How to Calculate Video Storage Capacity for 90 Days

Start With Your Camera Count and Bitrate

The first step in sizing your system is defining exactly what you are recording. You need the total number of cameras and the bitrate for each one. Bitrate is the amount of data a camera generates per second. It depends on resolution, frame rate, and compression settings.

Do not guess these numbers. Check the camera specifications or the NVR configuration screen. A 4K camera at 30 frames per second with H.265 compression will generate significantly more data than a 1080p camera at 25 frames per second. If your cameras support variable bitrate, use the maximum value for the calculation. This ensures the drives do not fill up faster than expected.

Many installers make the mistake of using the minimum bitrate listed in a datasheet. That figure usually applies to static scenes with low motion. In a busy environment, such as a warehouse dock or a retail entrance, the encoder pushes the bitrate higher to preserve detail. If you size the storage based on the minimum value, the drives will fill up quickly. The retention period will drop from 90 days to perhaps 30 days after just a few weeks of operation. Always pull the maximum sustained bitrate from the camera menu or the vendor’s technical support team.

For variable bitrate cameras, the maximum value is often listed as a peak figure. Use that peak for your calculation. It is better to have a little unused space than to lose footage because the disk filled up during a high-activity period. If you are unsure, call the manufacturer and ask for the worst-case bitrate for your specific model and scene type.

Understand the Difference Between Codec Standards

Compression standards affect how much space each second of video takes up. H.265, also known as HEVC, generally produces smaller files than H.264 for the same image quality. H.266, or VVC, is newer and often even more efficient.

If your NVR and cameras support H.265, use the bitrate figures from that mode. If you mix H.264 and H.265 cameras, the total storage requirement will be higher than if all cameras used the more efficient codec. Confirm the codec on every camera before finalizing your drive selection.

The difference between H.264 and H.265 can be substantial. For a given resolution and frame rate, H.265 often requires roughly 25% to 35% less bandwidth and storage to achieve the same visual quality. This means a camera that generates 8 Mbps in H.264 might only generate 5.5 Mbps in H.265. If you have a mix of older H.264 cameras and newer H.265 cameras, you must calculate the storage for each group separately.

Do not assume that a newer NVR will automatically handle older cameras efficiently. If the NVR records the stream in the format the camera sends it, the storage requirement depends on the camera’s output. If the NVR has a hardware transcoder, it can convert H.264 streams to H.265 before writing them to disk. However, transcoding uses processing power and can introduce latency. Check the NVR specifications to see if it supports hardware transcoding and if it is practical for your camera count. If you are mixing codecs, list each camera’s bitrate individually and sum them up. This is the most accurate method.

Calculate Daily Data Usage

Multiply the number of cameras by the bitrate of each camera. This gives you the total data per second for the entire system. Convert this to a daily figure.

The formula is:

Daily Data (GB) = (Number of Cameras x Bitrate per Second) x 86,400 Seconds / 8 / 1000

Dividing by 8 converts bits to bytes. Dividing by 1000 converts kilobytes to gigabytes. This daily figure is the baseline amount of video you will record in 24 hours of continuous operation.

It is helpful to visualize what these numbers mean in practice. A single 4MP camera recording at 4 Mbps generates about 43 GB of video per day. If you have ten of these cameras, the total is 430 GB per day. Over 90 days, that is nearly 39 terabytes of raw data. This is before accounting for system overhead.

When performing this calculation, ensure your bitrate is in megabits per second (Mbps). Some datasheets list it in kilobits per second (Kbps) or megabytes per second (MBps). Be careful with the distinction. Megabits per second is standard for video streams. Megabytes per second is ten times larger. If you accidentally use megabytes per second in the formula, your storage requirement will be ten times too high. Double-check your units before proceeding.

Determine Total Storage Requirement

For a 90-day retention period, multiply the daily data usage by 90. This gives you the raw storage required to store three months of footage.

Total Raw Storage (GB) = Daily Data (GB) x 90

This number is the theoretical minimum. It assumes 100% storage efficiency, which is not possible in real-world NVR systems. File systems, journaling, and metadata consume space. The NVR operating system also needs room for its own files.

