What is CPU load on a Mac?

Three numbers, not one

Open Activity Monitor and you will see three percentages at the bottom: User, System, and Idle. They always add up to 100%.

• User: work the CPU is doing for apps you launched. Safari rendering a page, Xcode compiling, Photoshop applying a filter.
• System: work for the macOS kernel and system daemons. Spotlight indexing, Time Machine, the window server.
• Idle: the CPU is doing nothing, waiting for work.

Total CPU load is essentially "100% minus Idle". On a healthy laptop sitting at the desktop, expect single-digit user and system, and 95%+ idle.

Per-core vs total

Activity Monitor's CPU window has a setting called "Show CPU history" with options for one graph per core. On an 8-core M-series chip you get 8 small graphs. Each graph maxes out at 100%.

Some other tools show a single number that goes up to 800% on an 8-core machine - that is the sum of all cores. Confusing the first time you see it. Both are legitimate ways to express the same data.

Apple Silicon: P-cores and E-cores

Since the M1, Apple Silicon chips have two kinds of cores:

• Performance cores (P-cores): fast, power-hungry. Used for foreground apps and demanding work.
• Efficiency cores (E-cores): slower, very low-power. Used for background tasks, mail fetching, indexing.

macOS schedules work between them automatically. When you see "100% on a few E-cores" while the P-cores stay idle, that is normal - it just means a background task is running and the system kept it off the fast cores to save battery.

What "high CPU" actually means

People panic when they see a process at 99% CPU. Usually it is fine.

• During compile, video export, or AI inference: 100% on all cores is what you want. The work is finishing as fast as possible.
• During a Spotlight reindex: high system CPU for an hour or two after a major update is expected.
• While streaming or on a video call: 30-60% across a few cores is normal.
• While the computer "is doing nothing": if user CPU is above 20% and you cannot identify what is running, that is worth investigating. Activity Monitor sorted by %CPU descending will name the culprit.

The numbers your monitor shows

A menu bar CPU monitor (the kind System Status Monitor adds) is showing the same numbers as Activity Monitor, sampled more frequently. The percentage can be:

• Average across all cores: 0-100%. Calm-looking but hides single-core spikes.
• Per-core, max: 0-100% from the busiest core. Spikier but more honest about what is happening.
• Total / sum: 0-N00% where N is your core count. The most "raw" view.

Pick whichever feels useful. The point of a menu bar monitor is to notice abnormal behaviour at a glance, not to be a perfectly calibrated instrument.