Filesystem Overhead Calculator
Estimate metadata and journaling overhead a filesystem subtracts from raw partition capacity.
About Filesystem Overhead Calculator
Formatting a raw block device with a filesystem always sacrifices some capacity to superblocks, inode tables, journals, checksums and reserved blocks. ext4 reserves 5% for root by default plus inode table overhead (roughly 1.5-5% total depending on inode ratio); XFS is leaner (0.5-1%) since it allocates metadata dynamically; Btrfs and ZFS trade more overhead (3-9%) for checksumming, copy-on-write metadata and reserved 'slop space' that keeps copy-on-write operations from deadlocking when nearly full.
Formula
Usable capacity
usable_capacity = total_capacity × (1 − overhead%)
- C
- total raw capacity
- o
- overhead fraction for the filesystem type
Worked example
With the default values below, Filesystem Overhead Calculator returns Estimated Usable Capacity (GB) of 967.50. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Total Raw Capacity (GB)minimum 0
- 1,000
- Filesystem Typeone of ext4 (1.5% - 5%), XFS (0.5% - 1%), Btrfs (3% - 6%), ZFS (5% - 9%, incl. reserved slop space), Custom
- ext4 (1.5% - 5%)
- Custom Overhead (%)accepts 0–20
- 3
Results
- Estimated Usable Capacity (GB)
- 967.50
- Overhead (GB)
- 32.50
- Overhead Percentage
- 3.25%
- Usable Capacity Range
- 950 - 985 GB
Step by step
- 1Values used
- 2Usable capacity
- 3Estimated Usable Capacity (GB)= 967.50
- 4Overhead (GB)= 32.50
- 5Overhead Percentage= 3.25
- 6Usable Capacity Range= 950 - 985 GB
Assumptions and limits
- The result is derived only from the 3 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 does df show less space than the disk's rated capacity?
Part of the gap is decimal-vs-binary unit labeling (a '1TB' drive is ~931 GiB), and part is genuine filesystem overhead: metadata structures, journal space, and — for ext4 — a default 5% root-reserved block pool that prevents the root user from being locked out when the disk fills.
Can I reduce ext4's reserved block percentage?
Yes, with tune2fs -m <percent> /dev/xxx. Lowering it from the 5% default reclaims usable space but removes the safety margin that lets root-owned daemons keep writing (e.g. logging) when regular users see a full disk.
Why does Btrfs have higher overhead than XFS?
Btrfs stores checksums for data and metadata, keeps multiple metadata copies (dup profile by default on single devices), and uses copy-on-write which temporarily needs extra free space for atomic updates — all of which XFS's simpler B+tree metadata layout avoids.