Abstract: Methods, apparatus, systems, and articles of manufacture are disclosed to determine digital audience reach across multiple platforms. An example apparatus includes audience data receiver circuitry to obtain first audience data from a first media platform; and processor circuitry to perform at least one of the first operations, the second operations, or the third operations to instantiate: activity analyzer circuitry to determine media activities in the first audience data; adjustment analyzer circuitry to: apply at least one adjustment factor to the first audience data based on a source of the audience data; apply a coverage factor adjustment to the first audience data; and output the adjusted first audience data as a deduplicated audience for the first media platform.