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Capacity Headroom Calculator

Work out capacity headroom instantly with clear inputs, formula shown and shareable results.

About Capacity Headroom Calculator

Capacity runs out on a compound curve, so headroom in months is the logarithm of the ratio between the safe ceiling and current utilisation, divided by the log of the growth factor. What turns that into an action is procurement lead time: if capacity takes six weeks to arrive, the order must be placed six weeks before the ceiling is reached, not when it is hit.

Formula

Headroom timing

months = ln(max safe / current) / ln(1 + monthly growth); order deadline = months - lead time in months

Worked example

With the default values below, Capacity Headroom Calculator returns Months until the safe ceiling of 4.4. Change any input above and every figure here is recalculated in your browser.

Inputs used

Current peak utilisationaccepts 1–200 %
62 %
Monthly growthaccepts 0.01–100 %
6 %
Maximum safe utilisationaccepts 10–100 %
80 %
Lead time to add capacityminimum 0.1 weeks
6 weeks

Results

Months until the safe ceiling
4.4 months
Headroom in utilisation points
18.0%
Order capacity within
3.0 months
Action
Headroom comfortable for now

Step by step

  1. 1Values used
  2. 2Headroom timing
  3. 3Months until the safe ceiling= 4.4 months
  4. 4Headroom in utilisation points= 18.0
  5. 5Order capacity within= 3.0 months
  6. 6Action= Headroom comfortable for now

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 cap utilisation at 80 percent?

Queueing delay grows sharply above that, and headroom is needed to absorb a failed replica or an unexpected burst. The exact number depends on how spiky the workload is.

Does cloud elasticity remove the need for this?

It shortens lead time but does not eliminate it. Quotas, instance-type availability and stateful services all have their own lead times.

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