Brief Note: Analytical Fit to the Luminosity Distance for Flat Cosmologies with a Cosmological Constant

We present a fitting formula for the luminosity and angular diameter distances in cosmological models with pressureless matter, a cosmological constant and zero spatial curvature. The formula has a relative error of less than 0.4% for 0.2<\Omega_0<1 for any redshift, and a global relative error of less than 4% for any choice of parameters.