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Memory Usage Breakdown Calculator

Work out memory usage breakdown instantly with clear inputs, formula shown and shareable results.

About Memory Usage Breakdown Calculator

Linux deliberately uses spare RAM for page cache, so a low free figure is healthy rather than alarming. The number that matters is available, which counts free memory plus the cache the kernel can reclaim without swapping — roughly 80 percent of it, since some pages are dirty or pinned. Reading used and free without available is the classic misdiagnosis.

Formula

Memory accounting

used = application RSS + kernel and slab; free = total - used - page cache; available ~ free + 0.8 x reclaimable cache

Worked example

With the default values below, Memory Usage Breakdown Calculator returns Memory available for new allocations of 19.40. Change any input above and every figure here is recalculated in your browser.

Inputs used

Total RAMminimum 0.1 GB
64 GB
Application resident memoryminimum 0 GB
38 GB
Page cacheminimum 0 GB
18 GB
Kernel and slab memoryminimum 0 GB
3 GB

Results

Memory available for new allocations
19.40 GB
Memory genuinely in use
41.00 GB
Completely unused memory
5.00 GB
Reclaimable page cache
14.40 GB

Step by step

  1. 1Values used
  2. 2Memory accounting
  3. 3Memory available for new allocations= 19.40 GB
  4. 4Memory genuinely in use= 41.00 GB
  5. 5Completely unused memory= 5.00 GB
  6. 6Reclaimable page cache= 14.40 GB

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

Why is free memory always low?

Because unused RAM is wasted RAM. The kernel fills it with cached file data, which is discarded instantly when an application needs the memory.

Which field should monitoring alert on?

MemAvailable from /proc/meminfo. Alerting on free produces constant false alarms on healthy systems.

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