface. The predictions of the simulations are compared with the experimental results from atom probe analysis as well as small angle neutron scattering.