Why is my CPU at 100 percent when I'm not doing anything?
Your CPU can reach 100 percent even when you are not touching your computer because apps and background tasks are still running. Updates, virus scans, a stuck browser tab, or mining software can all cause this—but high CPU use alone does not prove you have malware.
Find out what is using your CPU
Your CPU is the part of your computer that runs instructions. “100 percent” usually means its available processing time is fully occupied, not that it is broken.
Open your computer’s built-in activity viewer:
- Windows: Press Ctrl + Shift + Esc to open Task Manager. Select Processes, then click CPU to sort by usage.
- Mac: Open Activity Monitor, select the CPU tab, and sort by % CPU. On a Mac, an individual app can show more than 100 percent when it uses multiple processor cores.
Watch for a minute or two. A brief spike is normal. Sustained high use while you expect the computer to be idle deserves a closer look.
Note the busiest app or process—a process is a running program or part of one. Do not end unfamiliar system processes at random or delete files just because their names look strange.
Check ordinary background work first
Your computer may be busy with work you did not start directly, such as:
- Installing operating system updates.
- Scanning for viruses.
- Organizing files for faster searches.
- Syncing photos or cloud storage.
- Processing a video, game download, or backup.
A browser can also stay busy with video, animations, extensions, or a faulty webpage, even when its window is minimized.
Save your work, then close apps you recognize one at a time. If an update or security scan is progressing normally, let it finish. If the same app repeatedly gets stuck, update it or restart it.
Could it be a browser miner?
Mining uses computing power to help secure a cryptocurrency network. Monero’s mining method, called RandomX, favors CPUs, so mining can keep your processor busy even when you are doing nothing else.
If you deliberately started a browser mining session, high CPU use may be expected. Check its controls and lower the number of mining threads—the parallel workers using your CPU—or pause mining when you need your computer.
If you did not agree to mining:
- Close unfamiliar tabs, then fully quit your browser.
- Check whether CPU use falls.
- Review installed browser extensions and remove ones you do not trust.
- Use the browser’s own task manager, if available, to identify a busy tab or extension.
A drop after closing the browser points toward browser activity, but it does not prove mining. Unauthorized mining is often called cryptojacking: someone uses your computing power without permission.
If the high usage continues
Restart your computer, install trusted updates, and run a full scan with your built-in security software or another reputable tool. Review apps that launch automatically when you sign in.
Avoid pop-ups claiming to have found a “hidden miner” and demanding payment for a cleanup. Do not disable security tools because a mining site tells you to.
Constant CPU use can raise electricity costs, increase fan noise, and make your computer hot. Pause heavy tasks if it becomes unusually hot or shuts down unexpectedly. On a work or school device, contact IT rather than changing security settings yourself.
The bottom line
Start by identifying the busiest process. High CPU use is often ordinary background work, but unexplained, persistent activity is worth investigating—especially if you never agreed to mining.
