Abstract: A quantitative refill device is mounted on the top end of an overflow pipe in a water tank of a water saving toilet and comprises a first pipe, a second pipe and a float device. The second pipe defines a second chamber. Initially, water flows to the overflow pipe to create a water closing through a water flow passage between the second pipe and the float pipe, especially a conical passage defined between a conical surface of the second pipe and a conical portion of the float device. Meanwhile, the float device moves upward along with the rise of the water level of the second chamber to close the conical passage. By the time the conical passage is closed, the water closing is formed, and the refilled surplus water will overflow into the water tank, thus creating a quantitative water refilling effect.