I'm running a Solaris server to serve PHP through Apache. What tools can I use to measure the bandwidth my server is currently using? I use Google analytics to measure traffic, but as far as I know, it ignores file size. I have a rough idea of the average size of the pages I serve, and can do a back-of-the-envelope calculation of my bandwidth usage by multiplying page views (from Google) by average page size, but I'm looking for a solution that is more rigorous and exact.

Also, I'm not trying to throttle anything, or implement usage caps or anything like that. I'd just like to measure the bandwidth usage, so I know what it is.

An example of what I'm after is the usage meter that Slicehost provides in their admin website for their users. They tell me (for another site I run) how much bandwidth I've used each month and also divide the usage for uploading and downloading. So, it seems like this data can be measured, and I'd like to be able to do it myself.

To put it simply, what is the conventional method for measuring the bandwidth usage of my server?