Disk Write Time Calculator
How long to fill a disk based on capacity and write throughput (e.g. surveillance cameras).
Disk Capacity and Write Rate
Table of Contents
Comprehensive Disk Write Time Guide
Why Calculate Write Time?
When you record continuous data (surveillance cameras, DVR, NVR, logging), you need to know how long a disk will last before it fills up. This calculator estimates the time to fill a disk based on capacity and sustained write throughput. Essential for planning storage retention, choosing disk size, and configuring overwrite/rotation policies.
How the calculation works
You enter disk capacity (in MB, GB, or TB) and write throughput in bits or bytes per second (Kbps, Mbps, Gbps, or KB/s, MB/s, GB/s). The calculator converts units consistently and divides capacity by write rate to get time in seconds, then displays days, hours, and weeks. For surveillance, use the sum of all camera bitrates; the result is the time until the disk is full at that sustained rate.
- Disk capacity: Total usable space (GB or TB). Subtract ~10% for filesystem and overhead.
- Write throughput (bitrate): Sustained data rate in Mbps or MB/s. For multiple streams, sum all bitrates.
- Mbps vs MB/s: 1 MB/s = 8 Mbps. Video specs often use Mbps; storage I/O uses MB/s.
- Continuous write: Assumes 24/7 recording. Motion-only or scheduled recording extends effective retention.
Surveillance vs other continuous write
Surveillance / NVR / DVR
- Bitrate = sum of all camera streams (e.g. 8 × 4 Mbps = 32 Mbps).
- Use main stream for 24/7 recording; check NVR or camera settings for actual bitrate.
- H.265 halves bitrate vs H.264 at similar quality.
Logging / telemetry / data acquisition
- Single sustained write rate in MB/s or Mbps from your application or device.
- Use peak or average depending on whether writes are constant or bursty.
- Example: 100 MB/s sustained = 1 TB in about 2.8 hours.
Benefits of estimating fill time
- Retention planning: Know how many days of footage or data you can keep before overwrite.
- Disk sizing: Choose capacity to meet retention targets (e.g. 30 days) for your camera count and resolution.
- Overwrite policy: Configure ring buffer and alerts; avoid running full without planning.
- Budget: Compare cost of larger disk vs more cameras or higher resolution.
Limitations and considerations
- Assumes 100% sustained write; real NVR may have gaps, compression variance, or motion-only recording.
- VBR (variable bitrate) can spike; add 20–30% buffer over average bitrate for peaks.
- Usable capacity is less than advertised (filesystem, manufacturer units); subtract 5–10% for planning.
- Disk throughput must exceed write rate; surveillance bitrates are usually low enough that any modern disk suffices.
Use surveillance-grade drives (WD Purple, Seagate SkyHawk) for 24/7 recording—consumer drives are not rated for continuous write and may fail early. Plan for peak bitrate and subtract 5–10% from capacity for filesystem overhead.
Quick formula reference
Time (seconds) = Disk capacity (bytes) ÷ Write rate (bytes per second)
With capacity in GB and rate in Mbps: Time (s) ≈ Capacity × 8 × 1024 ÷ (Mbps ÷ 8). See the Calculation Formula section for exact conversion and examples.
Estimating disk fill time from capacity and write rate is the first step to sizing surveillance or logging storage. Use this calculator with the sum of your camera bitrates or application throughput, then apply the bitrate table and best practices below. Prefer H.265 and surveillance-grade disks for reliable long-term recording.
Surveillance Camera Bitrates
Use these typical bitrates per camera when you don't have exact values from your NVR or camera. Sum all camera bitrates and enter the total in the calculator.
Typical bitrates per camera (H.264/H.265, variable bitrate):
| Resolution / Codec | Typical Bitrate | Notes |
|---|---|---|
| 720p (H.264) | 1-2 Mbps | Standard definition |
| 1080p (H.264) | 2-4 Mbps | Full HD, common |
| 1080p (H.265/HEVC) | 1-2 Mbps | ~50% less than H.264 |
| 4K (H.264) | 8-16 Mbps | High bandwidth |
| 4K (H.265) | 4-8 Mbps | Recommended for 4K |
Example: 8× 1080p H.264 cameras at 4 Mbps = 32 Mbps total. A 4 TB disk at 32 Mbps fills in about 12 days.
Finding Your Camera Bitrate
Check the NVR/DVR web interface, camera settings, or datasheet. Bitrate is often configurable (CBR or VBR with min/max). Use the main stream bitrate for continuous recording; sub-stream is lower and used for preview.
