  Empirical fitting functions, describing the behaviour of the \lambda 4000 \AA
break, D_4000, in terms of effective temperature, metallicity and surface
gravity, are presented. For this purpose, the break has been measured in 392
stars from the Lick/IDS Library. We have followed a very detailed error
treatment in the reduction and fitting procedures, allowing for a reliable
estimation of the break uncertainties. This calibration can be easily
incorporated into stellar population models to provide accurate predictions of
the break amplitude for, relatively old, composite systems.