The raw calculation assumes that the disk is 100% full of video data. In reality, the file system used by the NVR (often ext4 or a proprietary variant) reserves space for block allocation, inodes, and journaling logs. These structures allow the system to recover from sudden power loss or crashes. Without them, a power cut during a write could corrupt the entire drive. The NVR also stores configuration files, logs, and system images. These take up a few gigabytes, which is negligible for large systems but can matter for smaller setups.

If you are using RAID, the calculation changes slightly. RAID 5 or RAID 6 provides redundancy, meaning you lose some capacity to create parity blocks. RAID 5 requires one drive’s worth of capacity as overhead. RAID 6 requires two. This is a separate consideration from system overhead, but it is important to factor in when selecting drive sizes and quantities. For this guide, we are calculating the raw video storage requirement. RAID redundancy will be addressed when selecting the drives.

Add a Safety Margin for System Overhead

Add a buffer to the raw storage figure. A standard practice is to add 15% to 20% to account for system overhead and file system fragmentation.

Final Storage Requirement (GB) = Total Raw Storage (GB) x 1.15

If your NVR runs a full operating system with a large journal, use the higher end of that range. This buffer prevents the NVR from reporting “disk full” errors before the 90-day retention window has actually elapsed.

The 15% buffer covers file system fragmentation and minor system growth. Over time, the file system becomes fragmented as data is written and deleted. Fragmentation reduces the efficiency of disk space usage. The NVR may also accumulate temporary files during updates or diagnostics. Adding 15% to 20% ensures that the 90-day retention period is fully usable.

Some NVRs have a feature called “overwrite protection” or “write protection.” This prevents the NVR from overwriting existing footage until the retention period has passed. If you have a legal requirement to store footage for 90 days without interruption, this feature is critical. However, it can cause issues if the disk fills up before the retention period ends. The safety margin helps prevent this. Without it, the NVR might stop recording new footage to protect old footage, which defeats the purpose of continuous surveillance.

Select the Right Hard Drives

Now convert the final storage requirement into hard drive sizes. Hard drives are sold in terabytes. Convert your requirement from gigabytes to terabytes by dividing by 1024.

Choose drives that are rated for video surveillance. These drives are designed to run 24/7/365. They have higher tolerance for heat and vibration than standard desktop drives. Look for models that support continuous recording workloads.

Consider the physical size of your NVR. Most NVRs use 3.5-inch enterprise drives. Some smaller units support 2.5-inch drives, but these are generally less reliable for continuous recording. Check the NVR manual for the supported drive form factor.

Video surveillance drives are often labeled with terms like “SATA 6Gb/s,” “CMR,” or “Surveillance.” CMR stands for Conventional Magnetic Recording. Most surveillance drives use CMR. Avoid drives with “SMR” or “Shingled Magnetic Recording” if possible. SMR drives can have slower write speeds and may struggle with the constant high-throughput writes of an NVR. They are better suited for less demanding applications.

The drive must also be rated for continuous operation. Check the warranty terms. Standard desktop drives are often rated for 15 hours of operation per day. Surveillance drives are rated for 24 hours a day, 365 days a year. This difference in duty cycle is a major factor in long-term reliability. If you use desktop drives in an NVR, you risk data loss and downtime. The cost of replacing a drive and restoring a backup is higher than the cost of buying the correct surveillance drive in the first place.

Verify the Retention Time

After you select the drives, verify the actual retention time. Divide the total usable capacity of your selected drives by the daily data usage.

Actual Retention Time (Days) = Total Usable Capacity (GB) / Daily Data (GB)

If the result is less than 90 days, you need larger drives or more drives. If the result is significantly more than 90 days, you may have overspent. Aim for a result that is close to 90 days to keep costs down while meeting the requirement.

Usable capacity is the actual amount of space available for data. It is slightly less than the advertised terabyte rating. A 10 TB drive typically offers about 9.3 TB of usable space due to file system overhead and the difference between binary and decimal terabytes. Always check the drive manufacturer’s specifications for the actual usable capacity.

