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Autoscaling Threshold Calculator

Work out autoscaling threshold instantly with clear inputs, formula shown and shareable results.

About Autoscaling Threshold Calculator

An autoscaler must trigger early enough that new capacity arrives before the target is breached. If traffic grows 25 percent a minute and an instance takes two minutes to become ready, load rises about 56 percent during startup, so the threshold has to sit well below the target. Long startup times force low thresholds, which is why image and warm-up optimisation is a capacity lever.

Formula

Scale-out threshold

growth during startup = (1 + per-minute growth)^(startup minutes) - 1; threshold = target / (1 + growth during startup)

Worked example

With the default values below, Autoscaling Threshold Calculator returns Scale-out threshold of 38.4%. Change any input above and every figure here is recalculated in your browser.

Inputs used

Scaling target utilisationaccepts 10–95 %
60 %
Instance startup timeminimum 1 s
120 s
Fastest traffic growthaccepts 0.1–500 %
25 %
Current instancesminimum 1
10

Results

Scale-out threshold
38.4%
Load growth while an instance boots
56.3%
Instances to add per scaling step
6
Startup time
2.00 min

Step by step

  1. 1Values used
  2. 2Scale-out threshold
  3. 3Scale-out threshold= 38.4
  4. 4Load growth while an instance boots= 56.3
  5. 5Instances to add per scaling step= 6
  6. 6Startup time= 2.00 min

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 does startup time matter so much?

It is dead time during which load keeps rising. Halving startup roughly halves the safety margin you must give up in the threshold.

How is flapping avoided?

Use a cooldown after scaling, asymmetric thresholds so scale-in is more conservative than scale-out, and scale in by one instance at a time.

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