When a server serves web pages and/or software application pages with digital ads to client devices, a system determines viewability scores for individual ad insertion spaces on the pages. The system determines viewability scores for each field based on the time at which at least a threshold percentage or ratio of the field's pixels where viewable and not off-screen or obscured by another open window. The system or the server may then select digital ads to serve to each field based on the field's viewability score.