A new approach to constraining or determining the spin of a massive black hole is proposed .
A key parameter in the analysis is the dimensionless ratio , r , of the energy released to the mass of the hole .
It is shown that the black hole spin j may be written as a function of the dimensionless ratio of the black hole spin energy to the black hole mass .
When extraction of the black hole spin energy powers an outflow from the hole , the ratio r provides an indication of the black hole spin .
The method is applied to a sample of 19 very powerful classical double radio galaxies with a range of size and redshift .
The sources are found to have remarkably similar values of r and j , implying that the sources have very similar physical conditions at the time the outflow is generated .
The weighted mean value of the spins is 0.12 \pm 0.01 .
A sample of 29 central dominant galaxies is also studied ; most of these sources have amorphous or FRI radio structure .
The sources have a broad range of ratios r , and correspondingly broad range of black hole spins j with values from about 0.001 to 0.4 , and a median value of about 0.03 .
The broad range probably results from the fact that each source is observed at a different stage in its lifetime .
The intrinsic range of parameters could be as tight as it is for the powerful classical double radio galaxies .