  We present an analytical theory on the heights of the quantum steps observed
in the hysteresis loops of molecular magnets. By considering the dipolar
interaction between molecular spins, our theory successfully yields the step
heights measured in experiments, and reveals a scaling law for the dependence
of the heights on the sweeping rates hidden in the experiment data on Fe$_8$
and Mn$_4$. With this theory, we show how to accurately determine the tunnel
splitting of a single molecular spin from the step heights.
