We derived an equation for saturation in carbonate reservoirs based on the electrical effi ciency model in the case of lacking core data. Owing to the complex pore structure and strong heterogeneity in carbonate reservoirs, the relation between electrical efficiency and water porosity is either complex or linear. We proposed an electrical effi ciency equation that accounts for the relation between electrical effi ciency and water porosity. We also proposed a power-law relation between electrical effi ciency and deep-formation resistivity and analyzed the factors controlling the error in the water saturation computations. We concluded that the calculation accuracy of the electrical effi ciency is critical to the application of the saturation equation. The saturation equation was applied to the carbonate reservoirs of three wells in Iraq and Indonesia. For relative rock electrical effi ciency error below 0.1, the water saturation absolute error is also below 0.1. Therefore, we infer that the proposed saturation equation generally satisfi es the evaluation criteria for carbonate reservoirs.