If you’ve ever shopped for an NVR or a UniFi Cloud Key with cameras, you’ve probably stared at the drive bay options and tried to do the math in your head. Four cameras at 4K, 30 days of footage, motion-only or 24/7 — how many terabytes is that, really? The answer varies wildly depending on a handful of choices most people don’t know they’re making.
This is the calculation post. If you’re speccing an NVR for a new build in Lehi or upgrading the dead hard drive in a five-year-old system in Sandy, this is what you actually need to know to pick the right disk.
The starting point: bitrate is what matters
Storage usage is just bitrate × time. A camera streaming at 8 Mbps for 24 hours produces:
8 Mbps × 86,400 seconds = ~691,200 megabits ≈ 86 GB per day per camera.
Bitrate is the actual unit you have to estimate. It depends on resolution, frame rate, codec, scene complexity, and the camera’s encoder quality. We have a separate post on how many megapixels you actually need — read that first if you’re still picking cameras. This post assumes you’ve already settled on resolution and need to budget the disk.
Typical bitrates by resolution and codec
Real-world bitrates from cameras we install — UniFi Protect, Hikvision, Dahua — at default quality settings with normal scene complexity:
- 1080p (2 MP) H.264 at 15 fps: ~2–3 Mbps
- 1080p H.265 at 15 fps: ~1–1.5 Mbps
- 4 MP H.264 at 15 fps: ~4–6 Mbps
- 4 MP H.265 at 15 fps: ~2–3 Mbps
- 4K (8 MP) H.264 at 15 fps: ~10–14 Mbps
- 4K H.265 at 15 fps: ~5–7 Mbps
- 4K H.265 at 30 fps: ~10–14 Mbps
Two things jump out. First, H.265 (also called HEVC) roughly halves the bitrate compared to H.264 at the same quality. Second, frame rate matters as much as resolution — 4K at 30 fps is double the bitrate of 4K at 15 fps. For most residential surveillance, 15 fps is plenty. You’re trying to identify people, packages, and vehicles, not analyze NBA replays.
The math: 4 cameras, 30 days, three scenarios
Scenario A: 4 cameras at 1080p H.265, 24/7 recording
Bitrate per camera: ~1.2 Mbps average. Total system bitrate: 4.8 Mbps. Per day: 4.8 × 86,400 / 8 = ~52 GB. For 30 days: ~1.5 TB.
A 2 TB drive comfortably handles this with room for growth. This is a typical “four-corner” small home setup — front door, back door, driveway, garage.
Scenario B: 4 cameras at 4K H.265 at 15 fps, 24/7 recording
Bitrate per camera: ~6 Mbps average. Total: 24 Mbps. Per day: 24 × 86,400 / 8 = ~260 GB. For 30 days: ~7.8 TB.
Round up to an 8 TB drive minimum. A 10 TB or 12 TB drive gives you margin for the inevitable case where you forget you set quality to “high” on one camera.
Scenario C: 4 cameras at 4K H.265 at 15 fps, motion-only
Motion-only recording dramatically reduces storage — but by how much depends entirely on how busy the scene is. A driveway camera in a quiet cul-de-sac in Holladay might trigger 30–60 minutes of recording per day. A camera covering a street-facing yard on a busy Cottonwood Heights road might trigger 6–10 hours a day.
Realistic averages we see: motion-only cuts effective recording time to roughly 15–25% of 24/7. Apply that to scenario B:
7.8 TB × 0.20 = ~1.6 TB for 30 days at 4K motion-only across 4 cameras.
A 4 TB drive is plenty, with margin. The catch is that motion-only saves storage at the cost of missing things — if a person stands still on the porch, or if motion detection is poorly tuned, you have gaps in the timeline. We almost always recommend 24/7 recording with motion flagging for review, not motion- only recording. Disk is cheap. Missed footage during a break-in isn’t.
The numbers nobody tells you
Audio adds ~5–10%
If your cameras record audio (and most should — audio is often more useful than video for context), add roughly 64–128 kbps per camera. For 4 cameras over 30 days, that’s another 50–200 GB on top of whatever the video math gave you. Negligible at 4K, more noticeable at 1080p.
The drive’s usable capacity is less than the sticker
A “10 TB” drive shows up in your NVR as roughly 9.1 TB after binary-vs-decimal conversion (TB vs TiB) and filesystem overhead. UniFi Protect and most other NVRs reserve another small percentage for system use. Plan for ~88% of the sticker capacity being actually usable.
