Estimate disk usage from average file size and file count, find how many files fit in available storage, or check used and free drive space.
Disk Usage Formulas
The calculator uses a different formula for each mode.
Storage needed
Total = AvgSize * Count Buffered = Total * 1.10
Files that fit
Files = floor(Available / AvgSize)
Drive status
Used% = Used / Capacity * 100 Free = Capacity - Used
- AvgSize — average size of one file
- Count — number of files
- Available — free space on the target drive
- Capacity — total drive size
- Used — space already occupied
All values are converted to bytes using binary units (1 KB = 1,024 bytes, 1 MB = 1,048,576 bytes, and so on). The 10% buffer in the first formula covers filesystem overhead and small variations in file size. Drive manufacturers list capacity in decimal units, so a “1 TB” drive shows about 931 GB in your operating system.
Typical File Sizes and Drive Health Thresholds
Use these as defaults when you do not have an exact average.
| File type | Typical size |
|---|---|
| Word document or short PDF | 0.1 to 1 MB |
| MP3 song (3 min) | 3 to 5 MB |
| Phone photo (JPEG) | 2 to 5 MB |
| RAW photo (DSLR/mirrorless) | 20 to 50 MB |
| 1 minute, 1080p video | ~130 MB |
| 1 minute, 4K video | ~375 MB |
| PC game install | 30 to 150 GB |
| Disk image / VM | 5 to 100 GB |
| Used | Status | What to do |
|---|---|---|
| Below 80% | Healthy | No action needed. |
| 80 to 90% | Getting full | Clear caches, archive old files, or upgrade. |
| Above 90% | Critical | Free space now. SSDs slow down and updates can fail. |
Worked Example
You want to back up 1,200 phone photos averaging 4 MB each.
Total = 4 MB × 1,200 = 4,800 MB ≈ 4.69 GB.
With buffer = 4,800 × 1.10 = 5,280 MB ≈ 5.16 GB.
An 8 GB drive fits this comfortably. A 4 GB drive does not.
FAQ
Why does my new “1 TB” drive show 931 GB?
Drive makers count 1 TB as 1,000,000,000,000 bytes. Operating systems count 1 TB as 1,099,511,627,776 bytes. The drive is the right size; the unit is different.
Why include a 10% buffer?
File sizes vary, filesystems use space for metadata, and you usually need headroom for temporary files during writes. A flat buffer is simpler than tracking those individually.
Should I keep an SSD below 80% used?
Yes. SSDs need free blocks for wear leveling and garbage collection. Performance drops noticeably as the drive fills past about 85%.
