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EC2 vCPU Requirement Calculator

Size vCPUs from request rate and CPU time per request, including peak factor, target utilisation and spare instances.

About EC2 vCPU Requirement Calculator

Requests per second multiplied by CPU seconds per request gives CPU-seconds consumed per second, which is exactly a vCPU count — the utilisation law behind every capacity model. Dividing by the target utilisation adds queueing headroom, because response time climbs sharply as a processor-sharing system approaches saturation, and adding spare instances afterwards keeps the target intact when one node or AZ disappears. Sizing from CPU time rather than latency avoids the classic mistake of provisioning for I/O wait, which needs threads and memory but no additional vCPU at all.

Formula

EC2 vCPU Requirement

vCPU busy = requests/s × CPU seconds per request; vCPUs required = ceil(peak vCPU busy ÷ target utilisation).

CPU seconds
CPU time per request, milliseconds ÷ 1000
peak factor
How much higher the busiest minute runs than average
target
Utilisation you plan to hit at peak, leaving queueing headroom

Worked example

With the default values below, EC2 vCPU Requirement Calculator returns vCPUs Required at Peak of 312. Change any input above and every figure here is recalculated in your browser.

Inputs used

Steady-state request rateaccepts 0–10,000,000 requests/s
2,500 requests/s
CPU time per requestaccepts 0.1–60,000 ms
45 ms
Peak to steady-state ratioaccepts 1–20 x
1.8 x
Target vCPU utilisation at peakaccepts 10–100 %
65 %
vCPUs per instanceaccepts 1–448 vCPU
8 vCPU
Spare instances for redundancyaccepts 0–100 instances
1 instances

Results

vCPUs Required at Peak
312
Instances Required
40
Peak Request Rate
4,500 requests/s
vCPUs Busy at Steady State
112.5
vCPUs Provisioned
320
Utilisation at Peak
63.3%

Step by step

  1. 1Values used
  2. 2EC2 vCPU Requirement
  3. 3vCPUs Required at Peak= 312
  4. 4Instances Required= 40
  5. 5Peak Request Rate= 4,500 requests/s
  6. 6vCPUs Busy at Steady State= 112.5
  7. 7vCPUs Provisioned= 320
  8. 8Utilisation at Peak= 63.3

Assumptions and limits

  • The result is derived only from the 6 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

How is EC2 vCPU Requirement calculated?

vCPU busy = requests/s × CPU seconds per request; vCPUs required = ceil(peak vCPU busy ÷ target utilisation). Requests per second multiplied by CPU seconds per request gives CPU-seconds consumed per second, which is exactly a vCPU count — the utilisation law behind every capacity model. Dividing by the target utilisation adds queueing headroom, because response time climbs sharply as a processor-sharing system approaches saturation, and adding spare instances afterwards keeps the target intact when one node or AZ disappears.

Why does EC2 vCPU Requirement matter?

Sizing from CPU time rather than latency avoids the classic mistake of provisioning for I/O wait, which needs threads and memory but no additional vCPU at all.

What values do I need to enter?

This calculator takes 6 inputs: Steady-state request rate, CPU time per request, Peak to steady-state ratio, Target vCPU utilisation at peak, vCPUs per instance, Spare instances for redundancy. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

Why keep the target below 100%?

Queueing theory: for a processor-sharing server, waiting time scales as 1 ÷ (1 − utilisation). At 65% you pay about 3× the service time in latency; at 90% you pay 10×. Targets of 60–70% are the usual compromise between cost and tail latency.

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