How to Calculate IOPS (I/O’s per Second)
IOPS (I/O’s per Second, 
Data Transfer Rate 
Minimum Number of Disk Drives to Support Workload 

Random I/O (10k RPM) 
125 
0.5 
n = (%R + f (%W))(tps)/125 
Random I/O (15k RPM) 
150 
0.5+ 
n = (%R + f (%W))(tps)/150 
Sequential I/O 
2000 
50 
n = (MB/sec)/50 
Where:
%R = the percentage of disk I/O’s that are reads.
%W= the percentage of disk I/O’s that are writes.
f = 1 for ordinary disks, 2 for mirrored disks, 4 for Raid 5 disks.
Assumes data is distributed evenly across all disk drives.
Using the above formula, here’s the minimum number of disks required to support a random I/O workload, at 1000 IOPS, 80% read, 20% write on 10K RPM disk drives.
Ordinary disks: (0.8 + 1*0.2)(1000 IOPS)/(125 IOPS/disk) = 8
Mirrored disks: (0.8 + 2*0.2)(1000 IOPS)/(125 IOPS/disk) = 10
Raid 5 disks: (0.8 + 4*0.2)(1000 IOPS)/(125 IOPS/disk) = 13