Coefficient method and sunlight assessment are two major approaches to calculate building interval. The algorithm based on equal sunlight duration time (ESDT) principle and models can be used to calculate a legitimate spacing distance, according to sunlight on reference days. Relations of four key variables including the N-S ESDT distance, building facade, elevation difference of obstructive and overshadowed building, and the threshold of building's height are concluded from simulation tests and mathematical analysis. The ESDT technique combines convenient computation with precise analysis so that the algorithm is perfect for building interval calculation without setting up CAD models.