ROI, or return on investment, in paid media campaigns is typically measured by comparing the revenue generated by the campaign to the total cost of the campaign. To calculate ROI, advertisers need to track the conversion rate, or the percentage of users who take a desired action, such as making a purchase or filling out a form. They also need to track the cost per conversion, or the total cost of the campaign divided by the number of conversions. By comparing the revenue generated by the campaign to the total cost, advertisers can determine the ROI and adjust their campaigns accordingly for better performance.