Calculate free disk space percentage from total and free or used space, or estimate cleanup needed to reach a target free-space percentage.

GB, MB and TB are decimal (drive labels). GiB, MiB and TiB are binary; Windows shows binary sizes labelled GB.

Cleanup target
Blank omits cleanup. A reached target needs no cleanup.

Free Disk Space Percentage Formula

The calculator uses one of three formulas depending on the calculation 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 = max(0, (Target Free % / 100) × 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.

Amounts are converted to bytes internally. Decimal units use 1 TB = 1000 GB and 1 GB = 1000 MB; binary units use 1 TiB = 1024 GiB and 1 GiB = 1024 MiB. Choose units matching each reading. Free space cannot exceed total capacity, and the target percentage must be between 0 and 100.

Reference Tables

These percentage bands and targets are illustrative planning examples, not universal disk-health or performance thresholds. Required headroom depends on the operating system, drive and workload.

Free % Status What it means
< 10%CriticalLittle available headroom; check workload and update requirements.
10–15%LowReview the workload and available capacity.
15–20%FairSuitability depends on the drive and workload.
> 20%HealthyMore headroom, without a performance guarantee.
Disk type Illustrative free %
Consumer SSD (Windows / macOS)15–20%
HDD with paging file10–15%
Server / database volume20–25%
Boot / system partition20%+

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? Decimal GB and binary GiB are different units. Compare total and available readings for the same volume and select their actual units; reserved space and reporting conventions can also differ.

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? More free space provides capacity for future writes, but there is no universal percentage at which performance gains stop. Consult the drive and operating-system guidance.