If your website has been around for a while, you probably know how much cash flow it generates on a monthly basis. You probably also know how much money you make per ad (on a CPM or CPC basis) and approximately how many visitors come to your site each month and how many ad impressions they generate. By doing some simple math and factoring in realistic assumptions for your site's traffic growth, you should be able to put together a pretty accurate cash flow forecast for your business.