Methods and apparatus to predict demographics are disclosed. An example apparatus includes a prediction manager to access matched demographic impression data when a database proprietor subscriber does not match user data corresponding to a media delivery device, the matched demographic impression data including first demographic data stored by a database proprietor; a linear scaler to linearly scale the matched demographic impression data when the matched demographic impression data covers all demographic categories under consideration; a differential scaler to differentially scale the matched demographic impression data when the matched demographic impression data does not cover at least one of the demographic categories; and a modeler to: generate a model representative of the matched demographic impression data linearly scaled or the matched demographic impression data differentially scaled; and generate predicted demographic impression data by applying the model to second impression data to correct for the database proprietor not storing second demographic data.