How to Calculate CCTV Storage: NVR Hard Drive Sizing Guide
One of the most common mistakes in CCTV planning is underestimating storage. Run out of space and your NVR either stops recording or overwrites footage before you can review it. This guide gives you a simple formula to calculate exactly how much hard drive space you need.
The Storage Formula
The amount of storage required depends on four variables:
Storage (GB) = Bitrate (Mbps) × Cameras × Seconds per day × Retention days ÷ 8,000,000
Let's break down each variable.
Bitrate by Resolution
Bitrate is the amount of data each camera generates per second. Typical values at standard compression (H.265):
| Resolution | Bitrate (H.265) | Bitrate (H.264) |
|---|---|---|
| 1080p (2MP) | 1–2 Mbps | 2–4 Mbps |
| 4MP | 2–3 Mbps | 4–6 Mbps |
| 5MP | 3–4 Mbps | 6–8 Mbps |
| 8MP (4K) | 6–8 Mbps | 12–16 Mbps |
Always use H.265 compression where supported — it delivers the same image quality at roughly half the storage of H.264.
Worked Example
Let's calculate storage for a typical office installation:
- 16 cameras at 1080p (H.265) — 1.5 Mbps each
- 24 hours recording per day = 86,400 seconds
- 30 days retention
Calculation:
1.5 × 16 × 86,400 × 30 ÷ 8,000,000 = 7.78 TB
Round up to 8 TB and add 20% headroom → use a 10 TB drive.
Practical Tips
- Always add 20–30% headroom above your calculated figure — never fill an NVR drive to 100%.
- Use motion-triggered recording for cameras in low-activity areas — this can reduce storage needs by 40–60%.
- H.265+ or Smart Coding can further cut storage by analysing static background and compressing unchanged areas more aggressively.
- Use surveillance-grade hard drives (such as Seagate SkyHawk or Western Digital Purple) — standard desktop drives are not designed for 24/7 write cycles and will fail prematurely.
Key Takeaways
- Use the formula: Bitrate × Cameras × Seconds/day × Days ÷ 8,000,000
- H.265 compression uses roughly half the storage of H.264 at the same quality
- Always add 20-30% headroom above your calculated requirement
- Motion-triggered recording can reduce storage needs by 40-60%
- Use surveillance-grade drives (Seagate SkyHawk, WD Purple) for 24/7 operation