Calculating out the conversion rate of a campaign is a great way to measure the total number of users who are taking a action on your website that is desirable. Tracking the conversion rate of these activities (form fill, views of a key web page, video views, white paper downloads, etc) allows for campaign analysis of which tactics / channels are sending you more of what you value.
Measuring the conversion rate of a campaign varies depending on the channel. For advertising channels which are measured based on clicks (ie: paid search, organic, direct, referral, paid social, etc) the conversion rate formula is very straightforward.
Conversion Rate = (Conversions / Clicks) x 100
Advertising channels which are measured based on the total number of times the ad was displayed, and potentially viewed. For a view through conversion to occur, the users must first:
1) See the ad, but not actually click on the ad
2) Complete a conversion on your website
The challenge here lies in the fact that the dsp (demand side platform) typically measures this activity via cookies, so there's no real formula for it. But if you wanted to take try and calculate it, the formula to calculate a view-through conversion would look something like this:
Conversion Rate = ((Total Conversions - Click Through Conversions) / Impressions)) x 100