To calculate required storage space for cameras, you only need to know two things: The average bit rate, and the amount of time. Then multiply them together with Google, being careful to use the correct units.
The Correct Units
When describing streaming bit rates and network speed, we typically use "bits" per second. Bits is represented by a lower case 'b'. Hard drive capacity is usually discussed in "bytes" abbreviated with a capital 'B'. Don't confuse them. People get this wrong all the time. Even internet providers.
Mbps = megabits per second. Typically used for describing internet and LAN speeds or audio and video bit rates.
MB/s = megabytes per second. Typically used for describing hard drive or SSD speed and file transfer rates.
MB = megabyte (1 million bytes)
Mb = megabit (1 million bits)
1 byte = 8 bits
How to Calculate Disk Space Requirements
For this example, we will calculate the required disk space to store one day's worth of video from one 8MP camera. Assuming we run it at 15 FPS, an appropriate bit rate might be around 8 Mbps. To calculate the required storage space for 1 day of video, we just multiply the bit rate by the amount of time.
Correct calculation: 8 Mbps * 1 day
Incorrect calculation: 8 MBps * 1 day
Now lets assume you have 8 of these cameras. You can just multiply by 8 to get results for 8 cameras.
8 Mbps * 1 day * 8
= 691.2 gigabytes
Different Time Spans
Of course 1 day isn't very long. Lets assume you want to record for a week.
8 Mbps * 1 week * 8
= 4.8384 terabytes
You can do other neat things too, like take a hard drive size and divide by a bit rate. Lets assume you have a 6 TB drive.
6 TB / (8 Mbps * 8)
= 8.68 days
Of course we all know that formatted, usable capacity is always lower than the advertised capacity. You usually get to store around 90% of the advertised capacity. So we can improve the accuracy of the calculation:
(6 TB * .9) / (8 Mbps * 8)
= 7.81 days
Hikvision Bit Rate Chart
Motion-triggered recording environments can usually retain video for months, if not years, using one modestly sized hard drive, so in such a situation there is little reason not to use the maximum possible bit rate on all your cameras. However for continuous-recording, it is often necessary to limit your bit rates in order to attain the desired retention time, and it can be difficult for users to decide what bit rate is appropriate.
Hikvision has published a chart of recommended bit rates which they felt would deliver good video quality at various resolutions and frame rates. Feel free to go higher or lower to suit your needs, or ignore this chart altogether.