CPU usage more than 100% on iMac

It all depends on what is reporting the CPU usage, and whether they want to count all cores and hyperthreading as fraction of 100% or if each thread of exection can consume 100% of a core or hyperthread.

If you look at your own Activity Monitor screen shot you can see that mds is using 20.7%, so even if Firefox was listed as using "Just" 100%, the sum of Firefox and mds would be 120+% so you are still over 100%. Thus Activity Monitor is allowing each core and hyperthread to have its own 100%.

A different reporting service may count things differently.

Anyway, depending on how many cores your Mac has AND whether the chip supports hyperthreading will determine Whether you have 200% (dual core no hyperthreading), 400% (quad core no hyperthreading), 800% (quad core with hyperthreading), 1600% (8 core with hyperthreading), etc...

Multiple CPUs (cores) and an app having multiple execution threads, means each available CPU could be used in part or 100% for a sum in the Activity Monitor display greater than 100%.

Firefox is is a multi-threaded app.

How can this be :

It shows 200 % so I assume 2 cores are maxed out ?

But how does the cpu load monitor on the right match to this ?

I see there 4 cores running at 50%?

**confused**

I agree that this is confusing there should never be > 100% especially for us 1 button mouse Apple users. To add to the confusion the cpu system, user and idle plot does not correlate with the list of process running and how much CPU% is reported. In the end I have no idea if my CPU is busy or not? Thanks for the super helpful activity monitor Apple. If I have 4 CPUs can I run at 400% reported CPU usage? Who knows? Does "turbo boost" change how much percentage is available? it sure would be nice to have some kind of tip strip that tells you what is going on here. I think windows 10 is more better at reporting multiple core system activity and CPU speed.

How to identify a Mac Pro (8-core) computer