What are marketing metrics & formulas for websites?

I think this can be a very helpful thread for website owners. Post the metrics and formulas you know and let's build a huge list here. I did the CTR one now its your turn.

Copy The formulas from above your post then add your formulas and repost them all, let's build a good list.

Marketing Formulas:

CTR - Click Through Rate = Total Clicks / Total Impressions
Example: I have a banner ad that was shown 500 time and I had 50 people click my banner. My CTR would = .1%