Content marketing creates tangible outputs for your business, it helps engage existing and future customers and leaves them changed, such that they are informed, educated or entertained. And are now more aware of your offering.
To measure the return on investment, first, you need to establish holistically how much you have spent on it. Including time, salaries, production, development, hosting, advertising.
Then look at the outputs, what did we receive back? There are tangibles and intangibles. The tangibles are how many people saw it, read it, watched it, the intangibles are how did they feel after reading it? Did they go and consider a purchase.
Using a dashboard like Nudge provides you with this transparency.