Quick Answer
To calculate the flow rate of a well pump, determine the well's static water level, the pump's rated flow rate, and the desired pressure at the outlet. Multiply these values to achieve the desired flow rate.
Calculating Static Water Level
To estimate the static water level, measure the depth to water (DTW) in the well casing using a tape measure or a calibrated depth tape. The DTW is the distance from the ground surface to the water level in the well casing. For example, if the DTW is 20 feet, this is the minimum depth the pump will need to lift water from the well.
Selecting a Pump
To select the right pump for your well, consider the desired flow rate, the static water level, and the pressure requirements at the outlet. A typical residential well pump has a flow rate ranging from 5 to 20 gallons per minute (GPM). Select a pump with a higher flow rate than the required rate to account for friction losses in the piping and fittings.
Pump Sizing and Efficiency
To ensure the pump is sized correctly, calculate the total dynamic head (TDH) of the system, which includes the static water level, the pressure drop in the piping, and the pressure requirements at the outlet. A typical pump efficiency ranges from 40 to 70% and should be considered when selecting the correct pump size. For example, a pump with a 6 GPM flow rate and 50% efficiency would move 3 GPM of water to the desired pressure.
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