If you are using RAID, the usable capacity is further reduced. For RAID 5 with four drives, the usable capacity is the sum of the drives minus one drive’s capacity. For RAID 6, it is minus two drives’ capacity. Factor this into your final calculation. If the result is close to 90 days, you have a well-sized system. If it is 180 days, you have double the storage you need and are wasting money. If it is 45 days, you are short and will need to replace drives or reduce camera count.

Common Mistakes in Storage Calculation

Several errors can lead to undersized storage.

• Using average bitrate instead of maximum: Cameras often use variable bitrate. If you use the average, you will underestimate the peak data usage and fill the drives faster than expected.
• Ignoring NVR overhead: Forgetting to add a buffer for system files and file system overhead is a frequent mistake. This leads to short retention periods.
• Mixing codecs without checking: Assuming all cameras use the same compression standard can skew the total bitrate. If some cameras use H.264 and others use H.265, calculate the total based on the actual mix.
• Choosing consumer-grade hard drives: Standard desktop drives are not designed for the constant write workload of an NVR. They fail more often and can corrupt data during continuous recording.

Another common mistake is ignoring the NVR’s maximum drive capacity limit. Some NVRs cannot manage drives larger than a certain size, even if the hardware could physically support them. For example, an NVR might have a limit of 8 TB per drive slot. If you install a 16 TB drive, the NVR may only recognize 8 TB of it. This can lead to unexpected data loss and confusion. Always check the NVR’s compatibility list for supported drive models and capacities.

Example Calculation

Consider a site with 12 cameras. Each camera records at 8 megabits per second using H.265 compression.

First, calculate the total bitrate:
12 cameras x 8 Mbps = 96 Mbps

Next, convert to daily data:
96 Mbps x 86,400 seconds / 8 / 1000 = 1,036.8 GB per day

For 90 days:
1,036.8 GB x 90 = 93,312 GB

Add a 15% safety margin:
93,312 GB x 1.15 = 107,308.8 GB

Convert to terabytes:
107,308.8 GB / 1024 = 104.8 TB

You would need approximately 105 terabytes of usable storage. In practice, you would select drives that sum to at least 105 TB of usable space. This might mean four 24 TB drives, depending on the specific models available and their usable capacity.

If you use four 24 TB surveillance drives, the total raw capacity is 96 TB. However, the usable capacity is likely around 90 to 92 TB due to file system overhead. This would fall short of the 104.8 TB requirement. You would need to use larger drives or add more drives. For instance, four 28 TB drives would provide about 105 TB of raw capacity and roughly 98 to 100 TB of usable capacity. This might still be slightly short. Five 24 TB drives would provide 120 TB of raw capacity and around 115 TB of usable capacity, which would meet the requirement. Always verify the usable capacity of the specific drive models you are considering.

Final Verification Step

Before finalizing the purchase, run the numbers one last time. Check the NVR manufacturer’s documentation for the maximum supported drive capacity. Some NVRs have limits on the total storage they can manage, even if the hardware could physically fit larger drives.

Confirm that the NVR supports the number of drives you plan to install. Check the power supply rating to ensure it can handle the heat and power draw of the selected drives. Review the warranty terms for the hard drives to confirm they are rated for continuous operation.

Once these checks are complete, you can confidently size your storage system for a 90-day retention period.

The power supply is often overlooked. Each 3.5-inch drive draws between 3 to 6 watts depending on the model and workload. If you have ten drives, that is 30 to 60 watts just for the drives. Add the power draw of the NVR’s CPU, RAM, and fans, and the total power requirement can be significant. If the NVR’s power supply is undersized, it may shut down under load, causing data loss and potential drive failure. Check the NVR’s power specifications and ensure the power supply has enough headroom for the number of drives you are installing.

Quick Reference Table

Camera Count	Bitrate per Camera (Mbps)	Daily Data (GB)	90-Day Raw (GB)	With 15% Buffer (GB)	Approximate TB Needed
4	8	345.6	31,104	35,769.6	34.9
8	8	691.2	62,208	71,539.2	69.8
12	8	1,036.8	93,312	107,308.8	104.8
16	8	1,382.4	124,416	143,078.4	139.7
20	8	1,728.0	155,520	178,848.0	174.6