Serverload Calculations

I am curious about how the server load calculations are made, and if they can be retrieved through a server side script.

I have made scripts in the past that have calculated CPU usage of individual accounts, Memory usage ect. So I assume server load is calculated using possibly the average of running processes over a given period of time (or something along those lines).

I would like to be retrieve server load information to give to my clients, so I am interested to know if anyone knows how the server load is calculated. I also assume memory and cpu usage play a part in determining it.

If anyone has any information regarding how server load is calculated I would love to know about it.

Thank you in advance to anyone who has any information.