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Auto Scaling Instance Count Calculator

Work out auto scaling instance count instantly with clear inputs, formula shown and shareable results.

About Auto Scaling Instance Count Calculator

Autoscaling bounds are set from the baseline and peak request rates divided by per-instance capacity at a safe utilisation of around 70 percent. The saving comes from the shape of the day: a five-hour peak means average running capacity is far below the maximum, and provisioning for peak all day wastes the difference.

Formula

Scaling bounds

min = ceil(baseline / (per-instance x 0.7)); max = ceil(peak / (per-instance x 0.7)); average = min + (max - min) x peak hours / 24

Worked example

With the default values below, Auto Scaling Instance Count Calculator returns Maximum instances needed of 18. Change any input above and every figure here is recalculated in your browser.

Inputs used

Baseline request rateminimum 0.001 req/s
400 req/s
Peak request rateminimum 0.001 req/s
2,200 req/s
Capacity per instanceminimum 0.001 req/s
180 req/s
Hours per day at peakaccepts 0.1–24 h
5 h

Results

Maximum instances needed
18
Minimum instances
4
Average instances running
6.9
Capacity saved versus fixed provisioning
61.6%

Step by step

  1. 1Values used
  2. 2Scaling bounds
  3. 3Maximum instances needed= 18
  4. 4Minimum instances= 4
  5. 5Average instances running= 6.9
  6. 6Capacity saved versus fixed provisioning= 61.6

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 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 divide by 0.7?

Instances should not run at their absolute ceiling. Reserving 30 percent covers request bursts and gives the scaler time to add capacity before latency degrades.

Should the minimum be higher than the baseline requires?

Usually yes, at least two instances across two availability zones, so a single instance or zone failure does not take the service down.

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