Determinations of the start, duration, and strength of flood and drought are important in the operational monitoring and to decision-makers, but cannot be made through the commonly-used Standardized Precipitation Index.Physical considerations are provided in this note for measuring the daily flood and drought extent with precipitation.The rationale is that, because of the memory of the soil-land surface system, the flood and drought extent of a day is contributed by the precipitation of both the day and the earlier days, while the contribution of the earlier day precipitation is decayed due to the demands of the water balance, including the runoff, evapotranspiration, and percolation.The proposed index has the same unit and magnitude as precipitation, and is easy to calculate.It can be used to evaluate the interannual variabilities and long-term changes of the start, duration, and strength of flood and drought.