Everyone wants “30 days of footage.” Almost nobody checks whether their NVR can actually hold it — until the recordings silently roll off at day 12 and nobody notices for a month.
Retention math is not complicated. It’s just multiplication, and the two numbers that matter are bitrate and capacity.
The formula
storage needed (GB) = (total bitrate in Mbps ÷ 8) × retention days × 86400 seconds ÷ 1000Simplify: 1 Mbps of sustained recording ≈ 10.8 GB per day.
So a rule of thumb:
| Camera load | Bitrate | Per day |
|---|---|---|
| 1 × 1080p @ 4 Mbps | 4 Mbps | ~43 GB |
| 4 × 1080p @ 4 Mbps | 16 Mbps | ~173 GB |
| 8 × 1080p @ 4 Mbps | 32 Mbps | ~345 GB |
| 8 × 4K @ 12 Mbps | 96 Mbps | ~1 TB |
Thirty days of 8 × 1080p needs roughly 10 TB. That’s a UNVR with two drives — not a single 4 TB drive.
Where the estimates break
The bitrate column is where everyone gets surprised:
- Default codec settings — H.264 at high quality eats bandwidth. H.265 or H.265+ can cut bitrate 30–50% on static scenes.
- High motion scenes — a busy street or a retail floor has higher bitrate than a hallway. Cameras are not uniform.
- Frame rate — 30 fps doubles the data of 15 fps for marginal benefit on most scenes. 15 fps is usually fine for security.
- “Continuous + events” — UniFi records continuously by default; smart detection reduces the review workload, not the storage.
Sizing for reality
Do the math on the worst camera in the busiest scene, then multiply by your camera count and add 20% headroom. A safe practical workflow:
- Write down your target retention in days.
- Estimate bitrate per camera (ask the installer, or measure in Protect for a week).
- Compute total TB needed.
- Buy storage that fits — or adjust the codec/fps/resolution until it fits.
The NVR is a network citizen, too
Storage planning is half the battle. The other half is the network path: NVRs are chatty, and cameras on a saturated switch port will drop frames before you see it in the retention stats. That’s why surveillance gets its own VLAN with enough bandwidth — it’s not just security segmentation, it’s performance.
The takeaway
Retention isn’t a setting you hope for — it’s a number you engineer. If you don’t know your current bitrates, your retention estimate is a guess. We size NVRs to the goal, not to the drive that happened to be in stock.
Want your retention math done properly? Scope it here — we’ll audit bitrates, size storage, and put it in the documentation so it stays true.