Abstract In general, a horizontal well is more productive than a vertical well, due to the fact that there is a larger area of contact with the reservoir and that there is less pressure drop per unit of length in the production section. The two main factors to obtain more reserves in a reservoir are the increase in drainage area or an increase of the recovery factor using some enhanced recovery method. The productivity as well as the drainage area of a horizontal well may be limited by the pressure drop within the wellbore, especially when the pressure drop is comparable to the drawdown from the reservoir to the wellbore. Over the past few years, horizontal well technology has acquired quite significant importance and thousands of wells are drilled around the world every year, which is why there is a need for more information on the factors that affect pressure drop within the well. Total pressure drop in horizontal wells has been analyzed in terms of four separate effects: wall friction, flow acceleration, perforation roughness, and fluid mixing.1 Thus, the pressure drop in horizontal wells is made up of a gravity term, a frictional term and another due to flow acceleration.2 The literature available about calculating drainage areas of horizontal wells only covers geometric methods, which assume that the horizontal well drains an elliptic area or a combined area made up of a central rectangle and two semicircles on the ends. 2,3 This paper proposes an expression to calculate the drainage area of horizontal oil wells that produce at high reservoir flow rates, exhibiting a frictional pressure drop throughout the wellbore between 15 to 60% of the reservoir drawdown at the heel of the well.