The optical outputs of single-section quantum-dash and quantum-dot mode-locked lasers (MLL) are well-known to exhibit strong group velocity dispersion. Based on careful measurements of the spectral phase of the pulses from these MLLs, we confirm that the difference in group delay between the modes at either end of the MLL spectrum equals the cavity roundtrip time. This observation allows us to deduce an empirical formula relating the accumulated dispersion of the output pulse to the spectral extent and free-spectral-range of the MLL. We find excellent agreement with previously reported dispersion measurements of both quantum-dash and quantum-dot MLLs over a wide range of operating conditions.