I calculate this stuff using an Excel spreadsheet I created.

You can read details and download the spreadsheet

here.
The one I use is actually updated a bit from that one, but I'm not going to post the newer version until I get a chance to do some testing to tune its accuracy. I'll probably ask that it be made a sticky then.

The biggest issues affecting its accuracy are: how well the C/W of the hotside cooling system is known, and how well the loaded CPU power consumption is known. Another big issue is the C/W of cooling/heating through the CPU pins and motherboard. I just kind of tack that on as a fudge factor to the number the spreadsheet calculates.

There are so many variables in watercooling systems and CPU temperature sensors, that a C/W guess I would make from a description of someone's WC components probably has at least a +100%/-50% margin of error. And the C/W value for the WC system has an enormous effect on the final result.

The best I can do is say that;

**IF** the CPU temp sensor is accurate, and

**IF** the

Computer Nerd Thermal Calculator is accurate about loaded power consumption, then the actual CPU temperature will probably be between 3 and 10 degrees higher than the number my spreadsheet calculates. (Those are both very big "ifs".)

So, to answer your question, yes there is an equation, but it is one big ugly MOFO.