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Docker Cache Efficiency Calculator

Measure how much build time you save from Docker layer cache hits versus misses.

About Docker Cache Efficiency Calculator

Docker caches layers by instruction hash. If a layer's inputs haven't changed, it's served instantly from cache. Time saved = (cachedLayers × avgBuildTime) − (cachedLayers × cacheLoadTime). Higher cache hit rates yield dramatic build speedups.

Formula

Time Saved

saved = cachedLayers × (avgBuildTime − cacheLoadTime)

cachedLayers
Number of cache hits
avgBuildTime
Seconds to build a layer from scratch
cacheLoadTime
Seconds to load from cache

Worked example

With the default values below, Docker Cache Efficiency Calculator returns Time Saved by Cache of 116.0. Change any input above and every figure here is recalculated in your browser.

Inputs used

Total Layersaccepts 1–500 layers
12 layers
Cached Layersaccepts 0–500 layers
8 layers
Avg Layer Build Timeaccepts 0–36,000 seconds
15 seconds
Cache Load Time per Layeraccepts 0–60 seconds
0.5 seconds

Results

Time Saved by Cache
116.0 seconds
Speedup Factor
2.8 ×
Cache Hit Rate
66.7%
Cached Build Time
64.0 seconds

Step by step

  1. 1Cache hit rate= 66.7%
  2. 2Full build (no cache)= 180s
  3. 3With cache= 64.0s
  4. 4Speedup factor= 2.8×

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 or compounding frequency into account.
  • Figures are rounded for display to 1 decimal place, 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 does one changed layer invalidate all subsequent layers?

Docker's layer cache is linear — if layer N changes, layers N+1, N+2, etc. must all rebuild because they could depend on N's output. This is why ordering matters: put stable layers first.

How can I improve cache hits in CI?

Use BuildKit's --cache-from with a registry cache, or mount package manager caches (e.g. --mount=type=cache for npm, pip) so dependencies aren't re-downloaded on every build.

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