CPU Utilization Calculator
Calculate CPU utilization percentage from user, system and idle time measurements.
About CPU Utilization Calculator
The Linux kernel exposes cumulative CPU time counters in /proc/stat, measured in jiffies since boot. Taking two snapshots and computing the delta of each field (or using cumulative totals as a rough single-point estimate, as here) lets you derive utilization: the busy time is user + nice + system + irq + softirq + steal, and total time additionally includes idle and iowait. Dividing busy by total gives the percentage of CPU time spent doing work.
Formula
CPU usage percentage
usage% = 100 × (user+nice+system+irq+softirq+steal) / (user+nice+system+idle+iowait+irq+softirq+steal)
- u
- user
- n
- nice
- s
- system
- i
- idle
- w
- iowait
- q
- irq
- f
- softirq
- t
- steal
Worked example
With the default values below, CPU Utilization Calculator returns CPU Usage of 31.72%. Change any input above and every figure here is recalculated in your browser.
Inputs used
- userminimum 0
- 5,000
- niceminimum 0
- 100
- systemminimum 0
- 2,000
- idleminimum 0
- 15,000
- iowaitminimum 0
- 500
- irqminimum 0
- 50
- softirqminimum 0
- 50
- stealminimum 0
- 0
Results
- CPU Usage
- 31.72%
- Idle
- 66.08%
- I/O Wait
- 2.20%
- Total Jiffies (sum of all counters)
- 22,700
Step by step
- 1Values used
- 2CPU usage percentage
- 3CPU Usage= 31.72
- 4Idle= 66.08
- 5I/O Wait= 2.20
- 6Total Jiffies (sum of all counters)= 22,700
Assumptions and limits
- The result is derived only from the 8 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
Should I use cumulative counters or deltas between two samples?
For an accurate instantaneous reading, take two /proc/stat snapshots a second or two apart and feed the *differences* of each field into this calculator — that is what `top` and `mpstat` do internally. Using raw cumulative totals from boot gives an average over the machine's entire uptime.
What does the steal field represent?
Steal time is CPU time a virtual machine's guest OS was ready to run but the hypervisor gave the physical CPU to another guest instead — a useful signal of noisy-neighbor contention on shared virtualized or cloud hosts.
Why is iowait excluded from busy time?
iowait represents idle time where the CPU had no runnable task but was waiting on a pending disk I/O — the CPU wasn't actually executing instructions, so it counts toward idle capacity, not utilization.