New equations for computing the electrical conductivity of powder aggregates and sintered compacts are proposed. In both cases, the effective or apparent conductivity is a function of the bulk material conductivity, the porosity of the sample and the tap porosity of the starting powder. Additional parameters are required for powder aggregates, such as the conductivity of the oxide covering the particles, the thickness of the oxide layers and the ease of descaling them. The new equations are valid from zero porosity to the tap porosity. Links between the equations and the percolation conduction theory are stated. Measurements of electrical resistance on sintered compacts and powder aggregates subjected to different pressures were performed. The proposed equations have been validated with these data. The electrical conductivity of both sintered compacts and powder aggregates of aluminium, bronze, iron and nickel was determined and compared to the equation predictions, resulting in notably good agreement.