Calculation Formula
Fill time is capacity divided by sustained write rate. Units must match: convert capacity and rate to the same base (e.g. bytes and bytes per second) before dividing.
Time (seconds) = Disk capacity (bytes) ÷ Write rate (bytes per second)
With Mbps: Time = (Capacity in GB × 8 × 1024³) ÷ (Mbps × 1024² ÷ 8) = Capacity × 8 ÷ (Mbps ÷ 8)
Example: 4 TB disk = 4096 GB. At 64 Mbps: Time = 4096 × 8 × 1024³ / (64 × 1024² / 8) = 524,288 seconds ≈ 6.07 days.
Mbps vs MB/s Conversion
1 MB/s = 8 Mbps. A 32 Mbps stream = 4 MB/s. Video bitrates are usually in Mbps; storage benchmarks in MB/s. Use the calculator's unit selector to match your source.
Approximate Retention (Days) by Capacity and Bitrate
Quick reference for common capacity and total bitrate combinations. Values assume continuous write at the given rate; motion-only or lower average bitrate extends retention.
| Capacity | 16 Mbps | 32 Mbps | 64 Mbps | 128 Mbps |
|---|---|---|---|---|
| 1 TB | ~6 days | ~3 days | ~1.5 days | ~18 h |
| 4 TB | ~24 days | ~12 days | ~6 days | ~3 days |
| 8 TB | ~48 days | ~24 days | ~12 days | ~6 days |
| 16 TB | ~96 days | ~48 days | ~24 days | ~12 days |
Use Cases Beyond Surveillance
The same formula—capacity ÷ write rate—applies wherever you have a sustained data stream. Use the calculator with your application's throughput in Mbps or MB/s.
This calculator applies to any continuous write scenario:
- Surveillance / NVR / DVR: Sum of all camera bitrates. Primary use case.
- Logging / telemetry: Servers, IoT, industrial. Write rate in MB/s or Mbps.
- Scientific / data acquisition: Sensors, oscilloscopes. Often MB/s or GB/s.
- Security / forensics: Capture cards, network taps. Sustained throughput.
- Time-lapse / dashcam: Lower bitrate than real-time. Multiply by frame interval.
For burst writes (backups, batch jobs), use average throughput or peak; fill time depends on sustained rate.
CBR vs VBR and Motion Detection
Bitrate mode and recording schedule affect how quickly the disk fills. Plan with average bitrate for VBR and adjust for motion-only or scheduled recording.
CBR (Constant Bitrate) vs VBR (Variable Bitrate)
CBR keeps a fixed bitrate; easier to plan. VBR adapts to scene complexity: low bitrate for static scenes, spikes during motion. Use average VBR for planning, but add 20-30% buffer for peak spikes. Most surveillance uses VBR for efficiency.
Motion Detection and Scheduled Recording
If you record only on motion or during business hours, effective fill time extends. Example: 50% duty cycle (12 h/day) = roughly 2× retention. 20% (motion-only, low activity) = 5× retention. Use the calculator with average bitrate × duty cycle for motion-based systems.
Surveillance-Grade Disks
For 24/7 recording, use drives built for continuous write. Consumer desktop drives have lower workload ratings and can fail prematurely under NVR/DVR load.
Consumer drives are not designed for 24/7 continuous write. Surveillance-grade drives (WD Purple, Seagate SkyHawk, Toshiba S300) have:
- Optimized firmware for sequential writes and multiple streams
- Higher workload rating (e.g. 180 TB/year vs 55 TB/year)
- Better vibration tolerance for multi-drive NVR
- TLER/ERC to avoid drive dropouts in RAID
Using consumer drives for 24/7 surveillance can lead to premature failure. Plan capacity with surveillance drives for reliable retention.
Best Practices
Apply these tips to get accurate fill-time estimates and reliable long-term recording.
- Subtract 5-10% from disk capacity for filesystem, metadata, and manufacturer vs usable size.
- Use H.265/HEVC instead of H.264 when possible - roughly half the bitrate for same quality.
- Motion detection or scheduled recording extends retention - multiply fill time by duty cycle (e.g. 50% = 2× retention).
- Plan for peak bitrate, not average; VBR can spike during motion. Add 20-30% buffer.
- Use WD Purple, Seagate SkyHawk, or similar surveillance-grade drives for 24/7 write workloads.
- Configure overwrite/ring buffer to avoid filling; most NVR/DVR overwrite oldest footage automatically.
- Mbps vs MB/s: 1 MB/s = 8 Mbps. Video specs use Mbps; storage I/O uses MB/s. Use the unit that matches your source.
- Monitor disk usage; set alerts before reaching 90% full to avoid overwrite issues.