Methods, apparatus, systems and articles of manufacture to estimate deduplicated total audiences in cross-platform media campaigns are disclosed. An example method includes determining a first audience reach for a television audience, the television audience representative of audience members exposed to the media campaign via television media delivery, and determining a second audience reach for a digital audience, the digital audience representative of audience members exposed to the media campaign via digital media delivery. The example method also includes an overlap multiplier based on the media campaign, the overlap multiplier an odds ratio, and determining a duplication factor for the media campaign based on the first audience reach, the second audience reach and the overlap multiplier. The example method also includes determining a total audience for the media campaign based on the first audience reach, the second audience reach and the duplication factor.