The video surveillance storage formula
Storage (GB) = cameras × bitrate (Mbps) × 3600 × hours per day × days ÷ 8000. Dividing by 8 turns bits into bytes and dividing by 1000 turns MB into GB. Bitrate matters most: if it doubles, the drive you need doubles too.
| Resolution | Typical H.265 bitrate | GB per day per camera | 4 cameras, 30 days |
|---|---|---|---|
| 1080p (2 MP) | 2 Mbps | 21.6 GB | 2.592 TB |
| 4 MP | 4 Mbps | 43.2 GB | 5.184 TB |
| 4K (8 MP) | 8 Mbps | 86.4 GB | 10.368 TB |
Calculated examples; rounded values.
How to choose the drive from the result
The result is the space the footage takes, not the drive you should buy. Add at least 15-20% headroom: the NVR reserves space, drive makers sell decimal TB while the system shows TiB, and real bitrate rises at night or in the rain because of image noise. For 5.2 TB a 6 TB drive is tight; an 8 TB drive leaves room to add cameras.
What bitrate is and where to find it
Bitrate is the amount of data per second each camera sends. You will find it in the video settings of the camera or recorder, next to resolution, frames per second (FPS) and codec. If the camera uses variable bitrate (VBR), note the average the NVR reports over several days, not the configured maximum.
Three settings change almost everything. Resolution: more pixels mean more data. FPS: 15 FPS is usually enough for surveillance and 30 FPS nearly doubles usage. And codec: H.265 takes 30% to 50% less space than H.264 at similar quality, and many brands add «smart» modes that cut bitrate further when the scene is still.
Continuous or motion recording
Recording 24 hours a day is the safest choice, but it also uses the most space. If you record on motion only, enter the real hours of activity per day in the calculator. A home entrance may see one or two hours of motion a day; a street or a shop, ten or more.
To estimate it without guessing, record for a week, check how much space was used and divide by seven. You get the real GB per day, which already includes the effect of night, wind or car headlights.
How many days of footage to keep
For a home, 7 to 15 days is usually enough: if something happens, you normally review it within days. Businesses often keep 30 days. Data protection rules in many European countries limit how long surveillance footage may be kept; in Spain the general maximum is one month unless the images are needed to prove a specific incident.
More days is not always better: besides the cost of the drive, keeping footage longer than needed increases your data protection obligations.
RAID and drives for an NVR
If the recorder takes two or more drives, you can use RAID 1 so a drive failure does not erase the footage. You then need twice the raw capacity: two 6 TB drives in RAID 1 give 6 TB usable. With three or more drives RAID 5 uses space better; the RAID calculator gives the exact usable capacity.
Choose drives designed for surveillance, such as the WD Purple or Seagate SkyHawk ranges. They are built to write non-stop 24 hours a day and handle several video streams at once better than a desktop drive.
Full example: 8 cameras in a shop
Eight 4 MP cameras with H.265 at 4 Mbps, recording 24 hours for 30 days: 8 × 4 × 3600 × 24 × 30 ÷ 8000 = 10,368 GB, about 10.4 TB. With 20% headroom that is about 12.4 TB.
Two 8 TB drives without RAID (16 TB) cover it, but one failure takes half the footage with it. Three 8 TB drives in RAID 5 also give 16 TB usable and keep working if one fails: the most balanced choice for a business.
Frequently asked questions
What happens when the drive is full?
Almost every NVR overwrites the oldest footage. That is why retention depends directly on drive size: with half the space you keep half the days.
Does camera quality affect storage?
Yes. Two 4 MP cameras can have different bitrates depending on sensor, compression and scene. Use the bitrate your equipment reports, not just the resolution.
Can I record to the cloud instead of a drive?
Yes, but each camera uploads video non-stop. Four cameras at 4 Mbps need a stable 16 Mbps upload just for video; check it with the transfer time calculator.