EC2 CPU Utilization Calculator

Convert CloudWatch CPU averages into busy vCPUs, idle headroom and the monthly cost of the capacity you never use.

Inputs

vCPUs on the instancevCPU

Average CPU utilisation%

CloudWatch CPUUtilization, averaged over a representative window.

Peak CPU utilisation%

Use the p99 rather than the maximum so one spike does not drive the sizing.

Target utilisation for sizing%

On-demand rate per hourUSD/hour

Hours running per monthhours

Monthly Cost of Idle vCPU

$218.65

vCPUs Actually Busy

1.76

vCPUs Idle on Average

6.24

vCPUs Needed at the Target

Headroom Above Peak

35.0%

Sizing Verdict

Reasonably sized against the target utilisation

Step by step

Values used
vCPUs on the instance = 8 vCPU; Average CPU utilisation = 22 %; Peak CPU utilisation = 65 %; Target utilisation for sizing = 60 %; On-demand rate per hour = 0.3840 USD/hour; Hours running per month = 730 hours
EC2 CPU Utilization
busy vCPU = vCPUs × average CPU%; idle cost = hourly rate × hours × (idle vCPU ÷ total vCPU); vCPU at target = ceil(busy vCPU ÷ target%).
Monthly Cost of Idle vCPU
= 218.65
vCPUs Actually Busy
= 1.76
vCPUs Idle on Average
= 6.24
vCPUs Needed at the Target
= 3
Headroom Above Peak
= 35.0
Sizing Verdict
= Reasonably sized against the target utilisation

How it works

CloudWatch reports CPU as a percentage of the whole instance, so multiplying by the vCPU count converts it into absolute compute. Costing the idle share is a first-order approximation because EC2 charges for the instance, not the cycles — the idle fraction is money spent on capacity that produced nothing. Sizing on the p99 rather than the mean is what keeps a resize from causing a latency regression. Average CPU under 20% with a modest peak is the classic right-sizing signal, and putting a dollar figure on the idle share is what gets a resize prioritised over a backlog item.

Formula

EC2 CPU Utilization

busy vCPU = vCPUs × average CPU%; idle cost = hourly rate × hours × (idle vCPU ÷ total vCPU); vCPU at target = ceil(busy vCPU ÷ target%).

average CPU%: CloudWatch CPUUtilization mean over the window
peak CPU%: p99 utilisation, the figure sizing must survive
target: Utilisation you are willing to run at steady state

Frequently Asked Questions

How is EC2 CPU Utilization calculated?

busy vCPU = vCPUs × average CPU%; idle cost = hourly rate × hours × (idle vCPU ÷ total vCPU); vCPU at target = ceil(busy vCPU ÷ target%). CloudWatch reports CPU as a percentage of the whole instance, so multiplying by the vCPU count converts it into absolute compute. Costing the idle share is a first-order approximation because EC2 charges for the instance, not the cycles — the idle fraction is money spent on capacity that produced nothing. Sizing on the p99 rather than the mean is what keeps a resize from causing a latency regression.

Why does EC2 CPU Utilization matter?

Average CPU under 20% with a modest peak is the classic right-sizing signal, and putting a dollar figure on the idle share is what gets a resize prioritised over a backlog item.

What values do I need to enter?

This calculator takes 6 inputs: vCPUs on the instance, Average CPU utilisation, Peak CPU utilisation, Target utilisation for sizing, On-demand rate per hour, Hours running per month. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

You might also need

EC2 Instance Right-Sizing Calculator EC2 Memory Utilization Calculator EC2 vCPU Requirement Calculator