NVR Storage Calculator: How Many Days Will It Record?
Eight 4K cameras recording continuously in H.265 at about 6 Mbps each fill a 4 TB surveillance drive in roughly 7 days. Holding 30 days from those eight cameras needs about 16 TB.
Values update as you change the inputs. Where a figure cannot be computed honestly from what manufacturers publish, this calculator says so rather than showing a zero.
Drives that meet this retention target
This grid re-orders itself against the number you just calculated, smallest adequate option first.
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.
Seagate Skyhawk AI 8TB Video Internal Hard Drive
$329.99Built for recorders running analytics on many streams at once, where the drive is being read for detection while it is still writing.
- Capacity
- 8 TB
Prices change often.
Western Digital 8TB WD Purple Pro Surveillance Internal Hard Drive
$399.99The higher workload rating in the WD surveillance line, intended for the largest camera counts.
- Capacity
- 8 TB
Prices change often.
The formula
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 0.93 factor is the part of a drive you actually get after decimal-to-binary conversion and the recorder database.
Common questions
How many days will 8 cameras record on a 4TB drive?
Eight 4K cameras encoding in H.265 at a typical 6 Mbps each, recording continuously, write about 518 GB a day between them. A 4 TB surveillance drive holds roughly 7 days of that footage once the 93 percent usable fraction is applied, which accounts for decimal to binary conversion and the space the recorder database itself takes up.
How big a drive do I need to keep 30 days of footage?
For eight 4K cameras at a typical 6 Mbps H.265 bitrate, recording around the clock, holding 30 days of footage needs roughly 16 to 17 TB of usable capacity. That figure already accounts for the 93 percent usable fraction of a drive marketed capacity, so buy a drive rated at or above that number, not exactly at it.
Does switching from H.264 to H.265 change how long a drive lasts?
Yes. H.264 typically runs about double the bitrate of H.265 for the same resolution, for example 12 Mbps against 6 Mbps on an 8MP camera, so switching a recorder from H.264 to H.265 roughly doubles how many days of footage the same drive holds. The setting costs nothing and most modern recorders and cameras already support it.
Why does a 4 TB drive not hold 4,000 GB of actual footage?
Drives are marketed in decimal terabytes, 1,000,000 MB per TB, while a recorder reserves space for its own database and loses some capacity to decimal to binary conversion. In practice about 93 percent of the marketed figure is usable for footage, so a 4 TB drive gives roughly 3.7 TB of real recording space once retention is calculated.
What happens if my setup does not match a typical bitrate figure?
This calculator uses typical H.265 or H.264 bitrates by resolution as a convention, not a measured figure, since real bitrate depends on scene motion and lighting and can run higher at night. If a recorder own settings page already shows its actual bitrate, the bitrate to storage calculator uses that measured Mbps figure directly instead of the typical convention used here.
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Keep going
A calculated figure is a planning number, not a promise. Every answer here comes from published standards and manufacturer specifications, and real installations vary with light, weather, scene motion and how a camera is actually aimed. Where a figure cannot be computed honestly from what manufacturers publish, this calculator says so rather than showing a zero.
Before you point a camera at anything. Aim at your own property and your own boundary, never into a neighbour's windows. Cameras in bedrooms, bathrooms or anywhere with a reasonable expectation of privacy are a serious legal problem, including for guests 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 enabling it. This is researched general information and not legal advice.