How many days will 2 8MP (4K) cameras record on an NVR?
2 8MP (4K) cameras recording continuously in H.265 use roughly 12 Mbps between them, which fills a 4 TB surveillance drive in about 28.7 days. Holding 30 days of continuous footage from 2 cameras at this resolution needs about 4.2 TB, and 14 days needs about 2 TB.
How this is worked out
days = (drive TB x 1,000,000 MB x 0.93 usable) / (cameras x bitrate Mbps x 450 MB per hour x hours per day)
A stream at 1 Mbps writes 450 MB every hour. The 93 percent factor is the part of a drive you actually get: drives are sold in decimal terabytes while the recorder reports binary and reserves space for its own database, which is the difference between a predicted 30 days and an actual 27.
| Drive size | Continuous, 24 h a day | Motion only, about 8 h a day | Motion only, about 4 h a day |
|---|---|---|---|
| 1 TB | 7.2 | 21.5 | 43.1 |
| 2 TB | 14.4 | 43.1 | 86.1 |
| 4 TB | 28.7 | 86.1 | 172.2 |
| 6 TB | 43.1 | 129.2 | 258.3 |
| 8 TB | 57.4 | 172.2 | 344.4 |
| 10 TB | 71.8 | 215.3 | 430.6 |
| 12 TB | 86.1 | 258.3 | 516.7 |
| 16 TB | 114.8 | 344.4 | 688.9 |
| Codec | Per camera bitrate | Total bitrate | Days on 4 TB | Days on 8 TB |
|---|---|---|---|---|
| H.265 (HEVC) | 6 Mbps | 12 Mbps | 28.7 | 57.4 |
| H.264 (AVC) | 12 Mbps | 24 Mbps | 14.4 | 28.7 |
| Retention target | Continuous recording | Motion only, about 8 h a day |
|---|---|---|
| 7 days | 1 TB | 0.3 TB |
| 14 days | 2 TB | 0.7 TB |
| 30 days | 4.2 TB | 1.4 TB |
| 60 days | 8.4 TB | 2.8 TB |
| 90 days | 12.5 TB | 4.2 TB |
Why the bitrate figure is a convention and not a specification
The 6 Mbps used above is a typical sustained H.265 bitrate for a 8MP (4K) camera, and it is an industry habit rather than a published standard. Real bitrate moves with the scene: a hedge thrashing in wind at night can push a camera well above this figure, and a still indoor hallway can sit at half of it. Variable bitrate encoding, which almost every camera uses by default, means the number changes minute to minute.
The practical consequence is to treat these retention figures as a planning estimate with perhaps 30 percent either way, and to buy the next drive size up if the difference between the two is a week of retention you would miss.
Use a surveillance rated drive, not a desktop one
A recorder writes from every camera continuously and forever, which is a duty cycle a desktop drive is not rated for and fails early doing. Surveillance drives such as the Seagate Skyhawk and WD Purple lines are built for sustained multi-stream writes and are tuned to keep writing rather than to pause for error recovery, which on a recorder would mean dropped frames.
They are not dramatically more expensive, and a drive failure on a security recorder is discovered at exactly the moment the footage was needed.
Drives sized for this system
Ordered by how closely the capacity matches the 4.2 TB this configuration needs for 30 days.
Seagate Skyhawk 4TB Video Internal Hard Drive
$159.99Rated for continuous write from multiple camera streams, which is the entire difference between a surveillance drive and the cheaper desktop drive beside it.
- Capacity
- 4 TB
Prices change often.
Western Digital 4TB WD Purple Surveillance Internal Hard Drive, 256 MB Cache
$189.99The direct WD equivalent of the Skyhawk, with a larger cache that helps when many streams are writing at once.
- Capacity
- 4 TB
Prices change often.
Seagate Skyhawk 8TB Video Internal Hard Drive
$309.99Doubling the drive doubles the retention, and retention is the specification people regret after the one event they needed it for.
- Capacity
- 8 TB
Prices change often.
Western Digital 8TB WD Purple Surveillance Internal Hard Drive, 256 MB Cache
$354.99Eight terabytes of surveillance rated storage, the usual pairing with a sixteen channel recorder.
- Capacity
- 8 TB
Prices change often.
Common questions
What size drive do I need for 2 8MP (4K) cameras?
For 30 days of continuous recording you need about 4.2 TB, so the next standard drive size up from that is the sensible purchase. For 14 days, which covers a week away from home with margin, about 2 TB is enough. Recording on motion only rather than continuously roughly triples both figures in your favour.
Does the recorder overwrite old footage automatically?
Yes. Every network video recorder writes in a loop and overwrites the oldest footage once the drive is full, so it never stops recording because of a full disk. That is why retention is measured in days rather than in free space, and why the drive size question is really a question about how far back you want to be able to look.
Is it better to record continuously or only on motion?
Continuously, if the drive allows it. Motion detection can miss a slow approach, a person already in frame when recording starts, or anything the detection zone did not cover, and footage that was never written cannot be recovered. Motion-only recording multiplies retention by roughly three at an 8 hour duty cycle, which is worth taking only when a larger drive is genuinely not an option.
Can I add a second drive later?
It depends on the recorder. Most eight channel units take a single drive, while many sixteen channel units take two. Check the bay count before buying, because the upgrade path on a one-bay recorder is to replace the drive rather than add to it, and that means copying or losing the footage already on it.
Get the system planning sheet
The camera count, storage and PoE budget worksheet, plus the wiring checklist, as one printable page.
Keep going
- NVR storage calculator
- Bitrate to storage calculator
- Typical bitrate by resolution
- Best surveillance hard drives
- How many days will 2 12MP cameras record on an NVR?
- How many days will 2 5MP cameras record on an NVR?
- How many days will 4 2MP (1080p) cameras record on an NVR?
- How many days will 6 8MP (4K) cameras record on an NVR?
Before you point a camera at anything. Aim cameras at your own property and your own boundary, never into a neighbour's windows or across their garden. Cameras in bedrooms, bathrooms, or anywhere else a person reasonably expects privacy are a serious legal problem, and that includes guests, lodgers and anyone who works in your home. Audio is legally different from video, many US states require the consent of every party to a recorded conversation, and several recorders capture audio by default, so check your own state before you enable it. Some places require visible notice that recording is taking place, and landlords and homeowner associations often impose their own rules on top. This is researched general information and not legal advice, and the law varies by state and by country.
And once it is installed. Change every default password, keep the firmware updated, and be clear with yourself about where the footage goes: a cloud camera means a third party holds recordings of your home, while a local recorder keeps them in your house. That is a genuine buying consideration, not a technicality.