CPU Utilization Calculator

Convert raw busy time measurements into a CPU utilization percentage by comparing active CPU time to the total observation window—perfect for profiler traces, performance dashboards, or /proc/stat snapshots.

Elapsed time the CPU (or core) spent executing work during the sample window.
Total sample duration, including both busy and idle time. Use the same units as the busy value.

Actual operating systems may use moving averages, weighted samples, or different clock sources. Always confirm interpretation with your monitoring tool's documentation.

Examples

  • 42 ms busy during a 120 ms sample ⇒ 35% utilization
  • 520 ms busy within a 1,000 ms interval ⇒ 52% utilization
  • 8.4 ms busy out of a 16 ms frame budget ⇒ 52.5% utilization
  • 2,400 µs busy out of 3,000 µs total ⇒ 80% utilization

FAQ

Can the observation window be zero?

No. Provide a non-zero window length to avoid division by zero and to represent a real sampling interval.

How should I handle multi-core processors?

Collect busy time per core and either average the resulting percentages or sum the busy times and divide by (core count × observation window).

My monitoring tool already shows a percentage—should I still use this?

If you only have the percentage, multiply it by the observation window to reconstruct busy time, then plug both values in to verify other calculations.

Does turbo boost or CPU frequency scaling change the math?

No. Utilization compares scheduled busy time against total time. Frequency changes affect throughput but not this ratio.

Additional Information

  • Utilization (%) = (Busy time ÷ Observation window) × 100. Keep both inputs in the same time units (ms, µs, or seconds).
  • Values above 100% indicate the busy time exceeds the window—this can happen if you sum per-core metrics without dividing by core count.
  • For multi-core CPUs, divide combined busy time by (cores × window) to obtain average utilization per core.

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