Security Camera Storage Calculator — NVR & CCTV Sizing
A security camera storage calculator multiplies camera count, bitrate and retention days into the drive capacity an NVR needs — the calculation every CCTV quote should show but rarely does.
Quick answer: Storage = cameras × bitrate × 24 h × days ÷ 8. Four 4 MP cameras at 4 Mbps recording continuously need about 173 GB per day, 2.4 TB for 14 days — or roughly half that with H.265 and motion-triggered recording.
Count every camera that records to this storage
Check the camera/NVR spec sheet
24 = continuous; motion-only is often ~4–8
14–30 days is common
| Storage | Retention |
|---|---|
| 32 GB | 0.353 days |
| 64 GB | 0.705 days |
| 128 GB | 1.41 days |
| 256 GB | 2.82 days |
| 512 GB | 5.64 days |
| 1 TB | 11 days |
| 2 TB | 22 days |
| 2 MP (1080p) | ~2 Mbps per camera → 21.6 GB/day continuous |
|---|---|
| 4 MP | ~4 Mbps per camera → 43.2 GB/day continuous |
| 4K (8 MP) | ~8 Mbps per camera → 86.4 GB/day continuous |
| 4 cameras 4 MP, 14 days | ≈ 2.4 TB (≈ 1.2 TB with H.265 + motion) |
| 8 cameras 4K, 30 days | ≈ 20.7 TB continuous — tier the bitrates instead |
Continuous versus motion-triggered
24/7 recording is the safest but the most expensive: a camera watching an empty corridor burns the same bits as one watching a busy street. Motion-triggered recording with pre-roll typically captures 10–30% of the day and cuts storage by 60–80%, at the cost of gaps if the detector misses an event. Many NVRs allow a hybrid: continuous low-bitrate substream plus full-quality event clips.
H.265 is free capacity
At matched visual quality, H.265 (HEVC) needs roughly 40–50% fewer bits than H.264, and smart codecs (H.265+, Smart H.265) cut idle-scene bitrate further still. Switching a four-camera system from H.264 to H.265 often removes the need to buy a bigger drive — the single highest-value change in a CCTV budget.
Sizing the drive and the retention window
Retention requirements come from insurers, employers or common practice: homes usually want 7–14 days, retail 30 days, some regulated sites 90 days. Choose retention first, then calculate the drive, and always leave 10% headroom — NVR file systems slow badly past 90% full, and overwriting starts earlier than the arithmetic suggests.
When to use this calculator
- Quoting or checking an NVR hard drive size before purchase.
- Deciding whether to record continuously or on motion only.
- Estimating how much a resolution upgrade will cost in storage.
- Working out whether H.265 support justifies a newer NVR.
- Comparing cloud versus local CCTV storage costs.