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Block Size Overhead Calculator

Work out block size overhead instantly with clear inputs, formula shown and shareable results.

About Block Size Overhead Calculator

Files occupy whole blocks, so a 3,400-byte file in a 4 KB block filesystem wastes 696 bytes of slack. Multiplied across hundreds of thousands of small files that becomes significant, and metadata adds more. Larger blocks improve sequential throughput but worsen slack, which is why 4 KB remains the general-purpose default.

Formula

Block allocation

blocks per file = ceil(size / block size); allocated = blocks x block size + metadata; slack = allocated - actual size

Worked example

With the default values below, Block Size Overhead Calculator returns Space actually consumed of 2.03 GiB. Change any input above and every figure here is recalculated in your browser.

Inputs used

Number of filesminimum 1
500,000
Average file sizeminimum 1 bytes
3,400 bytes
Filesystem block sizeaccepts 512–65,536 bytes
4,096 bytes
Metadata per fileminimum 0 bytes
256 bytes

Results

Space actually consumed
2.03 GiB
Space wasted on slack and metadata
453.95 MiB
Internal fragmentation per file
696 B
Overhead relative to file data
28.00%

Step by step

  1. 1Values used
  2. 2Block allocation
  3. 3Space actually consumed= 2,176,000,000
  4. 4Space wasted on slack and metadata= 476,000,000
  5. 5Internal fragmentation per file= 696
  6. 6Overhead relative to file data= 28.00

Assumptions and limits

  • The result is derived only from the 4 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.

Frequently Asked Questions

Does any filesystem avoid slack for small files?

Btrfs and ext4 with inline data can store very small files inside the inode itself, and ZFS uses variable block sizes, all of which reduce slack considerably.

When are larger blocks worthwhile?

For large sequential files such as video or database extents, where fewer, larger allocations reduce metadata and improve throughput.

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