Calculate free disk space percentage from total and free or used space, or estimate cleanup needed to reach a target free-space percentage.
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Free Disk Space Percentage Formula
The calculator uses one of three formulas depending on the tab you select.
Free space to percentage:
Free % = (Free Space / Total Capacity) * 100
Used space to percentage:
Free % = ((Total Capacity - Used Space) / Total Capacity) * 100
Cleanup target:
Cleanup Needed = (Target Free % * Total Capacity) - Current Free Space
- Total Capacity — the full size of the disk or volume.
- Free Space — unallocated or available space on the disk.
- Used Space — space currently occupied by files and the OS.
- Target Free % — the percentage you want to keep free after cleanup.
- Cleanup Needed — amount of data to delete or move to hit the target.
All values are converted to GB internally using 1 TB = 1024 GB, 1 GB = 1024 MB, 1 MB = 1024 KB. Free space cannot exceed total capacity, and the target percentage must be between 0 and 100.
Reference Tables
Use these tables to interpret the result and pick a sensible cleanup target.
| Free % | Status | What it means |
|---|---|---|
| < 10% | Critical | Performance drops, updates may fail. |
| 10–15% | Low | Clean up soon to avoid issues. |
| 15–20% | Fair | Workable but tight on SSDs. |
| > 20% | Healthy | Comfortable headroom. |
| Disk type | Suggested free % |
|---|---|
| Consumer SSD (Windows / macOS) | 15–20% |
| HDD with paging file | 10–15% |
| Server / database volume | 20–25% |
| Boot / system partition | 20%+ |
Example
You have a 512 GB SSD with 64 GB free.
Free percentage = (64 / 512) × 100 = 12.5%. That is in the low range.
To reach a 20% target: cleanup = (0.20 × 512) − 64 = 102.4 − 64 = 38.4 GB to free up.
FAQ
Why does my OS show a different percentage? Operating systems sometimes report capacity in decimal GB (1 GB = 1,000,000,000 bytes) while showing free space in binary GiB. Small mismatches are normal.
Should I count reserved space? If your OS reserves space for snapshots or recovery, treat that as used. The calculator only sees the numbers you enter.
Is more free space always better? SSDs benefit from headroom for wear leveling, but past 25–30% free the gains are minor.
