No. of days = Total storage capacity(in GB) / (No. of camera’s X Storage per camera per day(in GB))
Storage per camera per day (in GB)= Bit rate per FPS (in MB) X FPS X 60 X 60 X 24 / 1024
Example :
For 2 months recording of 1 camera at 720p
Storage per camera per day (in GB)= 0.02 X 15 X 60 X 60 X 24 / 1024 = 25 GB
60 days = Total storage capacity(in GB) / ( 1 X 25)
Total storage capacity(in GB) = 60 X 25 = 1500 GB
This is how capacity can be calculated.