What Does “% CPU” Really Mean in Activity Monitor on Mac?
If you’ve ever opened Activity Monitor on your Mac—whether you’re troubleshooting an issue or just curious about performance—you’ve probably noticed a column labeled “% CPU.” It’s easy to assume that this number tells you how much of your Mac’s total CPU power each app is using. So when you see an app using close to, or even over, 100% CPU, it’s natural to be confused. How can an app use more than 100% of your CPU? Let’s break it down.
Why “% CPU” Isn’t What You Think
Here’s the key thing to understand: the “% CPU” column in Activity Monitor doesn’t represent the percentage of the entire CPU being used by an app. Instead, it shows how much of a single CPU core is being used. Modern Macs have multi-core CPUs—meaning the processor is divided into several cores that can handle tasks independently or together. For example, the M4 MacBook Air has a 10-core CPU. So when an app shows 100% CPU in Activity Monitor, it means it’s fully using one core. If an app shows 200%, it’s using two cores at full capacity. And if it shows 50%, it’s using half of one core’s processing power.
A Quick Example
Let’s say you’re using an M4 MacBook Air with 10 CPU cores, and you see an app using 50% CPU. That doesn’t mean the app is using half of your total CPU power. Instead, it’s using 50% of a single core, which works out to just 5% of the total CPU capacity. So even though the numbers might seem high, they’re not necessarily alarming, especially if you’re using a modern Mac with plenty of cores.
But wait! That’s still not quite the whole story
Traditional CPUs are rated for a specific “clock speed” (you may have heard the terms MHz/Megahertz or GHz/Gigahertz to describe a computer’s speed), and all cores run at that speed.
However, when Apple introduced Apple Silicon with the M-series chips (ie M1, M2, M2 Pro etc), they split the processor’s cores into two groups: E-cores (efficiency) and P-cores (performance). If an app developer marks a task as having high priority, it will be run on the P-cores. Conversely, the E-cores can have their clock speed scaled right back so they use less energy for tasks which aren’t urgent.
What this means is even if an E-core is loaded up to 100% with low-priority tasks, it could still be using considerably less than 100% of that core’s potential capacity.
Why This Matters
Understanding how “% CPU” works helps you:
- Avoid unnecessary panic when you see high numbers
- Better diagnose performance issues
- Recognise normal behaviour vs. problematic apps
For instance, it’s completely normal for demanding apps like video editors or web browsers with many open tabs to show high CPU usage temporarily. But if a lightweight app is constantly using 300% CPU for no reason, it may be misbehaving.
Summary: Don’t Be Fooled by the Numbers
- “% CPU” shows usage per core, not the whole CPU
- Your Mac likely has multiple cores (e.g., 10 in an M4 MacBook Air)
- An app using 100% CPU is using one core fully, not your entire CPU
- High numbers aren’t always bad, but persistent, unexplained spikes are worth investigating
Understanding this quirk of Activity Monitor, and keeping an eye on the CPU usage trend over time, rather than a snapshot at any given moment, can save you a lot of confusion and help you get a better picture of your Mac’s performance.