NVRs treat “30 days” as a soft target
Most NVRs work as a circular buffer — when the disk is full, the oldest footage is overwritten. So if you spec for 30 days but the cameras actually generate more bitrate than you estimated (lots of motion, dark scenes, rain), you get fewer days. Build in 25–50% margin on the disk size for that exact reason. A drive that hits 95% full and recycles every 22 days instead of 30 is the most common “why don’t I have last month’s footage” complaint we field.
Single drive, mirror, or RAID?
Single drive — fine for most homes
A single surveillance-rated drive (Western Digital Purple, Seagate SkyHawk) is what most residential NVRs ship with, and for most homes it’s the right answer. The drive is a circular buffer; the worst case is you lose the last 30 days of footage if the disk fails, which is annoying but rarely catastrophic.
RAID 1 mirror — for properties where footage matters
If you’re running cameras at a short-term rental, a small business, or a property where footage might matter for an insurance claim or police report, RAID 1 doubles the cost but gives you survival when one drive fails. The UniFi UNVR and UNVR Pro support RAID natively; most generic NVRs do too.
RAID 5 / RAID 6 — large camera systems only
For 8+ camera installs at 4K — large Park City rental properties or commercial sites — RAID 5 or 6 across 4–8 drives gives you capacity and redundancy. The UNVR Pro with 7-bay support is the residential ceiling for this; beyond that you’re into rack-mount NVR territory.
SSD vs HDD: HDD for storage, SSD only for the OS
Surveillance is a write-heavy workload — every camera is constantly streaming data to the disk. Consumer SSDs (and even most prosumer ones) wear out fast under that workload. Surveillance-rated spinning drives (WD Purple, Seagate SkyHawk) are designed for 24/7 writes and last 5–7 years in this use case. Some NVRs use a small SSD or eMMC for the OS and a large HDD for footage, which is the right pattern.
Quick lookup table
Approximate disk size for 30 days of footage, 24/7 recording, H.265, with ~30% headroom for bitrate variance and filesystem overhead:
- 4 cameras at 1080p: 2 TB drive
- 4 cameras at 4 MP: 4 TB drive
- 4 cameras at 4K: 10 TB drive
- 8 cameras at 1080p: 4 TB drive
- 8 cameras at 4 MP: 8 TB drive
- 8 cameras at 4K: 18–20 TB drive (or RAID array)
- 16 cameras at 4K: RAID 5 across 4–6 large drives, ~40 TB usable
For 60 days instead of 30, double the numbers. For 90 days, triple. Beyond that, you’re looking at a second NVR or offsite backup.
What we actually install
For a typical 4–6 camera residential install on UniFi Protect, we use a Cloud Gateway Max or UNVR with a 4–8 TB WD Purple drive and 30-day retention at 4 MP H.265. This lands in the 60–75% disk utilization range with healthy headroom.
For larger installs that match the “$5,000 home security system” tier — 8–12 cameras, 4K, 60-day retention — we step up to a UNVR Pro with two large surveillance drives in RAID 1, or a four-drive RAID 5 if the client wants both capacity and redundancy.
For commercial properties or short-term rentals where losing footage is unacceptable, we mirror to a second disk and configure the NVR to alert on drive degradation. Western Digital Purple drives are warranted for surveillance use and report SMART data the NVR can surface — when one starts predicting failure, we replace it before it goes.
The local vs cloud question
One more thing worth saying: all of this only matters if you’re running a local NVR, which we strongly recommend over cloud-only systems. The reasons we walk through in our UniFi Protect vs Ring vs Nest comparison — privacy, no monthly fees, footage that survives an ISP outage, no subscription cliff if you stop paying — all lead to the same place: own the storage. The disk math in this post is the cost of doing it right, and it’s still less over five years than the equivalent cloud subscription.
Bottom line
For 4 cameras at 1080p H.265 over 30 days, a 2 TB drive is enough. For 4 cameras at 4K, plan for 8–10 TB. H.265 cuts storage in half versus H.264. 24/7 is always more reliable than motion-only. Surveillance- rated HDDs are the right disk. Add 25–50% headroom for real-world bitrate variance.
If the math feels like a lot, that’s normal — and it’s the kind of thing we sort out during a proper site survey so the system gets sized right the first time.
Keystone Integration installs UniFi Protect and surveillance systems across Sandy, Draper, Lehi, and the rest of the Wasatch Front — sized correctly the first time, with retention that matches what the client actually needs. See our full service list or get in touch for a camera-system scoping call.