Filesystem Utilization Calculator
Determine filesystem capacity utilization and headroom before hitting full disk alerts.
About Filesystem Utilization Calculator
Filesystem utilization is simply used blocks divided by total blocks, matching the Use% column reported by `df`. Because filling a filesystem completely can cause application failures, database corruption, or an inability to even write new log entries, most monitoring conventions apply staged alert thresholds — a soft warning at 80%, a high-priority alert at 90%, and a critical/page-now alert at 95% — giving operators time to react before the filesystem is completely full.
Formula
Utilization percentage
used% = used_blocks / total_blocks × 100
Worked example
With the default values below, Filesystem Utilization Calculator returns Used of 82.00%. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Total Blocksminimum 1
- 26,214,400
- Used Blocksminimum 0
- 21,495,808
Results
- Used
- 82.00%
- Alert Level
- Warning (≥80%)
- Free
- 18.00%
- Free Blocks
- 4,718,592
- Blocks Until 80% Warning
- 0
- Blocks Until 90% High
- 2,097,152
- Blocks Until 95% Critical
- 3,407,872
Step by step
- 1Values used
- 2Utilization percentage
- 3Used= 82.00
- 4Alert Level= Warning (≥80%)
- 5Free= 18.00
- 6Free Blocks= 4,718,592
- 7Blocks Until 80% Warning= 0
- 8Blocks Until 90% High= 2,097,152
Assumptions and limits
- The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Guides that use this calculator
Frequently Asked Questions
Why do monitoring tools warn at 80% instead of waiting until the disk is full?
Filesystems perform worse and fragment more as they approach full capacity, and many systems need free space for temp files, logs, and journaling even during normal operation — so 80% leaves a safety margin to investigate and remediate before things degrade further or writes start failing outright.
Why might df show 100% used even with some blocks reported free?
Most Linux filesystems reserve a small percentage of blocks (commonly 5% on ext4, via tune2fs -m) for the root user, so a non-root process can see 'no space left' before the raw block count hits zero.
Does this account for inode exhaustion?
No — a filesystem can report plenty of free blocks but still be unable to create new files if it has run out of inodes (common with filesystems storing huge numbers of tiny files); check inode usage with `df -i` separately.