Abstract: Conventional production formulas for horizontal wells only consider the outer planar radial flow and the inner vertical radial flow, and ignore the intermediate planar linear flow. So the flow resistance is underestimated and oil production is overestimated with these formulas. This paper presents a comprehensive production formula for horizontal wells, which covers all the three flows. In comparison with the conventional production formulas, this new formula can get a higher flow resistance and a lower production rate for horizontal wells. Calculations show that the longer the horizontal section is, the higher the ratio of the intermediate planar linear flow to the total flow of wells will be, and the larger the drop of the production rate from the conventional production rate formulae will be. As the horizontal section length is close to the length of oil drainage zone, the new formula tends to be the production rate formula for wells with relatively long horizontal section. Therefore, horizontal wells with long and short horizontal section lengths can be unified by this new formula