iops = 1000 (ms/s) / (average read seek time (ms) + (maximum rotational latency (ms) / 2))

maximum rotational latency = 60,000 (ms/min) / rotational speed (rpm)

Rotational Speed (rpm)		Max Rotational Latency (ms) 4200				14.3 5400				11.1 7200				8.3 10000				6.0 15000				4.0

For example, if we have a 73 GByte, 2.5" Seagate Saviio SAS drive which has a 4.1 ms average read seek and rotational speed of 10,000 rpm:  iops = 1000 / (4.1 + (6.0 / 2)) = 140.8   By comparison, a 750 GByte, 3.5" Seagate Barracuda SATA drive which has a 8.5 ms average read seek and rotational speed of 7,200 rpm:  iops = 1000 / (8.5 + (8.3 / 2)) = 79.0