CPU Utilisation Calculator

Work out cpu utilisation instantly with clear inputs, formula shown and shareable results.

About CPU Utilisation Calculator

CPU busy is user plus system time; I/O wait is idle time where at least one process is blocked on disk, so it indicates a storage bottleneck rather than CPU load. High system time relative to user time points at syscall, interrupt or context-switch overhead. Converting the percentage into core equivalents makes headroom concrete.

Formula

CPU breakdown

busy = user + system; idle = 100 - busy - I/O wait; cores busy = busy x cores / 100

Worked example

With the default values below, CPU Utilisation Calculator returns CPU busy of 67.0%. Change any input above and every figure here is recalculated in your browser.

Inputs used

User timeaccepts 0–100 %: 55 %
System timeaccepts 0–100 %: 12 %
I/O waitaccepts 0–100 %: 18 %
CPU coresminimum 1: 16

Results

CPU busy: 67.0%
Idle: 15.0%
Cores fully occupied: 10.72
Likely bottleneck: User bound - application code is the main consumer

Step by step

1Values used
2CPU breakdown
3CPU busy= 67.0
4Idle= 15.0
5Cores fully occupied= 10.72
6Likely bottleneck= User bound - application code is the main consumer

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

Is high I/O wait a CPU problem?

No. It means the CPU has nothing to do because processes are blocked on storage. Adding CPUs will not help; faster disks or fewer synchronous reads will.

What causes high system time?

Heavy syscall rates, network interrupt processing, page faults and lock contention in the kernel. Small-packet network traffic and inefficient I/O patterns are common causes.