Scalability is a key factor of the design of distributed systems and parallel algorithms and machines. However, conventional sealabilities are designed for homogeneous parallel processing. There is no suitable and commonly accepted definition of scalability metric for heterogeneous systems. Isospeed scalability is a well-defined metric for homogeneous computing. This study extends the isospeed scalability metric to general heterogeneous computing systems. The proposed isospeed-efflciency metric is suitable for both homogeneous and heterogeneous computing. Through theoretical analysis, we derive methodologies of scalability measurement and prediction for heterogeneous systems. Experimental results verify the analytical results and confirm that the proposed isospeed-efficiency scalability works well in both homogeneous and heterogeneous environments.