Every disk in your storage system has a maximum theoretical IOPS value that is based on a formula. Disk performance and IOPS is based on three key factors:

To calculate the **Rotational speed**. Measured in RPM, mostly 7,200, 10,000 or 15,000 RPM. A higher rotational speed is associated with a higher-performing disk.**Average latency**. The time it takes for the sector of the disk being accessed to rotate into position under a read/write head.**Average seek time**. The time (in ms) it takes for the hard drive’s read/write head to position itself over the track being read or written.**Average IOPS**: Divide 1 by the sum of the average latency in ms and the average seek time in ms (1 / (average latency in ms + average seek time in ms).

average IOPS = 1 / (average latency in ms + average seek time in ms).

Let's calculate the Rotational Delay - RD for a 10K RPM drive:

- Divide 10000 RPM by 60 seconds:
`10000/60 = 166 RPS`

- Convert 1 of 166 to decimal:
`1/166 = 0.006 seconds per Rotation`

- Multiply the seconds per rotation by 1000 milliseconds (6 MS per rotation).
- Divide the total in half (RD is considered half a revolution around a disk):
`6/2 = 3 MS`

- Add an average of 3 MS for seek time:
`3 MS + 3 MS = 6 MS`

- Add 2 MS for latency (internal transfer):
`6 MS + 2 MS = 8 MS`

- Divide 1000 MS by 8 MS per I/O:
`1000/8 = 125 IOPS`

Calculate the rotational delay (RD) for a 5400 RPM drive.